Precious metal alloys

Presented by
Dr. ARUNIMA UPENDRAN
1st year MDS
1

 Although popular press dental journals have
occasionally promoted "metal-free” dentistry as
desirable, the metals remain the only clinically proven
materials for many long-term dental applications
2

Alloy : a mixture of two or more metals or metalloids that are
mutually soluble in the molten state; distinguished as binary,
ternary, quaternary, etc., depending on the number of metals
within the mixture; alloying elements are added to alter the
hardness, strength, and toughness of a metallic element, thus
obtaining properties not found in a pure metal; alloys may also
be classified on the basis of their behavior when solidified
3

noble metal : those
metal elements
that resist
oxidation, tarnish,
and corrosion
during heating,
casting, or
soldering and
when used
intraorally;
examples include
gold and platinum
noble metal alloy:
as classified by the
American Dental
Association
(1984), any dental
casting alloy
containing a
minimum of 25%
by weight of Au,
Pt, and/or Pd
high noble metal
alloy: as classified
by the American
Dental Association
(1984), any dental
casting alloy with
at least 60% noble
metal (Au, Pt, Pd,
Rh, Ru, Ir, Os) by
weight with at least
40% gold;
4

 As early as the seventh century B.C Etruscan dental
prostheses made by passing thin strips of gold round
teeth on each side of a space from which a tooth or
teeth had been lost and riveting the strip so as to hold
the substitute teeth in place
5

 The discoveries were dated back to 550 B.C . A
canine tooth like object made of two piece of calcite
having hardness similar to natural teeth showing wear
on the chewing surface & secured with gold wires
wrapped around the neck of adjacent teeth
6

 The first printed book on dentistry, entitled 'Artzney
Buchlein' ('The Little Pharmacopoeia'), was
published by Michael Blum in Leipzig in 1530. Under
this title or as 'Zene Artznei' ('Dental Medicine')
“Scrape and clean the hole and the area of decay with a
fine small chisel or a little knife or a file, or with another
suitable instrument, and then to preserve the other
part of the tooth, fill the cavity with gold leaves.”
7

 Maggilio in 1809 , a dentist at the university of Nancy
, France, author of the book called “THE ART OF THE
DENTIST”. The first reference to modern style
implants. He has described the implant & placement.
 He made the tooth root shaped implant with 18 carat
gold with three prongs at the end to hold it in place in
the bone . The implant was placed in the freshly
extracted socket site retained with the prongs. After
the tissues healed the crown was attached with the
help of post placed into the hole of root section of the
implant. He placed the single stage gold implant
8

 In 1886 Harris treated a Chinese patient in Grass
valley , California . He placed the tooth root
shaped platinum post with lead coating, lasted for
27 yrs Reported in Dental Comos.
 In 1888, Charles Henry Land who fused
porcelain on thin platinum caps for use as crowns.
This technique is still used in making jacket
crowns.
 In 1890, a Massachusetts minister had his lower
jaw resected & was restored with an extensive
system of gold crowns soldered & joined to
hinged device attached to the remaining dentition
10

 Bonwill in 1895 reported on the implantation of
one or two tubes of gold or Iridium as a support
for individual teeth or crown.
 In 1898 R. E Payne at the National Dental
Association meeting gave the first clinical
demonstration by placing the silver capsule in the
extracted tooth socket.
 In 1896 B. F. Philbrook, attempted to make soft,
fusible metal inlays by a lost wax process, he
fitted several white metal inlays and one gold
inlay.
11

 In 1897 George B. Martin demonstrated gold
dummy or artificial teeth, called `pontics', for use
on fixed bridges; these were soldered to gold
crowns on the abutment teeth.
 In 1900, J. G.Schottler used a method to restore
the biting edges of front teeth by placing a
platinum wire in the root canal, building the
required shape on the tooth with wax. Invested
and casted it in gold.
 In 1906 John A. Lenz obtained a patent for
devising a method for lost wax casting a gold
chewing surface onto a gold band made to fit
around a tooth.
12

 At a meeting of the New York Odontological Society
on January 15, 1907, William H.Taggart of Chicago
read a lecture entitled `A New and Accurate Method
of Casting Gold Inlays' in which he described a lost
wax technique which can truly be said to have
revolutionized restorative and prosthetic dentistry
13

