Alloys used in dentistry/cosmetic dentistry course by Indian dental academy

S.NO CONTENTS PAGE NO.
1. Introduction 1
2. Metals 2
3. History of Metals 2
4. Properties of Metals 2
5. Classification of Metals 3
6. Inter Atomic Bonds 4
7. Microscopic Structure of Metals 5
8. Space Lattices 8
9. Lattice Imperfection 11
10. Heat Treatment 15
11. Strengthening of Metals 19
12. Alloys
1. classification
2. Solid solution
3. Eutectic Alloys
4. Peritectic Alloys
5. Intermetallic compound
6. Layer type system
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23
25
30
36
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13. Dental Casting Alloys
1. history
2. Properties
3. Casting Shrinkage
4. role of each ingredients
5. Alloys for all metal
6. Alloys for metal ceramic
7. Base Metal Alloys
40
40
41
42
45
52
61
67
14. Recent Advancements 80
15. Conclusion 81
References 82

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ALLOYS USED IN DENTISTRY
INTRODUCTION
Metals and alloys play an important role in dentistry. These form
one of the four possible groups of materials used in dentistry which
include ceramics, composites and polymers. These are used in almost
all the aspects of dentistry including the dental laboratory, direct and
indirect dental restorations and instruments used to prepare and
manipulate teeth. Although the latest trend is towards the “metal free”
dentistry, the metals remain the only clinically proven material for long
term dental applications.
WORK ORIGIN / MEANING
Metal : Latin– metallum = mine
Wrought : Old English – worhte = to work beaten to shape
Eutectic : gr-eu=well, tectos = to melt, easily melted
Anneal : Old English-aelan = to burn, heat
Grain : Latin – granum = seed
Alloy : Latin – alligere = to bind
Dendrite : gr-dendron=tree
Element : Latin – elementum = a first principle, a substance
that can not be resolved by chemical means into
simpler substances
Crystal : Any substance having regular shape and flat
Surfaces
Lattice : fr-lattis=lath=bar=network of crossed bars
Space lattice = a geometrically regular, 3 dimensional arrangement of
atoms in a space as it exits in a crystalline material and studied in x-
rays
Ingot = piece of cast metal sent to the work shop for rolling etc.
Base metal = metal which is easily oxidized when heated in air e.g.
copper, lead, iron, zinc.

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METALS :
Chemical elements in general can be classified as 1. Metals
2. Non-metals
3. Metalloids
Metalloids are those elements on the border line showing both
metallic and non metallic properties, e.g. carbon and silica. They do
not form free positive ions but their conductive and electronic
properties make them important.
Metals constitute about 2/3rd of the periodic table published by
DMITRI MEDELEYEV in 1868. Of the 103 elements which are categorized
in the periodic table according to the chemical properties, 81 are metals.
According to the metals hand book, they can be defined as “AN
OPAQUE LUSTROUS CHEMICAL SUBSTANCE, THAT IS A GOOD
CONDUCTOR OF HEAT AND ELECTRICITY AND WHEN POLISHED IS A
GOOD REFLECTOR OF LIGHT”
HISTORY OF METALS
Metals have been used by man ever since he first discovered
them. In ancient and pre-historic times, only a few metals were known
and accordingly these periods were called as “COPPER AGE”, “BRONZE
AGE” and “IRON AGE”. Today more than 80 metallic elements and a
large number of alloys have been developed. Ore is a mineral
containing one or more metals in a free or combined state.
PROPERTIES OF METALS :
All metals are solids except for mercury and gallium which are
liquid at room temperature and hydrogen which is a gas. The
properties of metals can be listed out as follows :
1. They have a metallic luster and mirror like surface
2. They make a metallic sound when struck

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3. Are hard, strong and dense
4. Ductile and malleable
5. Conduct heat and electricity
6. Have specific melting and boiling points
7. Form positive ions in solution and get deposited at the cathode
during electrolysis. E.g. copper in copper plating.
The outer most electrons of the atom are known as valence
electrons. These are readily given up and are responsible for most of
the properties.
Metals are tough and this is due to the fact that the atoms of the
metals are held together by means of metallic bonds.
The chemical properties of metals are based upon the
electromotive series which is a table of metals arranged in decreasing
order of their tendency to lose electrons. The higher an element is in
the series, the more metallic it is. This tendency of metals of lose
electrons is known as oxidation potential.
CLASSIFICATION OF METALS :
They can be done in many ways like :
1. Pure metal and mixture of metals (alloys)
2. Noble metals and base metals :
Noble metal is one whose compounds are decomposable by
heat alone, at a temperature not exceeding that of redness. E.g. Au,
Ag, and Pd.
Base metal is one whose compounds with oxygen are not
decomposable by heat Alone, retaining oxygen at high temperature.
E.g. Zn, Fe, and Al
3. Case metal and wrought metal
Cast metal is any metal that is melted and poured into the
mould
Wrought metal is a cast metal which has been worked upon in cold
condition

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4. Light metal e.g. Al and heavy metal e.g. Fe
5. High melting metal e.g. chromium and low melting metal e.g. tin
6. Highly malleable and ductile metal e.g. gold and silver
INTER ATOMIC BONDS :
The atoms are held together in place by atomic bonds or forces.
They may be
1. Primary
2. Secondary
Primary bonds or inter atomic bonds :
These are very strong bonds and may be of either type :
a. Ionic - These are seen in ceramics
b. Covalent - They are seen in organic compounds
c. metallic bonds - They are seen in metals and are non
directional
Secondary bonds or inter molecular bonds :
These are weak forces and are otherwise known as Vander waal‟s
forces. The various types are :
a. Hydrogen bonds
b. Dipole bonds
c. Dispersion bonds
Of all these, the most important one is the metallic bond which was
explained for the first time by LORENTZ, a Dutch scientist in 1916. It
can be explained by using the atomic and sub atomic structures.
The sub – atomic structures
1. Protons – positive charge
2. Neutrons – neutral charge
3. Electrons negative charge
The center or the nucleus of an atom consists of proton and
neutrons and are therefore positively charged. This is balanced by the

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revolving electrons which are negatively charged and arranged in
concentric shells with progressively increasing energy. The electrons in
the outer most shell are known as VALENCE ELECTRONS.
These are loosely bound and are therefore readily given up by the
atom to form positive ions. The cations thus formed behave like hard
spheres and the electron cloud formed by the freed valence electrons roam
about freely in the interstices formed by the arrangement of the solid
spheres. The electrons act like glue to hold all atoms together and are
known as INTER ATOMIC CEMENT. Because of this, the metals are strong,
hard, malleable, ductile and good conductors of heat and electricity.
MICROSCOPIC STRUCTURE OF METALS :
In the solid state, most metals have crystalline structure in which
atoms are held together by metallic bonds. This crystalline array
extends for many repetition in 3 dimensions. In this array, the atomic
centers are occupied by nuclei and core electrons. The ionisable
electrons float freely among the atomic positions.
The space lattice is a 3 dimensional pattern of points in space and
hence called as point lattice. In this the simplest repeating unit is called
as the UNIT CELL. The size and shape of the unit cell are described by
three vectors. They are a,b,c, and known as crystallographic axes. The
length and angle between them are known as LATTICE CONSTANTS
AND LATTICE PARAMETERS.
When a molten metal is cooled the solicitation process is one of
crystallization. These are initiated at specific sites called nuclei. These
in the molten metal are present as numerous unstable atomic
aggregates or clusters that tend to form crystal nuclei. These temporary
nuclei are known as EMBRYOS. These are generally formed from
impurities within the molten metal. In the case of pure metals, the
crystals grow as dendrites which can be defined as a three dimensional
network which is branched like a tree. The critical radius is the
minimal radius of the embryo at which the first permanent solid space
lattice is formed.

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The crystals are otherwise known as grains since they seldom
exhibit the customary geometric forms due to interference from
adjacent crystals during the change of state. The grains meet at grain
boundaries which are regions of transition between differently oriented
crystals. These are regions of importance as they are sites of:
1. Less resistance to corrosion
2. High internal energy and non crystalline
3. Collection of impurities
4. Barriers for dislocations
The nuclei can be homogeneous or heterogenous based upon
whether they are developed from the molten liquid or formed as a
result of foreign bodies incorporated into the molten metal. When the
crystals meet at the grain boundaries they stop growing further. The
grain boundaries are about 1-2 atomic distances thick. Grain
boundaries can be high angles (>10-15 degrees) or low angled (< 10
degree).
The grain structure can be fine where in, it contains numerous
nuclei as obtained during the rapid cooling process (quenching) or
refined when foreign bodies are added to obtain the fine grain
structure.
EQUALIXED GRAINS
When cooling occurs and grains are formed, the grains start
growing from the nuclei peripherally. This takes the shape of a sphere
and are equilaxed in structure meaning that they have the same
dimensions in any direction.
COLUMNAR AND RADIAL GRAINS
In a square mould, crystals grow from the edges towards the
centre to form columnar grains whereas in the cylindrical mould the
grains grow perpendicular to the wall surface and form radial grains.

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Columnar grains are weak due to interferences in the converging
grains. Sharp margins have columnar grains.

