Solidification & Microstructure of Metals Guide

Solidification & microstructure of metals
CONTENTS:
Page 1
1. Introduction
2. Metals
3. History of Metals
4. Properties of Metals
5. Classification of Metals
6. Inter Atomic Bonds
7. Microscopic Structure of Metals
8. Space Lattices
9. Lattice Imperfection
10. Heat Treatment
11. Strengthening of Metals
12. References

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..
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”
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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
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.
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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
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
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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
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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 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
repetitions 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
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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.
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
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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 equalized 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. 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.
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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:
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
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Base centered
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The arrangement of atoms in the crystal lattice depends 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.
Page 12

SIMPLE CUBIC LATTICE SYSTEM
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:
Absence of an atom from its position. This can be:
 Vacancy
 Divacancy
 Trivacancy
2. Interstitialcies:
Presence of extra atoms in the interstitial spaces.
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3. Impurities
4. Electronic defects
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.
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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 the deformation of the α
titanium alloys. The atoms have a mirror relationship.
3. Stacking fault
4. Tilt boundaries
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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.
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
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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:
1. Low ductility
2. Distorted and fibrous grains
When cold work is continued in these, they will eventually fracture. This
may be:
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
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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.
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
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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 result 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
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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 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
Page 22

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
solidification. e.g. iridium, rhodium.
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:
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- 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.
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
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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.
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. Phillips Science of Dental Materials (Eleventh Edition) – Anusavice
6. Restorative Dental Materials (Eleventh Edition) – Robert G. Craig
and John. M. Powers
7. Restorative Dental Materials – Floyd. A. Peyton
8. J.P.D. April 2002 Volume 87 No.4 Page 351 – 363.
Page 25

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