 In 1907, Dr. Solbrig, in Paris. introduced his casting
pliers which achieved enormous popularity for the
rapid production of small inlays.
 In 1913 Dr. Edward J. Greenfield, fabricated the
hollow cylindrical basket root of 20 gauge iridio
platinum soldered with 24 carat gold.
 In 1948,metallurgists experimenting with various alloys
were able to decrease the gold content while
maintaining their resistance to tarnish. This
breakthrough was due to palladium. It counteracted
the tarnish potential of silver.
14

 1950-Developments of resin veneers for gold alloys.
 1959-Introduction of porcelain fused to metal
technique.
 In the late 1950s, there was the successful
Veneering of a metal substructure with dental
porcelain. Until that time, dental porcelain had a
markedly lower coefficient of thermal expansion than
did gold alloys. This thermal mismatch often led to
impossible to attain a bond between the two
structural components.
15

 It was found that adding both platinum and palladium
to gold lowered the coefficient of thermal
expansion/contraction of the alloy sufficiently to ensure
physical compatibility between the porcelain Veneer
and the metal substructure.
 In 1968-Palladium based alloys as alternative to gold
alloys.
 In 1971-nickel based alloys as alternative to gold
alloys.
 1971 – The Gold Standard
16

 The United States abandoned the gold standard in
1971.
 Prices of gold increased, in response to that, new
dental alloys were introduced through the following
charges.
 In some alloys, gold was replaced with palladium.
 In other alloys, palladium eliminated gold entirely.
 Base metal alloys with nickel as the major element
eliminated the exclusive need for noble metals.
17

 Palladium-silver alloy type was introduced to the US
market in 1974 as the first gold free noble alloy
available for metal ceramic restorations.
 The first alloy of the gold palladium type (group V
according to ADA)Olympia was introduced in 1977 by
JF JELENKO AND CO.
18

 1980’s –Introduction of All ceramic technologies.
 Using a mesh screen pattern as a castability monitor,
WHITLOCK ET AL in 1985 measured percent
castability values of fourteen metal –ceramic alloys.
 1999-Gold alloys as alternatives to palladium based
alloys
19

 Thus the history of dental casting alloys has been
influenced by 3 major factors:
1.The Technologic changes of dental prosthesis.
2.Metallurgic Advancements
3.Price changes of Noble metals since 1968.
20

metal :any strong and relatively ductile substance
that provides electropositive ions to a corrosive
environment and that can be polished to a high
luster; characterized by metallic atomic bonding
21
GPT 9

 Of the 118 elements currently listed in the periodic
table, about 88 (74.6%) can be classified as
metals.
 Groups 3 to 12 - Transition metals.
 Groups 14 to 16 – Non metals
 Dental alloys are transition elements (typically 21
to 80, although groups 89 to 112 are also
included)
23

 The science and art of the extraction of metals
from their ores together with the refinement of
these metals and their adaption to various uses
24
CHEMICAL
PHYSICAL
MECHANICAL
TYPES

 Production and refinement of metals.
 “Process” metallurgy - processing of ores for the
production of metals.
25

 Physical metallurgy is newer science and deals
with the possible alteration in structure as well as
the characteristic physical properties of metals.
 In some respects physical metallurgy and
metallography are closely related.
 Investigates the effects of composition, casting
processes, deformation, and heat treatment on
the physical and mechanical properties of metals
26

 It includes various processes in the fabrication of
a structure such as the casting, rolling or drawing
operations.
 In restorative materials, physical metallurgy
combined with metallography and the mechanical
phase of metallurgy are of greatest importance.
27

 A metal - ionizes positively in solution.
 Typical and characteristic properties - lustre,
opacity, density, thermal and electrical
conductivity.
 Extreme ductility and malleability - often
desirable in metals used in dentistry -
predominate in pure metals rather than in alloys.
28

STRUCTURE
• Crystalline structures in the solid state.
• SPACE LATTICE / CRYSTAL - any arrangement
of atoms in space such that every atom is
situated similarly to every other atom.
• Types - Length of each of three unit cell edges
(called the axes) and the angles between the
edges.
29

 14 possible lattice types or forms
1. Simple cubic space lattice - Single cells of
cubic space lattice
 Simple cubic
 Face-centered cubic - Cu
 Body-centered cubic - Fe
30