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GRAIN SIZE :
The grain size can be altered by heating. When the metal is
heated above the solidus temperature to the molten state and rapidly
quenched, small grains are formed whereas, when they are allowed to
cool slowly to room temperature the grains tend to grow due to atomic
diffusion and this results in an increased grain size and subsequent
decrease in the number. The more fine the grain structure, the more
uniform and better are the properties.
ANISOTROPHY :
Alloys with uniform properties due to the presence of fine grain
structure are said to be anisotropic.
METHODS OF FABRICATION OF METALS AND ALLOYS
1. CASTING : It is the best and most popular method.
2. WORKING ON THE METAL : They can be worked in the hot or cold
conditions. They are known as wrought metals. They can be
pressed, rolled, forged or hammered.
3. EXTRUSION : A process in which a metal is forced through a die to
form metal tubing.
4. POWDER METALLURGY : It involves the pressing of the powdered
metal into the mould of desirable shape and heating it to a high
temperature to cause a solid mass.
SPACE LATTICES
The structure of the crystal can be determined using the BRAGG’S
LAW OF X-RAY DIFFRACTION. There are 14 lattices known as BRAVIS
LATTICES and these are grouped under six families. These vary
depending upon the crystallographic axes and lattice constants which
are the length of the vertices and the angle between them. The six
families are :

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1. Cubic
Simple
Body centered
Face centered
2. Triclinic
3. Tetragonal
Simple
Body centered
Rhombohedral
4. Orthorombic
5. Hexagonal
Simple
Body centered
Face centered
Base centered
6. Monoclinic
Simple
Base centered

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The arrangement of atoms in the crystal lattice depend on the atomic
radius and charge distribution of atoms.
The most commonly used metals in dentistry have one of the following
space lattices : body centered cubic, face centered cubic or hexagonal
lattice.
SIMPLE CUBIC LATTICE SYSTEM

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LATTICE IMPERFECTIONS AND DISLOCATIONS
Crystallization from the nucleus does not occur in a regular
fashion, lattice plane by lattice plane. Instead, the growth is likely to be
more random with some lattice positions left vacant and others
overcrowded with atoms being deposited interstitially. These are
called defects and can be classified as :
A. POINT DEFECTS OR ZERO DIMENSIONAL DEFECTS
1. Vacancies or equilibrium defects :
Absences of an atom from its position. This can be :
 Vacancy
 Divacancy
 Trivacancy
2. Interstitialcies :
Presence of extra atoms in the interstitial spaces.
3. Impurities
4. Electronic defects

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Point defects are responsible for increased hardness, increased
tensile strength, electrical conductance, and phase transformations.
B. LINE DEFECTS OR SINGLE DIMENSIONAL DEFECTS :
These can be
1. Edge dislocation
2. Screw dislocation
The planes along which a dislocation moves is called as slip
planes and when this occurs in groups it is called as slip bands. The
crystallographic direction in which the atomic planes move is called as
the slip direction and the combination of slip plane and slip direction is
called as slip system.
These are responsible for ductility, malleability, strain hardening,
fatigue, creep and brittle fracture.
The face centered cubic consists of large number of slip systems
and therefore is very ductile. This is seen in gold.
The hexagonal closely packed system seen in zinc possesses
relatively few slip systems and is therefore very brittle.
In between these is the body centered cubic with intermediate
properties.
The strain required to initiate movement is the elastic limit. The
method of hardening of metals and alloys is based on the impedance to
the movement of dislocations.

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C.SURFACE DEFECTS OR PLANE DEFECTS OR TWO
DIMENSIONAL DEFECTS :
1. Grain boundaries
2. Twin boundaries :
These are seen in the NiTi wires responsible for transformation
between the austenitic and martensitic phases. These are important for

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the deformation of the α titanium alloys. The atoms have a mirror
relationship.
3. Stacking fault
4. Tilt boundaries
D. VOLUME DEFECTS
These include cracks
ALLOTROPHY AND ISOMORPHOUS STATE :
ALLOTROPHY
This ability to exist in more than one stable crystalline form is
called as allotrophy. The various forms have the same composition but
different crystal structure.

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ISOMORPHOUS STATE
The ability to exist as a single crystal at any atomic composition
of binary alloys is known as iomorphous state e.g. Au-Ag, Au-Cu.
HEAT TREATMENT OR SOLID STATE REACTIONS
Heat treatment of meals (non-melting) in the solid state is known
as solid state reactions. This is a method to cause diffusion of atoms of
the alloy by heating a solid metal to a certain temperature and for a
certain period of time. This will result in changes in the microscopic
structure and physical properties.
Important criteria are :
1. Composition of the alloy
2. Temperature to which it is heated
3. Time of heating
4. Method of cooling slowly or quenching.
The purpose of heat treatment is :
1. Shaping and working on the appliance in the laboratory is made
easy when the alloy is soft. This is the first stage and called as
softening heat treatment.
2. To harden the alloy to withstand high oral stresses, it is again
heated and this is called hardening heat treatment.
i. ANNEALING OR SOFTENING HEAT TREATMENT
This is done for structures that are cold worked. These cold
worked structures are characterized by :

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1. Low ductility
2. Distorted and fibrous grains
When cold work is continued in these, they will eventually
fracture. This is may :
1. Transgranular – through the crystals and occur at room
temperature
2. Intergranular – in between the crystals and occurs at elevated
temperature
These can be reversed by annealing. The various phase are :
1. Recovery
2. Recrystallization and
3. Grain growth
Technique:
The alloy is placed in an electric furnace at a temperature of 700°
C for 10mins and then rapidly quenched. Annealing temperature
should be half that necessary to melt the metal in degrees Kelvin.
Recovery
During this phase, the cold work properties begin to disappear.
There is a slight decrease in tensile strength and no change in ductility.
The tendency for warping decreases in this stage.
Recrystallization
There is a radical change in the microstructure. The old grains
are replaced by a set of new strain free grains. These nucleate in the
most severely cold worked regions in the metal. The temperature at
which this occurs is the recrystallization temperature. During this the
metal gets back to the original soft and ductile nature.

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Grain growth
If the fine grain structure in a crystallized alloy is further heated,
the grains begin to grow. This is essentially a process in which the
larger grains consume the smaller grains. This process minimizes the
grain boundary energy. This does not progress until the formation of a
coarse grain structure.
Properties of an annealed metal
1. There is an increase in ductility
2. Makes the metal tougher and less brittle
Stress relief annealing is a process which is done after cold
working a metal to eliminate the residual stress. This is done at
relatively low temperatures with no change in the mechanical
properties.

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ii. HARDENING HEAT TREATMENT
This is done for cast removable partial dentures, saddles, bridges
but not for inlays. This is done for clasps after the try in stage so that
adjustments can be carried out during the try in when the metal is soft.
Technique
The appliance is heat soaked at a temperature between 200-450°
C for 15-30 minutes and then rapidly quenched. The results is :
1. Increased strength
2. Increased hardness
3. Increased proportional limit
4. Decreased ductility
Microscopic changes
Diffusion and rearrangement of atoms occur to form an ordered
space lattice. Therefore this is called as order hardening or
precipitations hardening.
iii. SOLUTION HEAT TREATMENT OR SOLUTION HARDENING
When the alloy is soaked at 700°C for 10 minutes and then
rapidly quenched like that for a softening treatment, any precipitation
formed during the earlier heat treatment will become soluble in the
solvent metal.
iv. AGE HARDENING
This is a process in which following solution heat treatment ; the
alloy is once again heated to bring about further precipitation as a
finally dispersed phase. This causes hardening of the alloy and it is

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known as age hardening because the alloy will maintain the quality for
many years. E.g. Duralium.
METHODS OF STRENGTHENING METALS AND ALLOYS :
All metals possess an inherent barrier to dislocations. This is
relatively small and known as pearls stress. This is imposed by the
bonds associated with the arrangement of atoms in a given crystal
structure. Thus to improve the mechanical properties, other methods
of hardening are used. These are :
1. GRAIN BOUNDARY HARDENING OR GRAIN REFINEMENT
HARDENING
A poly crystalline metal contains numerous grains or crystals.
These meet at the grain boundaries. The grain boundary is non –
crystalline and contains impurities. These act as barriers to dislocations
as it moves by slip planes from one grain to another.
Finely grained structure contains large grain boundaries and
hence the obstacle to motion of dislocations is higher. therefore
dislocation density rises rapidly due to plastic deformation. These
dislocations at the grain boundaries increase and therefore the stress
necessary to continue the plastic deformation also increases. Therefore,
there is an increase in the yield strength and ultimate tensile strength.
The yield strength varies inversely with the square root of grain size
(hall petch equation).
Grain refinement can be done by :
1. Heat treatment
2. Addition of grain refiners which act as nucleating agents.
Grains refiners are metals or foreign bodies of high melting
temperature. They crystallize out at high temperature and act as nuclei
or seeds for further solidication. e.g. iridium, rhodium.

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The best method to improve properties of alloys and metals is by
the addition of grain refiners. Finely reined grains structure contain
grain size >70µm.
2. SOLUTION HARDENING OR SOLID SOLUTION STRENGTHENING
An alloy is a solid solution ; it has a solute and a solvent. The
atomic diameter of a solute and solvent will never be the same.
The principle of solid solution hardening is by introducing either
tensile or compressive strain depending on whether the solute atom is
smaller or larger than the solvent respectively and finally distorting the
grain structure. This solute can be either :
- Substitutional
- Interstitial
3. PRECIPITATION HARDENING
Another method of strengthening alloys is by means of this
technique. In this, the alloy is heated so that precipitates are formed as
a second phase which blocks the movement of dislocations. The
effectiveness is greater if the precipitate is part of the normal crystal
lattice which is known as coherent precipitation.
4. DISPERSION STRENGTHENING
It is a means of strengthening a metal by adding finely divided
hard insoluble particles in the soft metal matrix as a result of which, the
resistance to dislocations is increased. This increases hardness and
tensile strength.

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The ideal properties are seen when the particles range from 2-
15% by volume with spacing at 0.1 – 1.0µm intervals and particle size
from 0.01 – 0.1µ.
The ideal shape of the dispersed particle is a needle like
LAMELLAR SHAPE which can intersect with the slip planes. Powdered
metallurgy makes use of this method for strengthening.
5. STRAIN HARDENING OR WORK HARDENING
This is seen in wrought metals. The metals are worked after
casting to improve their mechanical properties. They may be forged,
hammered, drawn as wires, etc. All this is done below the re-
crystallization temperatures. This working causes vast number of
deformations within the alloys or metals. These interact with each
other mutually, impeding the movements. The increased stress
required for further dislocation movement to achieve permanent
deformation provides the basis for work hardening. This result is
distorted grain structure with the grains being fibrous.