Other simple lattice types of dental interest.
A) Rhombohedral
B) Orthorhombic
C) Monoclinic
D) Triclinic
E) Tetragonal
F) Simple hexagonal
G) Close packed hexagonal –Ti, Zn, Zr
H) Rhombic.
31

 The plane along which an edge dislocation moves
is known as a slip plane
 The crystallographic direction in which the atomic
planes have been displaced is termed the slip
direction
 Combination of a slip plane and a slip direction is
termed a slip system.
33

 Point defects
 Line defect/ Dislocations
 Slip plane
 Slip direction
 Slip system
34

 Alloy—a mixture of two or more metals or
metalloids that are mutually soluble in the molten
state; distinguished as binary, ternary, quaternary,
etc., depending on the number of metals within
the mixture; alloying elements are added to alter
the hardness, strength, and toughness of a
metallic element, thus obtaining properties not
found in a pure metal; alloys may also be
classified on the basis of their behavior when
solidified.
 Alloy system—All possible alloyed combinations
of two or more elements at least one of which is a
metal.
35

 Dental amalgams – Hg, Ag, Sn, Cu
 High noble (HN) alloys
 At least 40 wt% Au and 60 wt% of noble metals.
 Noble (N) metal alloys
 Palladium (Pd) - main noble metal content - 25
weight percent
 Also contain Au, Ag, Cu, Ga, In, Pt, Sn.
 Predominantly base (PB) metal alloy
 Less than 25 wt% of noble metals,
 Ni-Cr; Co-Cr; Fe-C-Cr; CP-Ti; Ti-Al-V
36

 Microstructure—Structural features of a metal,
including grains, grain boundaries, phases, and
defects such as porosity, revealed by microscopic
imaging of the chemically or electrolytic ally
etched surface of a flat, polished specimen
37

 During solidification - liquid changes in to solid
- cooling
 Energy of liquid is < solid above the melting
point. Liquid is stable above the melting point
 Below the melting point, the energy of liquid
>solid. Solid becomes more stable
 At the melting point, liquid gets converted in to
solid during cooling.
 This transformation of liquid into solid below
melting point is known as solidification
38

 Thermodynamically, both liquid and solid equal
energy at melting point - equally stable at melting
point - no solidification or melting at the melting
point
 3.Under-cooling - essential for solidification
 4.Solidification occurs by two process :
nucleation and growth.
39

• During the super cooling process,
crystallization of the pure metal
begins. Once the crystals begin to
form, the release of the latent heat
of fusion causes the temperature to
rise to Tf, where it remains until
crystallization is completed at
point C
40

 Solidification begins with the formation of
embryos in the molten metal—(small clusters of
atoms that form nuclei of crystallization)
 At temperatures > Tf - embryos form
spontaneously in the molten metal - unstable,
since the liquid state has a lower free energy than
the solid state.
41

Nucleation and Growth Transformation
 EMBRYO
 Tiny particle of solid that forms from the liquid as
atoms cluster together.
 Unstable - either grow into a stable nucleus or re-
dissolve.
 NUCLEUS
 Large enough to be stable, nucleation has
occurred and growth of the solid can begin.
42

 I step - creation of tiny, stable, nuclei in the liquid
metal
 Cooling the liquid below its equilibrium freezing
temperature, or under cooling, provides the
driving force for solidification
 Once a cluster reaches a critical size, it becomes
a stable nucleus and continues to grow
 The mold walls and any solid particles present in
the liquid make nucleation easier
43
Cluster of atoms Embryo Nuclei Crystals Grains
r < Ro
r > Ro

 Heterogeneous nucleation—Formation of solid
nuclei on the mold walls or on particles within a
solidifying molten metal.
 Homogeneous nucleation—Formation of nuclei
that occur at random locations within a
supercooled molten metal in a clean, inert
container
 No external interface
45

 Crystals grow as dendrites, which can be described
as three-dimensional, branched network structures
emanating from the central nucleus
46

 Volume free energy ΔGV – free energy difference
between the liquid and solid
 Surface energy ΔGs – the energy needed to
create a surface for the spherical particles
 Total free energy Change, ΔGT = ΔGV + ΔGs
 At low temperatures atoms form small cluster or
groups.
 Embryos formed may either form into stable
nuclei or may re-dissolve in the liquid.
 Beyond the critical radius of the nuclei it will
remain stable and growth occurs
48