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ALLOYS
ALLOYS AS ALREADY SAID, MEAN IN LATIN = TO BIND
Alloys can be defined as
1. Alloy is a combination of two or more metals which are generally
mutually soluble in the liquid condition.
2. Alloy is a metallic material formed by the intimate blending of two
or more metals. Sometimes a non metal may be added.
3. Alloy is a substance composed of two or more elements, at least one
of which is a metal.
METHOD OF ALLOYING
1. By melting together the base metal and the alloying element,
mixing them thoroughly and allowing them to solidify. This is the
common method.
2. Sintering or powder metallurgy : Metals are powdered, mixed
and pressed to the desired shape and then heated but not melted
till the powders unite to form a solid mass.
OBJECTIVES OF ALLOYING
The subjects of alloying are :
1. To increase the hardness and strength
2. To lower the melting point
3. To increase the fluidity of the liquid metal
4. To increase the resistance to tarnish and corrosion
5. To make casting or working on metal easy
6. To change the microscopic structure of metal
7. To change the color of the metal

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8. To provide special electrical and magnetic properties.
The alloying treatment may be present in the main or base
element as a :
1. Substitutional type
2. Interstitial type
3. Chemically combined form
CLASSIFICATION OF ALLOYS
The alloys can be classified in many ways :
1. According to the uses - All metal inlays
- Crowns and bridges
- Metal ceramic restorations
- Removable partial dentures
- Implants
2. Major element present - Ferrous alloys : rich in iron
- Gold and silver alloys
- Babbit metals – tin and lead
based alloys
- Nickel alloys
3. Nobility - High noble metals : noble metal - 60wt%
gold – 40%
- noble metals : 25% wt%, no
stipulation for gold
- predominantly based metal : <25% of noble
metals
4. Principle three elements : - Au-Pd-Ag
- Pd-Ag-Sn
- Co-Cr-Mo
- Ti-Al-V

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5. Based on yield strength and - Soft
elongation - Medium
- Hard
- Extra hard
6. Based on the dominant phase - Isomorphous
- Eutectic
- Peritectic
- Layered
- Intermetallic compound
7. Based on the method of - Cast metal
of fabrication
- Wrought metal
8. Based on the number of metals - Binary
- Ternary
- Quaternary
- Quinary
The composition of alloys can be defined by :
- Weight percentage of each element
- Atomic fraction or percentage of each element
Usually the alloy properties relate more directly to the atomic
percentage rather than weight percentage. The atomic % is not always
equal to the weight %.
- In Au-Cu3, the wt% of Au is 51% of Au is 25%
- Beryllium is present in nickel alloys in a small amount of
1.8wt%, but by at % it constitutes about 10.7%.

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SOLID SOLUTIONS OR ISOMORPHOUS STATE OR SINGLE
PHASE :
Solid solution is nothing but solution in the solid state. The
alloys of this type exist in a single phase with two or more components.
It consists of a solute and a solvent. These are completely miscible in
any proportion in both the solid and liquid state. Solvent is that metal
whose space lattice persists and solute is the other metal. By far these
represent one of the simplest, most common and useful of all
combinations.
E.g. Au – Ag
Au – Cu
Au – Pt
Au – Pd
Ag – Pd
The solid solution can be either :
1. SUBSTITUTIONAL SOLID SOLUTION
In this the solvent atoms are replaced by the solute.
This can be either :
- Regular or Ordered
- Random or Disordered
The ordered arrangement is one in which the atoms of solute are
arranged in the solvent in an ordered fashion so that they are not
distinguishable from the solvent. E.g. Au-Cu3 obtained when 50.2
wt% of gold and 49.8wt% of copper is cooled to below 400°C. This
causes a distorted crystal structure leading to keying it and increasing
hardness. This ordered structure is called as super lattice.

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The random arrangement contains solute that is randomly
distributed in the solvent. E.g. Pd-Ag, in which the silver atoms
replace the palladium atoms randomly. This arrangements has higher
energy.
2. INTERSTITIAL SOLID SOLUTION
In this, the solute atoms are present in positions between the
solvent atoms. E.g. carbon is distributed interstitially in iron to form
steels. In this the atomic size of the solute atoms should be smaller
than the solvent atoms.
HOME ROTHER’S RULE OF SOLID SOLUBILITY :
For substitution solid solutions, the solubility limit of solute in
solvent depends on :
1. CRYSTAL STRUCTURE
Only metals with the same type of crystal lattice can form a series
of solid solutions particularly if the size factor is less than 8% most of
the metals used for dental restorations are face centered cubic.

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2. CHEMICAL AFFINITY
When two metals exhibit a high degree of chemical affinity, they
tend to form an intermetallic compound on solidification rather than a
solid solution.
3. VALENCE :
Metals of the same valency and size are more likely to form
extensive solid solutions than metals of different valencies. If the
valancies differ ; the metal with a higher valence may be soluble in a
metal of lower valence.
4. ATOM SIZE
If the sizes of the two metallic atoms differ by less than 15% they
posses a favourable size factor for solid solubility. If the size factor is
greater than 15% multiple phases appear during solidification. For
good solubility the size difference should be less than 8%.
COOLING CURVE OF A SOLID SOLUTION
A cooling curve of a solid solution type of an alloy is shown.

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The temperature is found to drop as in the case of a pure metal
from „e‟ to „f‟ by simple cooling of the molten solution. At the
temperature „f‟, crystals of the solid start to form throughout the liquid.
The alloy is partly liquid and partly solid in the stage of cooling from „f‟
to „g‟. During this time interval the composition of the remaining
liquid is changing slightly and the temperature continues to drop
slowly. The portion of the curve from „f‟ to „g‟ represents the
solidification or freezing range during cooling in contrast to the
freezing point seen in pure metals. Portion „g‟ to „h‟ represented the
cooling of the solidified alloy.
PHASE DIAGRAM OF A SOLID SOLUTION ALLOY
The phase diagram of an alloy of composition X (approximately
60% A and 40% B) is shown :
TmA and TmB represent the melting points of the pure metals A
and B. This alloy is rendered completely molten by heating it to a
temperature above T1 which is the liquidus temperature for that
particular composition.
When the alloy is cooled from above T1, it remains molten until it
reaches T1 where the first solid begins to form. The composition of the

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first solid to form is given by drawing a horizontal line or TIE LINE to
intersect the solidus. In this case, drawing such a tie line reveals that
the first solid to form has a composition Z (approx 90% A/10%/B) As
the alloy is further cooled, more crystallization occurs and between
temperatures T1 and T2 a mixture of solid and liquid exists.
Selecting one temperature Tsl within this region, the composition
of both solid and liquid can be predicted by noting where the tie line
intersects both solidus and liquidus. Thus, at temperature Tsl, the
composition of the solid is Y (approx 80%A/ 20%B) and the
composition of the remaining liquid is W (approx75%B/ 25%A). On
further cooling, the alloy becomes completely solid at temperature Ts.
The last liquid to crystallize has the composition V (approx
80%B/20%A). This confirms the previous observation for the solid
solution alloy, that a cored structure exists in which the first material to
crystallize is rich in the metal with the higher melting point (A), whilst
the last material to solidify is rich in the other metal (B).

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PROPERTIES OF A SOLID SOLUTION ALLOY
The solid solution possesses:
1. Increased hardness
2. Increased strength
3. Increased proportional limit
4. Decreased ductility
5. Decreased resistance to corrosion due to coring
6. Melting range rather than a point
In general the microstructure of a solid solution resembles that of
the parent metals with properties that resemble an average of the two
compounds. The properties keep increasing until the concentration of
each compound reaches 50%.
EUTECTIC ALLOYS
The eutectic alloy is one in which the components exhibit
complete solubility in the liquid state but limited solid solubility E.g.
Ag-Cu. The term eutectic means lowest melting point. The eutectic
alloy has the lowest melting point than either of the constituent metals.
In silver copper system the temperature of silver is around
960.5°C and that of copper is 1083° C. But that of the eutectic
composition is 779.4° C. In this, an intimate but heretogeneous mixture
of the component metals exist when the alloy solidifies. E.g. a mixture
of salt and ice although completely soluble in each other in the liquid
state solidifies as separate salt and ice crystal on solidification.
These in contrast to other alloys do not have a solidification range
; instead they have a solidification point. When the eutectic alloy
solidifies, the atoms of the constituent metals segregate to form regions
of nearly pure metals, which result in a layered structure.

31
It can be written as :
LIQUID = α SOID SOLUTION + ß SOLID SOLUTION
It is referred to as invariant transformation because it occurs at a
single temperature and composition. The first formed grains of the
above said equation are called as primary grains and they are larger
than that of the eutectic composition.
Partial eutectic is a system where in the metals exhibit solubility
in liquid state and limited solubility in the solid state.
COOLING CURVE OF A EUTECTIC ALLOY
The solidification of an alloy of eutectic composition may present
the same curve as that of a pure metal, except that the solidification
temperature is lower than that for either of the pure metals. The
cooling curves of eutectic alloy, pure metal and a composition between
that of a metal and pure eutectic composition is given below :
During the cooling of such a mixture, the first break in the curve
represents the separation of some crystals of excess pure metal,
resulting in a change in the shape of the cooling curve. As the metal
crystals separate, the composition of the remaining liquid alloy changes
until the true eutectic composition is reached. At this time, the freezing

32
of the eutectic mixture occurs without further change in the
composition and at a constant temperature.
The cooling curve of an alloy of 50% tin and 50% lead through the
temperature range from near 300°C to about 120° C is shown. This
composition does not represent the eutectic composition of lead tin.
The cooling curve for this eutectic type of an alloy with excess
lead present can be divided into five distinct parts, each of which
represents a change in condition, or liquidsoid phase equilibrium of the
system. These changes in the curve may be observed by simple
inspection. The simple cooling of the liquid alloy is represented from
the starting temperature of about 270°C to 210°C. This section of the
curve is the same as that found in the uniform cooling of any liquid,
and the temperature drop here represents a simple function of the time
of cooling.
The second portion of the curve from 210°C to 176°C represents
the separation or freezing out of pure lead from the molten mass.
Within this range the whole mass is beginning to crystallize, but the
crystals that separate are pure lead floating in a liquid bath of lead and
tin, which is continually becoming richer in tin as a result of the lead
separation. Lead continues to separate as a crystalline metal until a
temperature of 176°C is reached.
At this point, the change in direction of the curve represents a rise
in temperature from 176°C to 183°C, which is from the under cooled
condition. This is due to the liberation of the latent heat of fusion. The
first irregularity of the curve at 210°C was brought about by the
liberation of the heat due to crystallization of lead. The final solidified
mass consists of a heterogeneous mass of lead crystals surrounded by a
matrix of lead tin alloy mixture of definite eutectic composition of 62%
tin and 38% lead. The matrix alloy has had its composition developed
through the process of separation of 12% of pure lead (50% minus 38%)
during the cooling from 210° C to 183°C. This final matrix alloy is
called the eutectic mixture.

33
Finally, from the temperature of 183°C downward, the curve
represents the simple cooling of the solid alloy.
PHASE DIAGRAM OF A EUTECTIC ALLOY
The phase diagram is obtained as for the pure metal.
In this diagram, on the left is shown the melting point of lead
(327°C) and on the right the melting temperature of tin (232°C). The
melting temperature (183°C) of the eutectic alloy (62%) tin is shown to
be lower than that of either ingredient metal.

34
By connecting the portions of the cooling curves which represent
the eutectic freezing temperature and the portions of the cooling curves
which represent the first separation of the excess ingredient metal in
different alloy compositions, a diagram is obtained.
From this it is evident that any alloy composition will be in the
liquid phase when heated to a temperature above that represented by
the lines from 327°C for pure lead, to 183°C at 62% tin for the eutectic,
to 232°C for pure tin. Below these lines, the excess metal will start to
crystallize out when an alloy of any composition is cooled, and the
mass will entirely crystallized below the temperature of 183°C. The
composition of 62% tin and 38% lead represents the lowest melting
mixture of tin and lead and is described as the eutectic composition. At
this composition no excess lead or tin separates, but instead a
homogenous mixture of lead and tin crystallizes simultaneously from
the liquid state.
To the right of the eutectic composition, at 80% tin for example,
the excess in separates during the cooling from 200°C to the eutectic
melting temperature, at which time the eutectic mixture crystallizes to
surround the separated tin. In the lower right portion the solid alloy is
described as solid eutectic and tin.
In the left portion, on cooling, the excess lead in a composition of
60% lead and 40% tin will separate before reaching 183°C after which
the eutectic will surround the lead crystals.
The phase diagram of the eutectic composition of Ag-Cu is given
below :

35
This has a composition of 28.1% Cu and 71.9% Ag. It can be seen
that a small amount of solid solution exists at each end of the diagram,
indicating, that silver is slightly soluble in copper and that copper is
slightly soluble in silver. The eutectic structure does not appear in
alloys of less than 8.8% copper. Only the α solid exists with varying
amount of ß solid solution depending on the temperature.
A photomicrograph of this alloy is interesting, since it indicates
that silver and copper have separated as mixtures rather than as
homogeneous solutions of silver and copper. Such an appearance is
typical of eutectic alloys.
PROPERTIES OF EUTECTIC ALLOYS
Alloys with composition less than that of the eutectic are called as
hypoeutectic and those with a composition greater than that of the
eutectic are known as hyper eutectic alloys. The primary crystals of
hypoeutectic are composed of α – solid solution and those of hyper
eutectic are composed of ß solid solution.
Therefore :
1. A linear variation between the composition and the physical
properties cannot be expected.
2. Since there is a heterogeneous composition, they are susceptible to
electrolytic corrosion.
3. They are brittle, because the present of insoluble phases inhibits
slip.
4. They have a low melting point and therefore are important as
solders.

36
PERITECTIC ALLOYS
Peritectic is a phase where there is limited solid solubility. They
are not of much use in dentistry except for silver tin system. Like the
eutectic, this is also an invariant transformation since this occurs at a
particular temperature and composition. The reaction is written as :
Liquid + ß = α
This type of reaction occurs when there is a big differences in the
melting points of the components. The peritectic phase diagram is
given below.
The α phase is a silver rich phase, the ß phase, a platinum rich,
and α + ß, a two phase region resulting from limited solid solubility.
The peritectic transformation occurs at the point P at which the liquid,
plus the platinum rich ß phase transforms into the silver rich α phase.
The substantial composition change involved can lead to large amounts
of coring if rapid cooling occurs. If the alloy has a hypoperitectic

37
composition, as does alloy 1 in the figure, cooling of the alloy through
the peritectic temperature results in the transformation.
LIQUID + ß = LIQUID +α
Rapid cooling results in precipitation of α phase around the ß
grains before diffusion can occur. The solid α phase inhibits diffusion,
and substantial coring occurs. The cored structure is more brittle and
has corrosion resistance inferior to that of the homogenous α phase.
These alloys undergo phase reactions and transformations upon
solidification because of partial solubility of the constituent metals.
INTERMETALLIC COMPOUNDS
These are compounds that are soluble in the liquid state but unite
and form a chemical compound on solidification E.g. Ag3 – Sn,
- Ag2 – Hg3
- Sn7 – Hg8
These are called as intermetallic compounds because ; the alloy is
formed by a chemical reaction between a metal and a metal. At space
lattice level, the atoms of one metal, instead of appearing randomly in
the space lattice of another metal, occupy a definite position in every
space lattice.

38
The phase diagram of an intermetallic compounds is :
The most important feature in this diagram, from the stand point of
silver tin amalgam alloy, is the fact that when an alloy containing
26.85% tin is slowly cooled with a temperature of 480°C, there is
produced an inter metallic compound, (Ag3-Sn) known also as gamma
phase ( ). This silver tin compound is formed only at the lower
temperatures over a narrow composition range from about 25 to 27%.
The silver content for such an alloy would be 73.15% on the basis of the
presence of 26.85% tin.
These diagrams are generally more complex than those for
eutectic and solid solution alloys. Few general effects can be predicted
from alloys forming chemical compounds.

39
PROPERTIES OF INTER METALLIC COMPOUND
1. Very hard
2. Brittle
The properties do not resemble that of the pure metal.
LAYER TYPE SYSTEM
In this, the two metals are completely insoluble in both the liquid
as well as the solid state. The two metals appear to solidify at their
individual freezing points into two separate distinct layers. The phase
diagram of this is shown below :
All this while, the discussion was on binary alloys and their
phase diagrams. But the same can be obtained for ternary alloys. The
three pure metals may be represented as the vertices of an equilateral
triangle, with the temperature indicated by the length of the vertical

40
line perpendicular to the plane of the triangle. Ternary diagrams have
not been developed to the extent of binary diagrams because of the
difficulty in their preparation and interpretation
DENTAL CASTING ALLOYS
Metal restorations can be made by a number of methods like
direct compaction as in the case of pure gold, swaging of metal foils,
CAD-CAM process for pure titanium or titanium alloys, electroforming
and copy milling.
Thus, although a variety of methods are available, the best and
the most popular method in use is casting. In this, the impression of
the prepared tooth is replicated in a refractory die, and a required
pattern is done using wax. This is then invested in an investment
material and burned out. Now in the mold available, the molten metal
or alloy is casted under pressure using centrifugal force.
The major events in the history of dental casting alloys are given
below :
Event Year
Introduction of lost wax technique 1907
Replacement of Co-Cr for Au in removable partial dentures 1933
Development of resin veneers for Au alloys 1950
Introduction of the porcelain fused to metal technique 1959
Palladium based alloys as alternatives to Au alloys 1968
Ni based alloys as alternatives to Au alloys 1971

41
Introduction of all ceramic technologies 1980
Au alloys as alternative to palladium based alloys 1999
The history of the dental casting alloys have been influenced by
quite a number of factors which involve the following :
1. The technological changes of dental prosthesis
2. Metallurgic advancements
3. Price changes of the noble metals
The fabrication of the cast inlay restoration which was presented by
TAGGART in 1907 to the New York Odontological group has been
acknowledged as the first reported application of the lost wax
technique.
DESIRABLE PROPERTIES OF THE CASTING ALLOYS
The metals must exhibit
1. Biocompatibility
2. Ease of melting
3. Ease of casting, brazing, soldering, and polishing
4. Minimal reactivity with the mold material
5. Good wear resistance
6. High strength, stiffness and rigidity
7. Sag resistance
8. Excellent tarnish and corrosion resistance

42
9. Should be inert in the oral conditions
10.Should have fatigue resistance
11.Should be amenable to heat treatment
12.Little solidification shrinkage
CASTING SHRINKAGE
This includes both the solidification shrinkage and the thermal
contraction from the solidification temperature to room temperature.
The shrinkage occurs in three stages :
1. The thermal contraction of the liquid metal between the
temperature to which it is heated and the liquidus temperature.
2. The contraction of the metal inherent in its change from the liquid
to the solid state
3. The thermal contraction of the solid metal that occurs on further
cooling to room temperature.
The first mentioned one is not of much consequence, because this
is compensated by the molten metal that flows into the mold.
In order to obtain accurately fitting prosthesis, it is necessary to
obtain compensation for this casting shrinkage. This can be achieved
by either generating computer aided over sized dies or through
controlled expansion techniques, which include both setting or
hygroscopic expansion and thermal expansion.
Linear solidification shrinkage of casting alloys :
Alloy type Casting shrinkage (%)
Type I (Au based) 1.56
Type II (Au based) 1.37

43
Type III (Au based) 1.42
Type IV (Ni-Cr based) 2.30
Type V ( Co –Cr based) 2.30
Generally type 2 and type 3 gold alloys represent the standards
against which the performance of other casting alloys are judged.
The classification of alloys for all metal, metal ceramic and frameworks
for removable partial denture are given below.
Classification of casting metals for full metal and metal ceramic
prosthesis and partial dentures
Metal Type
All-metal
prostheses
Metal ceramic
prostheses
Partial denture
frameworks
High Noble
(HN)
Au-Ag-Pd Pure Au
(99.7%)
Au-Ag-Cu-Pd
Au-Pd-Cu-Ag Au-Pt-Pd
HN metal
ceramic alloys
Au-Pd-Ag
(5-12 wt % Ag)
Au-Pd-Ag
(>12 wt% Ag)
Au-Pd
Noble (N)
Ag-Pd-Au-Cu Pd-Au
Ag-Pd Pd-Au-Ag
Noble metal
ceramic alloys
Pd-Ag
Pd-Cu-Ga
Pd-Ga-Ag
Predominantly
Base metal (PB)
CP Ti
Ti-Al-V
CP Ti
Ti-Al-V
CP Ti
Ti-Al-V
Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be
Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo
Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo

44
Co-Cr-W Co-Cr-W Co-Cr-W
Cu-Al
Solidus and liquidus temperature of the commonly used classes of
alloys :
Alloy type
ADA
classification
Solidus
temperature
(°C)
Liquidus
temperature
(°C)
Au-Pt High Noble 1060 1140
Au-Pd High noble 1160 1260
Au-Cu-Ag-Pd High noble 905 960
Au-Cu-Ag-Pd Noble 880 1270
Pd-Cu Noble 1145 1230
Pd-Ag Noble 1185 1045
Ag-Pd Noble 990 1270
Ni-Cr-Be
(Cr<20 wt %)
base metal 1160 1270
Ni-Cr (Cr<20
wt %)
Ni-Cr-Be
(Cr<20 wt %)
Co-Cr base metal 1215 1300