Atomic diffusion in random patterns within the crystal structure -
Structural imperfections
Extensions form - grow into cooler regions of the mold cavity
Rapid growth - adjacent super cooled regions that lie farther away
in the molten metal - certain crystallographic directions
Heat released - lowers the amount of super cooling - impedes
growth adjacent to the extensions - highly elongated crystals are
formed
Similar growth process at lateral sites along the extensions -
secondary branches, resulting in a three-dimensional dendritic
structure
52

 Dendrite formation occurs during solidification of
alloys because of constitutional supercooling
 Dendritic microstructures are not desirable in
cast dental alloys because interdendritic regions
serve as sites for crack formation and
propagation.
 Microcracks, called “hot tears,” form at elevated
temperatures in thin cast areas of these alloys.
53

 Crystal growth continue until all the material has
solidified and all the dendritic crystals are in
contact
 Grain—A single crystal in the microstructure of a
metal
 Grain boundary—The interface between
adjacent grains in a polycrystalline metal
 Dendritic microstructure—A cast alloy structure
of highly elongated crystals with a branched
morphology
 Equiaxed grain microstructure—A cast alloy
microstructure with crystal (grain) dimensions that
are similar along all crystal axes
55

 Material placed under sufficiently high stress -
dislocation is able to move through the lattice until
it reaches a grain boundary
 The plane along which the dislocation moves is
called a slip plane
 Stress required to initiate movement is the elastic
limit
60

 Grain boundaries - Natural barrier to the
movement of dislocations.
 Concentration of grain boundaries increases as
the grain size decreases.
 Metals with finer grain structure -
 Harder
 Higher values of elastic limit
61

 Fine grain structure achieved by rapid cooling of
the molten metal or alloy following casting.
 Referred as quenching - many nuclei of
crystallization are formed - large number of
relatively small grains
62

 Fine grain sizes - noble metal alloys
 Rapid solidification conditions – time is
inadequate for the growth of large crystals.
 Compositional uniformity and corrosion resistance
- superior for a fine grain size
 Less opportunity for microsegregation.
 Controls yield strength – inversely proportional
63

 Some metals and alloys are said to have a refined
grain structure.
 Achieved by seeding the molten metal with an
additive metal which forms nuclei crystallization
64

 Alloy
 Mixture of two or more metals.
 Alloy system
 Refers to all possible compositions of an alloy.
 Eg. silver-copper system refers to all alloys with
compositions ranging between 100% silver and
100% copper.
 Metals usually show mutual solubility, one within
another. When the molten mixture is cooled to
below the melting point:-
 The component metals may remain soluble in
each other forming a solid solution.
66

 When in a solid, the atoms of solute are present
in the lattice of the solvent, it is known as solid
solution.
 It is considered a solution rather than a compound
 Crystal structure of the solvent remains
unchanged by addition of the solutes.
67
Two types.
1. Substitutional solid solution
2. Interstitial solid solution

 When the atoms of solute substitute for the atoms
of the solvent in its lattice, the solution is known
as Substitutional solid solution.
 The solute may incorporate into the solvent
crystal lattice substitutionally by replacing a
solvent particle in the lattice.
 Substitutional element replaces host atoms in its
lattice
68

 Substitutional solid solutions can be of two types
1. Ordered solid solution
2. Disordered solid solution
69

Ordered solid solution
 Atoms of the solute occupy certain preferred sites
in the lattice of the solvent,
 Occur only at certain fixed ratios of the solute and
solvent atoms.
 In Cu – Au system, Cu atoms occupying the face-
centered sites and Au atoms occupying the corner
sites of the FCC unit cell.
70

 Atoms of the solute are present randomly in the
lattice of the solute
 Most of the solid solutions are disordered solid
solutions
71

 Atoms of the solute occupy the interstitial spaces
in the lattice of the solvent
 If the size of the solute is less than 40% that of
solvent, interstitial solid solution may be formed.
 The solute may incorporate into the solvent
crystal lattice interstitially, by fitting into the space
between solvent particles.
 Only H, Li, Na and B form interstitial solid solution.
72