45
Different Metals Used In Dentistry
Gold (Au)
 Gold provides a high level of corrosion and tarnish resistance
 increases an alloy's melting range slightly.
 Gold improves workability, burnish ability, and raises the
density .
 However, gold imparts a very pleasing yellow color to an alloy (if
present in sufficient quantity).
 Unfortunately, that yellow color is readily offset by the addition
of "white" metals, such as palladium and silver. Gold is a noble
metal.
Palladium
 Palladium is added to increase the strength, hardness (with
copper), corrosion and tarnish resistance of gold-based alloys.
 Palladium will also elevate an alloy's melting range and improve
its sag resistance.
 It has a very strong whitening effect, so an alloy with 90% gold
and only 10% palladium will appear platinum-colored.
 Palladium possesses a high affinity for hydrogen, oxygen, and
carbon.
 It lowers the density of the gold-based alloys slightly but has little
similar effect on silver-based metals. Palladium, a member of the
platinum group, is a noble metal

46
Platinum
 Platinum increases the strength, melting range, and hardness of
gold-based alloys while improving their corrosion, tarnish, and
sag resistance.
 It whitens an alloy and increases the density of non gold-based
metals because of its high density.
 Platinum is a member of the platinum group and is a noble metal
Iridium
 serves as a grain refiner for gold- and palladium-based alloys to
improve the mechanical properties as well as the tarnish
resistance.
 Iridium is a member of the platinum group and is a noble metal.
Ruthenium (Ru)
 Ruthenium acts as a grain refiner for gold- and palladium- based
alloys to improve their mechanical properties and tarnish
resistance (like iridium).
 Ruthenium is a member of the palladium group and is a noble
metal.
Silver
 Silver lowers the melting range, improves fluidity, and helps to
control the coefficient of thermal expansion in gold- and
palladium-based alloys

47
 Silver-containing porcelain alloys have been known to induce
discoloration (yellow, brown, or green) with some porcelains.
 Silver possesses a high affinity for oxygen absorption, which can
lead to casting porosity and/or gassing.
 However, small amounts of zinc or indium added to gold- and
silver-based alloys help to control silver's absorption of oxygen.
 Silver will also corrode and tarnish in the presence of sulfur.
Although silver is a precious element, it is not universally
regarded as noble in the oral cavity .
Aluminium
 Aluminum is added to lower the melting range of nickel-based
alloys.
 Aluminum is a hardening agent and influences oxide formation.
 With the cobalt - chromium alloys used for metal ceramic
restorations, aluminum is one of the elements that is "etched"
from the alloy's surface to create micromechanical retention for
resin-bonded retainers (Maryland Bridges).

48
Beryllium
 Like aluminum, beryllium lowers the melting range of nickel-
based alloys, improves castability, improves polishability, is a
hardener, and helps to control oxide formation.
 The etching of nickel-chromium-beryllium alloys removes a Ni-
Be phase to create the micro retention so important to the etched
metal resin-bonded retainer.
 Questions have been raised as to potential health risks to both
technicians and patients associated with beryllium-containing
alloys .
Boron
 Boron is a deoxidizer.
 For nickel-based alloys, it is a hardening agent and an element
that reduces the surface tension of the molten alloy to improve
castability.
 The nickel-chromium beryllium-free alloys that contain boron
will pool on melting, as opposed to the Ni-Cr-Be alloys that do
not pool.
 Boron also acts to reduce ductility and to increase hardness.
Chromium (Cr)
Chromium is a solid solution hardening agent that contributes to
corrosion resistance by its passivating nature in nickel- and cobalt-
based alloys

49
Cobalt (Co)
 Cobalt is an alternative to the nickel-based alloys, but the cobalt-
based metals are more difficult to process.
 Cobalt is included in some high-palladium alloys to increase the
alloy's coefficient of thermal expansion and to act as a
strengthener
Copper (Cu)
 Copper serves as a hardening and strengthening agent, can lower
the melting range of an alloy, and interacts with platinum,
palladium, silver, and gold to provide a heat-treating capability
in gold-, silver-, and palladium-based alloys.
 Copper helps to form an oxide for porcelain bonding, lowers the
density slightly, and can enhance passivity in the high palladium-
copper alloys.
Gallium (Ga)
 Gallium is added to silver-free porcelain alloys to compensate for
the decreased coefficient of thermal expansion created by the
removal of silver. (Concerns over silver's potential to discolor
dental porcelain have greatly limited its use in systems other than
palladium-silver )
Indium
 Indium serves many functions in gold-based metal ceramic
alloys.
 It is a less volatile oxide-scavenging agent (to protect molten
alloy);

50
 lowers the alloy's melting range and density; improves fluidity;
 Has a strengthening effect. Indium is added to non goldbased
alloy systems to form an oxide layer for porcelain bonding.
 Alloys with a high silver content (eg, palladium-silver) rely on
indium to enhance tarnish resistance.
Iron (Fe)
 Iron is added to some gold-based porcelain systems for
hardening and oxide production.
 Iron is included in a few base metal alloys as well.
Manganese (Mn)
 Manganese is an oxide scavenger and a hardening agent in
nickel- and cobalt-based alloys.
Molybdenum (Mo)
 Molybdenum improves corrosion resistance, influences oxide
production, and is helpful in adjusting the coefficient of thermal
expansion of nickel-based alloys.
Nickel (Ni)
 Nickel has been selected as a base for porcelain alloys because its
coefficient of thermal expansion approximates that of gold and it
provides resistance to corrosion.
 Unfortunately, nickel is a sensitizer and a known carcinogen.

51
 Estimates of nickel sensitivity among women in the United States
range from 9% to 31.9% and from 0.8% to 20.7% among men .
Tin (Sn)
 Tin is a hardening agent that acts to lower the melting range of an
alloy. It also assists in oxide production for porcelain bonding in
gold- and palladium-based alloys. Tin is one of the key trace
elements for oxidation of the palladium-silver alloys.
Titanium (Ti)
 Like aluminum and beryllium, titanium is added to lower the
melting range and improve castability.
 Titanium also acts as a hardener and influences oxide formation
at high temperatures.
Zinc (Zn)
 Zinc helps lower the melting range of an alloy and acts as a
deoxidizer or scavenger to combine with other oxides.
 Zinc improves the castability of an alloy and contributes to
hardness when combined with palladium.

52
ALLOYS FOR ALL METAL RESTORATION
As it can been seen from the table, the metals that can be used for
all metal restoration can be classified as highly noble, noble and base
metal alloys.
Among the highly noble metals are Au-Ag-Cu-Pd and metal
ceramic alloys. The metal ceramic alloys are dealt under a separate
section.
In the noble group are the Ag-Pd-Au-Pd and metal ceramics.
The base metal alloys that can be used for all metal restorations
are those that are used for metal ceramics and removable partial
denture frameworks. Since they are used for the latter two purposes,
they are discussed under that.
It can be seen that all of the metal ceramics can be used for all
metal restorations but it is not the same vice versa. The principle
reasons for this may be because the alloys of all metal restoration may
not be able to form metal oxides that is required for bonding to
porcelain, their melting temperature may be too low to resist sag
deformation at porcelain firing temperatures, and their thermal co-
efficient of contraction may not be close enough to match that of
porcelain.
Typical compositions of Casting Alloys for Full-Metal, Resin-
Veneered and Metal- Ceramic Prostheses
Alloy
type
Classification
Elemental composition (wt%)
Au Pd Ag Cu
Ga, In, and
Zn
I
High Noble
(Au-based)
83 0.5 10 6 Balance
II
High Noble
(Au-based)
77 1 14 7 Balance
III
High Noble
(Au-based)
75 3.5 11 9 Balance
III Noble 46 6 39 8 Balance

53
(Ag-based)
III
Noble
(Ag-based)
- 25 70 - Balance
IV
High Noble
(Au-based)
56 4 25 14 Balance
IV
Noble
(Ag-based)
15 25 45 14 Balance
Metal
Ceramic
High Noble
(Au-based)
52 38 - - Balance
Metal
Ceramic
Noble
(Ag-based)
- 60 30 - Balance
Metal
Ceramic
High Noble
(Au-based)
88 7 1 - Balance
Metal
Ceramic
Noble
(Ag-based)
0-6 74-88 0-10 0-15 Balance
The alloys used for all metal restoration are described below :
GOLD AND GOLD BASED ALLOYS
Gold in the as cast condition is very soft and can be easily cold
worked. The gold in the pure form is used for direct restorations
whereas the alloys of gold are used for casting purposes. The alloys of
gold are classified as :
Type Au% Ag% Cu% Pt/Pd% Zn%
I (soft) 85 11 3 - 1
II (Medium) 75 12 10 2 1
III (Hard) 70 14 10 5 1
IV (Extra hard) 65 13 15 6 1

54
It can be seen that the gold content or nobility of the alloys
decreases on going from type I to type IV. nobility of gold alloys is
often indicated by either carat value of fineness. Carat value represents
the number of parts by weight of gold per 24 parts of gold. Fineness
indicate the number of part per thousand parts of gold. Thus the
fineness rating is 10 times the gold percentage of the alloy. Fineness is
considered a more practical term than the carat value.
Their comparative properties, also are shown below
Type Hardness Proportional
limit
Strength Ductility Corrosion
resistance
I
II Increases Increases Increases Decreases Decreases
III Downwards Downwards Downwards Downwards Downwards
IV
Mechanical Property Requirements in ANSI/ADA Specification No.5
for Dental Casting Alloys (1997)
Alloy
type
Yield strength (0.2% offset) Elongation
Annealed Hardened Annealed Hardened
Minimum
(MPa)
Minimum
(MPa)
Minimum
(MPa)
Minimum
(%)
Minimum
(%)
Type 1 80 180 - 18 -
Type 2 180 240 - 12 -
Type 3 240 - - 12 -
Type 4 300 - 450 10 3
hardness, strength and the proportional limit increases from type I to
type IV whereas the ductility and the corrosion resistance decreases

55
from type I to type IV. This is due to the property of forming solid
solution by the alloying elements. The last two types can be further
hardened be hardening heat treatments. The corrosion resistance is
due to the effects of platinum and palladium which form a cored
structure on solidification de to their high melting points. There is a
consequent increase in the separation of the liquidus and the solidus
lines in the phase diagram.
USES
1. Type I : are sure for inlays which are well suppose and do not
have to resist high masticatory forces. The high ductility values allow
them to be burnished thus improving the marginal fit.
2. Type 2 : are the most widely used metals for inlays. They have
superior mechanical properties than type I.
3. Type 3 : are used when there is less support from tooth structure
and when the opposing stress are high like for crowns, bridges.
4. Type 4 : are used exclusively for construction of components of
partial dentures and for this reason are referred to as partial denture
casting alloys.
The functions of each of the ingredient metals in the casting alloy are
:
1. Gold - Yellow color, ductility, resistance to tarnish
and corrosion.
2. Silver - Hardness and strength. Whiten the alloy thus
reducing the reddening effect of copper, but
tarnishes the alloy.
3. Copper - Hardness and strength. Reddish color but
lowers tarnish resistance. Lowers fusion
temperature. Reduces the density of the alloy.

56
4. Palladium - Increases resistance to tarnish and corrosion.
Whitens the alloy Cheap. Absorbs gases
formed during casting, and thus reduces
porosity. Increases hardness.
5. Zinc - Acts as a scavenger and removes the oxides.
Makes the alloy more castable.
The classification based on the color of the allow :
1. Yellow gold – Those with more than 60% Au
2. Low gold or economy gold – With 42-55% Au, also has yellow
color
3. White gold – are those with gold more than 50%, but palladium
gives the white color
4. Silver palladium with or without gold but mainly silver – Has
white color
5. Palladium silver with mainly palladium gives white color.
6. Japanese gold – Also known as technique alloy used for training
students in casting technology - has yellow color. It has the
composition of
Cu - 53%
Zn - 37%
Al - 7%
Others - 3%
The grain refined alloys are those that contain iridium or
ruthenium in 100-150 parts per million. By this the grain size is
decreased to 150-50 microns. Therefore better physical properties can

57
be obtained since they depend on the smaller grain size for better
properties.
The advantages of the refined alloys are :
High yield strength
High elongation
Homogenous casting
More resistance to corrosion
HEAT TREATMENT OF GOLD ALLOYS
The heat treatments are :
1. Softening heat treatment :
In this, the alloy is heated in an electric furnace at a temperature
of above 700°C for 10 min and then quenched rapidly in water. The
normal procedure is to leave the mould until the gold is no longer at
red heat which is visible in the sprues of the casting. This ensures that
the internal metal temperature is about 600° C after which it is
quenched. This causes a fine grain structure. The ductility and the
corrosion resistance increase whereas the strength, hardness and the
proportional limit decrease.
2. Homogenization heat treatment :
This is done when platinum and palladium are present, to
remove coring. This involves heating to 700°C for ten minutes,
followed by quenching.
3. Stress relief anneal :
This is done when any adjustments are done to the appliance to
remove the stresses. This involves heating in a low temperature to
remove the stresses for a given period of time.

58
4. Hardening heat treatment :
This is done for type III and type IV alloys which contain
sufficient amount of copper. This is due to solid state transformations.
The casting is heated to above 450° C and allowed to cool slowly until
200°C, then quenching. This takes about 20 min. This causes an
increase in the strength, hardness and proportional limit with a
decrease in corrosion resistance and ductility.
Hardening heat treatment (theoretical considerations):
The hardening process can be explained by the consideration of
phase diagrams for silver copper and gold copper systems.
Silver and copper are immiscible in each other. They form
eutectic phase at a composition of 71.9% Ag and 28.1% Cu. Although
they are not soluble in each other, they tend to form little amount of
solid solution at room temperature in the eutectic mixture. When the
alloys are heated, the diffusion of atoms become possible and copper
tends to precipitate from the α solid solution. This occurs of the
precipitation hardening procedure used for type III and type IV alloys.

59
Gold and copper form a continuous series of solid solution with
face centered cubic lattices. The copper is randomly substituted in the
gold lattices. From the phase diagram it can be seen that the solidus
and the liquidus are close together and almost coincide at point M.
Two other areas on the phase diagram, at composition between 40%
and 90% gold, indicate regions in which the alloys are capable of
forming an ordered state from a random one. This ordered lattice is
known as super lattice.
This occurs by the rearrangement of atoms when their energy is
increased to allow diffusion as when heating to 200°C – 400°C. The
super lattice has a formula of Cu3-Au. This heat treatment is known as
ordered heat treatment. Similarly, when an alloy containing 75% gold
is heated, an ordered tetragonal structure of the formula Cu-Au is
formed.

60
LOW GOLD CONTENT ALLOYS
These contain about 45% - 50% gold and was introduced due to
rise in the price of gold. They have a high palladium content which
imparts a whitish color to them. The properties are similar to that of
type III and IV alloys, but the ductility is considerably lower. They
have an elongation percent of only 2% whereas, type III alloy has 20%.
SILVER PALLADIUM ALLOYS
These alloys, as the name suggest, contain predominantly silver
in composition but have substantial amounts of palladium (25%) that
provide nobility and promote the Silver tarnish resistance. They may
or may not contain Copper or Gold. These contain small amounts of
Zinc and Indium. They are whitish in color.
These have casting temperatures in the range of yellow gold
alloys. They have lower density than the gold alloys and therefore,
present difficulties in casting. Care must be paid to the casting
temperature and the mold temperature if no defects are to be expected.
Alloys containing palladium have a propensity to dissolve oxygen in
the molten state which may lead to a porous casting.
The copper free Ag-Pd alloys contain 70% - 72% Ag and 25% Pd.
These have properties of type III Gold alloys. Other silver based alloys
contain 60% Ag, 25% Pd and as much as 15% or more of Cu. These
have properties of type IV gold alloy. The major limitation of Ag-Pd
alloys in general and Ag-Pd Cu in particular is their greater potential
for tarnish and corrosion.

61
ALUMINIUM BRONZE ALLOYS
This is the only alloy that is based on Copper as its main
component and approved by the ADA. Although, Bronze is defined as
Copper rich Copper – Tin phase, Bronze alloys containing no Tin like
Aluminium bronze (Cu-Al), Silicon bronze (Cu-Si), and Beryllium
bronze the surface.
The aluminium bronze alloys contain 81-88wt% Cu, 7-11 wt% Ni
and 1-4 wt% Fe. This has the potential to react with Silver and form
copper sulphide which tarnishes the surface.
ALLOYS FOR METAL CERAMIC RESTORATIONS
Because of the poor tensile and shear bond strength, All porcelain
restorations are weak and brittle. Therefore, they break easily. But
porcelain is necessary for aesthetics. This problem is solved by, making
the restoration in metal and applying porcelain to labial and buccal
areas of the appliance in thin layers (veneers) for esthetics. Thus both
strength and appearance are met.
Metal ceramic alloys are also referred to as porcelain-fused-to-
metal or ceramometal alloys. But the preferred term is metal-ceramic.
Likewise the preferred acryonym is PFM rather than PBM (porcelain
bonded to metals) and PTM (porcelain to metal).
These are classified as high noble, noble and base metal, like in
All Metal Restoration Alloys.
Properties of metal ceramic alloys :
1. High fusing temperature of the alloys. This should be 100C
greater than the fusion temperature of porcelain.
2. The contact angle between the ceramic and metal should be
less than 60°

62
3. They should form oxides on the surface for bonding to
porcelain. For this purpose, base metals like Tin, Indium and
Iron are added.
4. They should have compatible co-efficient of thermal expansion.
For, this is added Palladium which tends to lower the co-
efficient of thermal expansion. Although they should be equal
theoretically, it is ideal to have the co-efficient of thermal
expansion of metal greater than that of the porcelain by 0.5 x
10-6/°C. Most metals have a coefficient of thermal expansion of
13.5 x 10-6 /°C and porcelains of about 13-144 x 10-6/°C.
5. Adequate stiffness and strength
6. High sag resistance
7. Accurate casting of the alloy even under high temperature.
The bond between metals and porcelain is that of
chemisorption and the most common failure that occurs in
metal ceramic is due to debonding of the metal.

64
The high noble alloys used for PFM are :
1. Au-Pt-Pd alloys :
These have a gold cement ranging up to 88% with varying
amounts of palladium, platinum, and small amounts of base metals.
These are yellow in color. They have a high melting range. The base
metals are tin, indium and iron, and these are added to form metal
oxides for bonding to porcelain. Rhenium is added as a grain refiner.
The hardening that occurs in this is due to the precipitate of Fe-Pt3.
The heat treatment consists of heating the alloy for 30min at 550°C.
They have high stiffness, hardness, strength and reasonable elongation
but low sag resistance. Because of the yellow color, producing esthetics
is easier.
2. Au-Pd alloys :
They have a Pd content of 35% - 45% and Au content of 44% -
45%. These have remained popular as metal ceramic alloys in spite of
their relatively high cost. Due to the absence of silver, there is freedom
from porcelain greening and also decreased thermal coefficient of
contraction. Since they do not contain Pt or Fe there is no possibility
for precipitation hardening to occur. Only solution hardening occurs.
Indium is added for bonding purposes and Gallium, for lowering of
the fusion temperature. Rhenium is the grain refiner and Ruthenium is
added for improving castability. This difficulty in casting is due to
their low density. These are white in color and therefore esthetics is
difficult to obtain. They are harder, stiffer and stronger than Au-Pt-Pd.
They are more ductile and easier to solder but have higher casting
temperatures.
3. Au-Pd-Ag Alloys :
These contain between 39% and 77% Au, up to 35% Pd and Silver
levels as high as 22%. Silver increases the thermal coefficient of
contraction, but it has the tendency to discoloration porcelain. Indium
and Tin are used for bonding the Rhenium for grain refining.
Ruthenium is used for improving the castability. The hardening is by

65
means of solution hardening and the properties are similar to that of
Au-Pd.
The noble metals used for metal ceramic restorations are :
1. Pd-Ag Alloys :
This was the first gold free noble metal to be marketed.
This contains about 53% - 61% Pd and 28% - 40% Ag. These have
the lowest noble content of the five noble metal alloys. These contain
Tin and Indium for oxide formation for porcelain bonding, and to
increase the alloy hardness and Ruthenium for improving the
castability, since they produce greening effect. This is due to the
escaping of the silver vapor to the surface of the alloys during the firing
which diffuses into the porcelain as silver ions and is reduced to
colloidal metallic silver in the surface layer of porcelain. This can be
minimized by using ceramic coating agents or gold metal conditioners.
In some of these alloys there is the formation of internal oxides
rather than an external oxide. This produces nodules on the surface
which causes a mechanical type of bond rather than a chemical one.
Because of the increased Pd content, there is a decrease in the
coefficient of thermal expansion but the increased silver content
increases this and lowers the melting range.
2. Pd-Cu-Alloys :
These are recent introduction to the market and the cost is similar
to that of the previous alloy type.
These contain 74% to 80% Pd and 9% - 15% Cu. They may
contain 2% Gold. These tend to form dark brown or black oxides
during porcelain firing. This should be eliminated by proper masking
of the oxide. It is necessary that a brown rather than a black oxide is
formed. Otherwise poor adherence to porcelain may occur. These are
susceptible to sag deformation at elevated firing temperatures. Indium
is added for oxide formation and Gallium for improving casting