 Solid solutions are generally harder, stronger and
have higher values of elastic limit than the pure
metals from which they are derived. This explains
why pure metals are rarely used.
 The hardening effect, known as solution
hardening, -atoms of different atomic radii within
the same lattice form a mechanical resistance to
the movement of dislocations along slip planes
73

 CONDITIONS FOR SOLID SOLUBILITY
 Atom size difference-diameters of the solute
atoms
 Valence- SS forms - same valence; chemical
affinity
 Type of crystal structure - same type of crystal
structure
 Potential for solvent atoms to become ordered
74

PHYSICAL PROPERTIES OF SOLID SOLUTIONS
 Solid solution strengthening
 Greater concentrations of the solute atoms
 Increasingly dissimilar sizes of the solvent and
solute atoms.
 For binary alloys - maximum hardness at
concentrations of approx. 50% for each metal
75

EQUILIBRIUM-PHASE DIAGRAMS
 Equilibrium- or constitution-phase diagrams
 Identify the phases present in an alloy system for
different compositions and temperatures.
77

 All possible combinations of 2 metals - completely
soluble at all compositions in both the solid and
liquid states.
 Liquid, liquid-plus solid, and solid regions
separated by the liquidus and solidus curves
 Liquidus temperature – Temperature at which
an alloy begins to freeze on cooling or at which
the metal is completely molten on heating.
 Solidus temperature – Temperature at which an
alloy becomes solid on cooling or at which the
metal begins to melt on heating
81

 INTERPRETATION OF THE PHASE DIAGRAM –
Determines -
 Composition of alloy
 Weight percentage of alloy
82

 PO - 65% Pd and 35% Ag
 PO intersects liquidus curve at point R - first solid
forms
 Temp 1400 ᵒC - Composition of the first solid
formed
 Tie line RM - R on the liquidus curve to M on the
solidus curve = 77% Pd
 Temp - 1370 °C Compositions of the solid and
liquid determined
 YW -liquid 57% Pd, and solid 71% Pd
 Temp – 1340 ᵒC
 last portion of liquid that solidifies - 52% Pd (point
N). The solid phase contains 65% Pd (point T).
83

 Percentages of two phases in equilibrium at a
given temperature – LEVER RULE
Length of the tie line segment opposite a given
boundary curve ÷
Total length of the tie line connecting the two
boundary curves is the percentage of the phase.
86
FULCRUM

 CORING AND HOMOGENIZATION HEAT
TREATMENT
 Coring process
 Under rapid solidification conditions, the alloy has
a cored structure.
 The core consists of dendrites composed of
compositions that developed at higher solidus
temperatures
 Matrix is the portion of the microstructure
between the dendrites that contains
compositions that developed at lower solidus
temperatures 87

88
• Indication of the degree of coring - Separation of the solidus
and liquidus lines on the phase diagram.
• The potential for coring is greater when there is wide
separation of solidus and liquidus lines

 Homogenization heat treatment
 Promotes atomic diffusion - eliminate as-cast
compositional difference
 Produce equiaxed grains
 Reduce microsegregation as can be confirmed by
the appearance of distinct grain boundaries in the
microstructure
90

 Homogenizing the cast structure
 Heated near its solidus temperature to promote
the most rapid diffusion without melting – about 6
hrs
 Little grain growth occurs because the grain
boundaries are immobilized by secondary or
impurity elements and phases
91

 Other adv. of homogenisation-
 Reduces tarnish and corrosion
 Increases ductility
 Decreases brittleness
92

 Wrought alloys are heat treated by Annealing -
Ductility increases
 Gold alloys are heat treated by softening
(solution heat treatment) or hardening (age
hardening heat treatment)
93

 The term eutectic greek word ‘euctectos’- easily
fused
 Correspond to a composition with the lowest
melting point in an alloy system.
 Used in joining metal components - brazing or
soldering
 The eutectic alloy is one in which the components
exhibit complete solubility in the liquid state
but limited solid solubility
 Eg; Ag – Cu system
Au – Ir system
 Reaction during the cooling process :
Liquid α solid solution + β solid solution
94