66
qualities. It has high strength and hardness, moderate modulus of
elasticity and elongation.
3. Pd-Co Alloys :
This is comparable in cost to the above previous groups. They
are often advertised as gold free, nickel free, beryllium free and silver
free alloys.
These are the most sag resistant of all noble metal alloys. These
have fine grain size. These tend to discolor porcelain in spite of the
absence of the silver due to the formation of cobalt oxides. But this is
not considered as a significant problem and no metal coating agents are
necessary to mask the oxide layer. Like the above three alloys, they
have a high coefficient of thermal expansion and can be used with
higher expansion porcelains.
4. Pd-Ga-Ag and Pd- Ga – Ag-Au Alloys :
These are the most resistant of the noble alloys. These were
introduced because of their tendency to form lighter oxides than Pd-Cu
or Pd-Co. They are compatible with lower expansion porcelains like
vita porcelain.
PHYSICAL PROPERTIES OF HIGH NOBLE AND NOBLE METAL
ALLOYS :
1. All are biocompatible
2. Good resistance to tarnish and corrosion
3. Melting temperature of around 1000°C. The casting temperature is
obtained by adding 75-150°C to the liquidus temperature.
4. Density of 15gm/cm3. This gives an idea of how many castings can
be done from a unit weight of metal and therefore the cost.
5. Hardness from soft to hard

67
6. Elongation which is a measure of ductility of about 20-39%
7. Linear coefficient of thermal expansion in the range of 14 – 18 x 10 -
6/°C.
8. Yield strength in the range of 103 – 572 MPa.
BASE METAL ALLOYS USED FOR PFM :
Base metals were introduced to the field of dentistry as an
alternative to the noble metals due to rise in the price of gold. They
were found to possess some good qualities which have made them a
commonly used material in dentistry. The two commonly used alloys
are Co-Cr and Ni-Cr.
According to the ADA the following combinations are available :
1. Cobalt – chromium
2. Nickel – chromium
3. Nickel – chromium – beryllium
4. Nickel – cobalt – chromium
5. Titanium – aluminium – vanadium
Although Ni-Cr is used for PFM, and Co-Cr for partial dentures,
these are described here because of the similarity in certain properties.
i. Co-Cr Alloys :
These were introduced by the name stellites by Eldwood Haynes
an automobile engineer in the early 1900. They were so named because
of their bright, lustrous, mirror like surface resembling starts at night.
The first introduced Co-Cr alloy in dentistry was called as
vitallium and it was introduced in 1928. It was nickel free. This closely
resembled satellites. This has been in use since 1930‟s. The first dental

68
application of this alloy is recognized as having been made by R.W.
Erdle and C.H.P range.
In 1943, a report appeared which described the properties of
these alloys including other products under various names like
ticonium, niranium and lunorium.
TYPES
There are two types of alloys and they are :
a. Type I which is high fusing with fusion temperature
greater than 2400F.
b. Type II which is low fusing with fusion temperature less
than 2400F.
COMPOSITION
These alloys generally contain 35% - 65% Co, 20-35% Cr, 0-23 Ni
and trace quantities of other elements such as molybdenum, silicon,
beryllium, boron and carbon.
Cobalt and nickel are strong metals and the purpose of the
chromium is to further strengthen the alloy by solution hardening and
to impart corrosion resistance by the passivating effect. This is because
of the chromium oxide that is formed when exposed to air. The
minimum percentage required to provide this protective coat is 12%.
Nickel increases the ductility.
The minor elements are added to improve the casting and
handling characteristics and modify the mechanical properties.
Molybdenum decreases the thermal co-efficient of expansion and
strengthens the alloy while Tungsten, when present also acts to
strengthen it. Beryllium causes grain refinement and uniformity of the
properties. It also lowers the melting point and strengthens and
harden the alloy. Ruthenium improves the castability, since the alloys
are low is density.

69
Carbon acts as a major strengthener and also affects the strength
and hardness when present at 0.2%. When this increases to above
0.25% it causes brittleness in the alloy. This is due to the formation of
carbide core. This concentration not only depends on the manufacturer
but also on the type of flame used. When oxyacetylene flame is used
there is possibility of introducing carbon inadvertently.
These are stronger than Ni-Cr and used mainly for partial
denture frameworks rather than PFM. They are stronger than noble
alloys.
PROPERTIES
1. Melting point :
Is between 1250°. Therefore, they cannot be melted using gas air
torch. Only induction method and oxyacetylene flame should be used.
While using care should be taken not to incorporate carbon in excess.
2. Yield Strength :
This is between 470-710 MPa which for gold is 320mpa. As a
result high stresses are required to deform the appliance. This is
important in constructing clasps.
3. Modulus of elasticity :
This is greater than the gold alloys and determines the thickness
and the thinness of the various parts of the denture framework. High
stiffness is an advantage since less undercut is involved but this can
also be damaging to the abutment tooth because of the excessive
stresses introduced when the clasp is taken out and inserted into the
mouth.
4. Tensile strength :
This is 685 to 870 MPa
5. Hardness :

70
This is in between 264 – 432 VHN. For gold it is around 264
VHN. Because of this it is difficult to grind, cut or polish. The
polishing of these is carried out by sand blasting using aluminium
oxide of size 50 microns or by electrolytic deposition. This is in
contrast to electroplating.
6. Ductility :
This denotes the elongation percentage which is less than gold
which has around the ductility of these is around 1.6 and 3.8 this is
related to the fracture of the clasp and how it occurs.
7. Specific gravity :
This is a measure of the weight of the appliance and is half of that
of gold.
8. High resistance to tarnish and corrosion
9. Solidification shrinkage :
This is greater than that of gold and is about 2.3. Therefore, it is
necessary that the investment compensates this shrinkage. For the
alloys, either silica or phosphate bonded investments are used.
10.Castability :
This does not produce very accurate castings because of the low
density which decreases the thrust of the molten metal during casting.
This is improved by alloying beryllium with it.
11.Cost :
This is less than that of gold and therefore economical.
OTHER USES :
i. As part of the implant denture
ii. For making surgical screws and plates

71
iii. Orthopedic surgery
ii. Ni-Cr Alloys :
These are base metal alloys with a composition of nickel -70 to
90% and chromium of about 13-20%. Other elements added are iron,
aluminum, molybdenum, beryllium, silicon and copper. These are
mainly used for PFM. These were developed after Co-Cr gained wide
spread popularity.
PROPERTIES
a. Higher modulus of elasticity
b. Increased hardness
c. High yield strength
d. Less density
e. Less costly
f. Superior sag resistance which is about 25 microns as compared to
225 microns for gold.
g. Ductility greater than that of Co-Cr.
MANIPULATION OF BASE METAL CASTINGS
Since the fusion temperatures of these are high, they cannot be
casted as for gold alloys in gypsum bonded investments. Instead they
should be casted in silica or phosphate bonded investments. Melting of
these should be done only by electrical induction or by
acetylene/oxygen flame.
These alloys have low density and therefore do not develop the
necessary thrust required for filling the mould. Therefore the casting
machines should be capable of producing this extra thrust. Because of
the increased hardness, these materials should be polished by
electrolytic method.

72
COMPARISON OF PROPERTIES OF THE VARIOUS TYPES OF
BASE METAL ALLOYS
Property
High
noble
alloy
Co-Cr Ni-Cr-Be CPTi
Biocompatibility Excellent Excellent Fair Excellent
Density (g/cm3) 14 7.5 8.7 4.5
Elastic Modulus
(GPa)
90 145-220 207 103
Sag resistance
Poor-
excellent
Excellent Excellent Good
Technique
sensitivity
Minimal
Moderately
High
Moderately Extremely
Bond to
porcelain
Excellent Fair High High
Metal High Low
Good
Excellent
Low
Fair
Low
COMPARISON WITH CASTING GOLD ALLOYS
The two main components of cast partial denture frameworks are
the connectors and clasps. The connectors should be rigid and should
not be permanently deformed. Thus it can be inferred from the above
comparison that Co-Cr alloys meet the requirements.
For a clasp, a high value of proportional limit is required in order
to prevent deformation. A lower value of modulus of elasticity would
enable the clasp to engage relatively deep undercuts due to its
increased flexibility. In addition the alloy used to construct clasps
should be ductile so that adjustments can be made to clasps without
fracture. Therefore the gold alloys most closely match the
requirements for a clasp. But in practice, both are cast from Co-Cr
alloys. When designing clasps from this, due regard must be paid to
the high modulus of elasticity and low ductility. Clasps should not be

73
designed to engage deep undercuts and alterations leading to fracture.
A reduction in thickness decreases the force necessary to push the clasp
over the bulge of the tooth but leaves it exposed to the dangers of
deformation during handling of the denture. This can be overcome by
reducing the undercut area and also the thickness of the clasp.
COMPARISON OF THE PROPERTIES OF TYPE IV AND Co-Cr
ALLOY :
Properties Co-Cr Type Gold IV Comments
Tensile Strength
(Mpa)
850 750 Both acceptable
Density (gms /
cu.cm)
8 15 More difficult to produce
defect the castings for CO-
Cr but dentures are lighter.
Hardness (Vickers) 420 (Hard
than
enamel)
250 (Softer
than enamel)
More difficult to polish but
retains polish during
services.
Stiffness Stiff More flexible
Ductility 2 15 (as cast)
8 (hardened)
Co-Cr clasps may fracture
if adjustments are made.
Modulus of
elasticity (GPa)
220 100 Co-Cr more rigid for the
same thickness
Proportional limit
(MPa)
700 500 Both resist stresses without
deformation.
Melting
temperature (oC)
As high as
1500
Lower than
1000
Co-Cr require electrical
induction or oxyacetylene
Casting shrinkage 2.3 1.25 – 1.65
Heat treatment
Tarnish resistance
price
Complicated
adequate
Reasonable
Simple
adequate
high
The success of the crown and bridge alloys depends to a great
extent on the accuracy of the restorations. The gold alloys have a
significant advantage from this point of view. The casting shrinkage is
less (approximately 1.5% when for base metal alloys it is around 2.3%).
This is well compensated by the mould whereas, for the base metals it