 3 phases
1. Liquid phase (L)
2. α Phase - A silver-rich substitutional solid
solution phase + small percentage of copper
atoms;
3. β Phase – A copper-rich substitutional solid
solution phase + small percentage of silver
atoms.
 α and β phases - Terminal solid solutions .
 Solidus curve - ABEGD.
 Liquidus curve - AED
 Below 780 °C - two-phase α-plus-β region - a
mixture of the silver-rich and copper-rich phases
in the alloy microstructures.
96

 Liquidus and solidus phases meet at composition
E
(Temp - 779 °C, corresponding to line BEG)
 This composition, of 72% Ag and 28% Cu by
weight, which corresponds to point E, is known
as the eutectic composition,
98

 Characteristics of this special composition
 Temp at which the eutectic composition melts –
779 ᵒC- Lower than fusion temp of either metal
 No solidification range for composition E
99

 Solidus slope - AB & DG
 AB - copper content of the silver-rich α phase -
0% to nearly 9%.
 DG - silver content of the copper-rich β phase -
0% to 8%.
 Solvus lines – CB & FG
 CB - solid solubility of copper in the α phase
increases - 1% at 300 °C to nearly 9% at B.
 FG - solid solubility of silver in the β phase
increases - extremely small value at 300 °C to a
maximum of approx. 8% at point G
100

 Phase diagram used to determine -
 Composition – Application of tie line
 Weight percentage – Application of lever rule
102

 PROPERTIES OF HYPOEUTECTIC AND
HYPEREUTECTIC ALLOYS
 Alloys with a composition less than that of the
eutectic are called hypoeutectic alloys
 Those with a composition greater than the
eutectic are known as hypereutectic alloys
104

 Hypoeutectic alloys – Primary crystals - α solid
solution
 Hypereutectic alloys – Primary crystals - β solid
solution.
 Hypoeutectic or hypereutectic alloys containing
eutectic constituent in their microstructures
(compositions between B and G) - brittle
 Alloys with microstructures lacking this
constituent (compositions to the left of B or to
the right of G) are ductile and tarnish resistant
 Alternating lamellae of the α and β phase
inhibit the movement of dislocation
increases strength and hardness,
decreases ductility (or increases brittleness).
 The tarnish resistance of these alloys without
the eutectic is superior
105

 Since there is a heterogeneous composition,
they are susceptible to electrolytic corrosion.
 They are brittle, because the present of
insoluble phases inhibits slip.
 They have a low melting point and therefore
are important as solders.
106

 PERITECTIC ALLOYS
 Another example of the limited solid solubility of
two metals is the peritectic transformation
 Eg ; Ag – Sn system
Ag – Pt system
Pd – Ru system
 Reaction during the cooling process :
Liquid + β solid solution α solid solution
107

 Peritectic is a phase where there is limited solid
solubility.
 Not of much use in dentistry except for silver tin
system.
 Reaction occurs when there is a big differences
in the melting points of the components.
108

 α phase - silver-rich
 β phase - platinum-rich
 Two -phase (α+β) region results from :
 Limited solid solubility of Ag in Pt at 700 °C - <
12% (point F)
 Limited solid solubility of Pt in Ag at 700 °C- 56
% (point G)
109

 Peritectic transformation occurs at point P
 Liquid composition at B and the platinum-rich β
phase (composition at point D) transform into
the silver-rich α phase (composition at point P)
111

 Extensive diffusion is required in these phases
for transformation,
 Peritectic alloys are susceptible to coring during
rapid cooling.
 Cored structure has inferior corrosion resistance
 More brittle than the homogeneous solid solution
phase.
112

COLD WORKING AND ANNEALING
113

 When the stress is greater than the elastic limit at
relatively low temperatures – Ductile and
malleable
 Produces a change in microstructure, with
dislocations becoming concentrated at grain
boundaries, but also a change in grain shape.
 The grains are no longer equiaxed - more fibrous.
114

 Cold working is sometimes referred to as work
hardening due to the effect on mechanical properties.
 When mechanical work is carried out - at a more
elevated temperature - object change shape without
any alteration in grain shape or mechanical properties.
115

 The temperature below which work hardening is
possible is termed the recrystallization temperature.
 If the material is maintained above the recrystallization
temperature for sufficient time, diffusion of atoms
across grain boundaries may occur, leading to grain
growth.
 Grain growth should be avoided if the properties are
not to be adversely affected.
116