74
is not so. But one advantage of the Ni-Cr alloys is that, the margins are
not destroyed during finishing and polishing procedure. These are
rarely used for all-metal but widely, for metal – ceramic restorations.
COMPARISON OF PROPERTIES OF TYPE III AND Ni-Cr ALLOY
Density
(gm/cu.cm)
8 15 More difficult to produce to
produce defect free castings
for Ni-Cr alloys.
Fusion
temperature
(oC)
as high
as 1350
Lower
than 1000
Ni-Cr alloys require electrical
induction or oxyacetylene
flame. Both adequate
Tensile
strength (MPa)
230 290 Both high enough to prevent
distortions when used.
Modulus
elasticity (GPa)
220 85 Higher modulus of Ni-Cr
advantage for larger
restorations.
Hardness
(Vickers
Ductility)
300 upto
30%
20 (as cast)
10
(hardened)
Ni-Cr more difficult to polish
but retains polish during
service. Burnishing is
possible but high forces are
required.
BIOCOMPATIBILITY OF BASE METALS
The main disadvantage of base metal alloys in from the beryllium
vapor. This is greatest for the dental technicians who are exposed to
the dust and vapor during the various processes of casting and
finishing. According to OSHA, the exposure to beryllium dust in air
should be limited to particulate beryllium concentration of 2 g/cu.m
determined from 8 hour time weighted coverage. The allowable ceiling
concentration is 5 g/cu.m not to be exceeded for a 15 minutes period.
For a minimum duration of 30 minutes a maximum ceiling

75
concentration of 2 g/cu.m is allowed. This vapor can be reduced
effectively by the use of exhaust fans.
Exposure to beryllium may result in acute or chronic forms of
beryllium disease. The symptoms may vary from contact dermatitis to
severe chronic pneumonitis which can be fatal. The chronic disease is
characterized by symptoms of severe coughing, chest pain and general
weakness to pulmonary dysfunction.
To other disadvantage of these base metal alloys is the allergy of
patients to nickel. This allergy can be tested by a patch test using 25%
nickel sulfate. Positive reactions were reported by 9.4% women and.
79% of men.
The effects of nickel exposure to humans have included
dermatitis, cancer of the lungs, cancer of the nasal sinus and larynx,
irritation and perforation of the nasal septum loss of smell, asthma like
lung disease, pulmonary irritation, pneumoconiosis, a decrease in lung
function and death.
NIOSH has recommended OSHA to adopt a standard to limit
employee exposure to inorganic nickel in the laboratory office to
15µg/cu.m of air determined as a time weighted average (TWA)
concentration for upto a 10 hr work shift (40 hr work week) the existing
OSHA standard specifies an 8 hr TWA concentration limit of
1000µg/cu.m of air.
Thus it is better to follow certain methods like using high speed
evacuation systems when procedures are performed intra orally and
using exhaust fans in the laboratory.
DISADVANTAGE OF THE BASE METALS
i. Difficult to grind and polish because of their hardness.
ii. They are technique sensitive
iii. Checking or delayed failure of porcelain due to difference
in the thermal co efficient of contraction.

76
iv. The greatest disadvantage lies in the variability in the
strength and quality of the brazed or pre soldered
connectors. These are susceptible to brittle fracture and this
is due to the fact that the pre soldered parts contain voids,
flux inclusions and localized shrinkage porosity. This can
be avoided using the cast joining process.
iii. Titanium
Commercially pure titanium is an element rather than an alloy.
But since it is also used, it is discussed here.
It is a slight weight metal with a density of 4.51g/cm3. It has a
low elastic modulus of 110 GPa, which is about half that of the other
base metal alloys. IT has a relatively high melting point of 1668°C and
a low coefficient of thermal expansion of 8.4 x 10-6/°C. This value is far
below that of porcelains. Therefore, low fusing porcelains should be
used. IT has a good passivating property. IT has a poor oxidation
resistance above 650°C. At room temperature, it exists as a low
strength but a ductile metal while heating to above 883°C, it forms a
hard, more brittle ß phase.
This is non toxic and found to be the most bio compatible of all
metals.
This is being used for crowns and removable partial dentures. It
is an excellent choice to patients with known allergy to nickel.
Titanium alloys
The most common alloy used is Ti-Al-Va. This contains 90% Ti,
6% Al, 4% Va. The major benefits of alloying are strengthening and
stabilization of the alloy against the formation of α and ß phases seen in
the pure metal. The former is formed by the addition of Aluminium
and the latter due to Copper, Palladium or Vanadium.

77
COBALT CHROMIUM NICKEL ALLOYS
These alloys which were first marketed for use in 1950s, were
originally developed as watch springs. They were known as elgiloy.
Composition
40% cobalt, 20% chromium, 15% nickel, 15.8% iron, 7%
molybdenum, 2% manganese, 0.16% carbon and 0.04% beryllium.
These exhibit excellent tarnish and corrosion resistance in the oral
environment.
Types
It is available in four tempers (soft, ductile, semi resilient and
resilient) which are color coded. The soft variety is color blue and the
most widely used. All can be heat treated.
Heat treatment
The softening heat treatment is at 1100°C to 1200°C followed by a
rapid quench.
The age hardening temperature is 260°C C to 650°C for elgiloy it
should be kept at 482°C for 5 hours.
Heat treatment is 482C for 7 to 12 minutes.
These stress relief heat treatment is at 370°C for 11 minutes. This
treatment not only improves the elastic properties but also decreases
the corrosion.
Properties
These alloys should not be annealed, since the softening effect
cannot be reversed by heat treatment. The hardness, yield strength and
the tensile strength are the same as the stainless steel alloys. Ductility
is greater than the stainless steels in the softened state whereas less in
the hardened state.

78
NICKEL-TITANIUM ALLOYS
It was introduced commercially during the 1970s following
research by Andreason and his colleagues. They were called as
NITINOL and this name came from the two elements nickel and
titanium and the Naval Ordinance Laboratory where these alloys were
developed first by duehler and associates.
Composition
These contain 54% nickel, 44%. Titanium and generally 2% or les
of cobalt. This result in the 1:1 atomic ratio of the two major
components.
As with the other systems this alloys can exist in various
crystallographic forms. At high temperature a BCC lattice referred to
as austenitic phase is table. Whereas appropriate cooling can induce
the transformation HCP martenistic phase. This transformation can
also be induced by the application of stress. There is a volumetric
change associated with the transition and an orientation relation is
developed between the phases. This phase transition results in two
unique features. Shape memory and super elasticity (Psuedoelasticity).
The cobalt is used to control the lower transition temperature
which can be near mouth temperature. The memory effect is achieved
by establishing a shape at temperature near 482°C and cooling it
followed by forming it into another shape. When this is heated
through the lower transition temperature the wire will return to its
original shape.
Inducing the phase transition by stress can produce super
elasticity. The strain developed due to the stress is caused by a phase
change that results from a change in the crystal structure. These alloys
have large working radius. They are difficult to form and have to be
joined by mechanical crimps as they can not be soldered or welded.

79
ß-TITANIUM ALLOYS
Pure titanium is polymorphic or allotrophic. At temperature
above 880°C, the HCP or the α crystal lattice is stable whereas at high
temperatures the metal rearranged into a BCC or ß crystal lattice.
Certain elements like Al, C, O and N stabilize the HCP structure
whereas other such as V, Mo and Ta stabilize the BCC structure.
The Ti-Al-V alloy contains both these crystal structures.
The Ti60% Al 40% alloy is based on the HCP lattice. An alloy to
the composition of Ti 79% Mo -115 and Sn 4% is produced as TMA and
is used for orthodontic purposes. These contain the ß crystal structure.
This can be cold worked and heat treated.
It can be joined by electrical resistance welding which need not be
reinforced with solder. This is the only orthodontic alloy which is
considered to possess true weld ability.
Both the forms of Ti have excellent corrosion resistance and
environmental stability. This is because of the oxide. B Ti is the only

80
major orthodontic alloy that is Ni free. These properties of Ti
stimulated its use in heart valves, hip implants and orthodontic wires.
RECENT ADVANCEMENTS
The recent advancement in the metal field is the development of
SINTERED COMPOSITE
These composites consist of sintered high noble alloy sponge
infiltrated with an almost pure gold alloy. The result is a composite
between the two gold alloys that is not cast, but fired onto a refractory
die. The porcelain does not bond through an oxide layer in these
systems, but it bonds mechanically to a micro rough surface.
The advantages of this that any stress concentration on the
ceramic is relieved by the excellent ductility of the metal.
It has been claimed that these systems support few periodontal
pathogens around the restoration have yet to be substantiated.

81
CONCLUSION
Thus a variety of metals and alloys are available. These possess
the main advantage over resins in that, they are able to transfer heat
which is due to the thermal conductivity. This is gives a more
acceptable appliance. But the main disadvantages as we all know is the
esthetics because of which the metal free dentistry is gaining wide
spread popularity.
But the use of all ceramic is not favored, since they require
extensive tooth preparation. More over they are susceptible to fracture
because their brittleness. Therefore the vast majority of restorations are
metal ceramic.
Finally the guidelines for the selection of an alloy for a restoration
should be based on :
1. A thorough understanding of the alloy
2. Avoid selecting an alloy based on its color unless all other factors
are equal
3. Know the complete composition of alloys, and avoid elements that
are allergic to the patient
4. Whenever possible use single phase alloys
5. Using clinically proven products from quality manufacturers
6. Use alloy that have been tested for elemental release and corrosion
and have the lowest possible release of elements.
7. Focus on long term clinical performance
8. Finally it is important for the dentist to remember and take up the
responsibility of being responsible for the safety and efficacy of any
restoration.

82
REFERENCES
1. Anderson‟s Applied Dental Materials – John F.Mc. Cabe
2. Dental Materials – Craig. O‟Brien – Powers
3. Essentials of Dental Materials – S.H. Soratur
4. Material and Metallurgical Science – S.R.J. Shantha Kumar
5. Materials Science and Engineering – V. Ragahavan
6. Phillips Science of Dental Materials (Eleventh Edition) –
Anusavice
7. Restorative Dental Materials (Eleventh Edition) – Robert G. Craig
and John. M. Powers
8. Restorative Dental Materials – Floyd. A. Peyton
9. J.P.D. April 2002 Volume 87 No.4 Page 351 – 363.

Alloys used in dentistry/cosmetic dentistry course by Indian dental academy

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