Process of heating a metal to reverse the effects
associated with cold working such as strain hardening,
low ductility and distorted grains.
In general it has 3 stages.
1) Recovery
2) Recrystallization
3) Grain growth.
117

 Recovery :
 Stage at which the cold work properties begin to
disappear before any significant visible changes are
observed under the microscope.
 Recrystallization :
 Occurs after the recovery stage.
 The old grains disappear completely
 Replaced by a new set of strain free grains.
 Grain growth:
 The crystallized structure has a certain average grain
size, depending on the number of nuclei .
 The more severe the cold working, the greater the
number of such nuclei.
 Grain size for completely recrystallized material can
range from rather fine to fairly coarse.
118

 Cold working may cause the formation of internal
stresses within a metal object. If these stresses are
gradually relieved they may cause distortion which
could lead to loss of fit of, for example, an orthodontic
appliance.
 For certain metals and alloys the internal stresses can
be wholly or partly eliminated by using a low
temperature heat treatment referred to as stress relief
annealing.
120

 This heat treatment is carried out well below the
recrystallization temperature and has no deleterious
effect on mechanical properties since the original grain
structure is maintained.
121

 Heat treatment of metals in the solid state is
called SOLID STATE REACTIONS.
 Results in diffusion of atoms of the alloy by
heating a solid metal to a certain temperature
and for certain period of time.
 This will result in the changes in the microscopic
structure and physical properties.
122

 Shaping and working on the appliance in the
laboratory is made easy when the alloy is soft. -
First stage called softening heat treatment.
 To harden the alloy for oral use, so that it will
withstand oral stresses. The alloy is again heated
and this time it is called hardening heat
treatment.
123

 Softening Heat treatment/ Solution Heat
treatment
 Hardening Heat treatment/ Age Hardening
124

 Also known as ANNEALING. This is done for
structures which are cold worked. Eg ; Gold
(High noble & noble metal alloys)
 Phase change – disordered solid solution
 Technique - alloy is placed in an electric furnace
at a temperature of 700°C for 10 minutes and
then rapidly cooled (quenched).
 Result of this is reduction in strength, hardness
and proportional limit but increase in ductility.
In other words the metal becomes soft. This is
also known as HOMOGENIZATION TREATMENT.
 Indication- Structures to be ground, shaped or
cold worked
125

 This is done for cast removable partial dentures,
saddles, bridges, but not for Inlays.
 Technique - The appliance (alloy) is heat soaked
at a temperature between 200-450°C for 15-30
minutes and then rapidly cooled by quenching.
 The result of this is increase in strength (yield
strength), hardness and proportional limit but
reduction in ductility(percent elongation).
 Also known as ORDER HARDENING or
PRECIPITATION HARDENING.
 Phase change – ordered solid solution
126

 After solution heat treatment, the alloy is once
again heated to bring about further precipitation
and this time it shows in the metallography as a
fine dispersed phase.
 This also causes hardening of the alloy and is
known as age hardening because the alloy will
maintain its quality for many years.
127

In order of increasing melting temperature, they include gold,
palladium, platinum,
rhodium, ruthenium, iridium, and osmium. Only gold, palladium, and
platinum, which have the lowest melting temperatures
of the seven noble metals, are currently of major importance in
dental casting alloys.
129

ALLOY TYPE BY MAJOR ELEMENTS:
Gold-based, palladium-based, silver-based, nickel-based,
cobalt-based and titanium-based .
ALLOY TYPE BY PRINCIPAL THREE ELEMENTS:
Such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V
and Fe-Ni-Cr.
(If two metals are present, a binary alloy is formed; if three
or four metals are present, ternary and quaternary alloys,
respectively, are produced and so on.)
 ALLOY TYPE BY DOMINANT PHASE SYSTEM:
Single phase [isomorphous], eutectic, peritectic and
intermetallic
132

NOBLE METALS
• Noble Metal are corrosion and oxidation resistant
because of inertness and chemical resistance.
• Basis of inlays, crowns and bridges because of
their resistance to corrosion in the oral cavity.
• Gold, platinum, palladium, rhodium, ruthenium,
iridium, osmium, and silver
135

 Precious metal: a metal containing primarily
elements of the platinum group, gold, and silver.
 Precious metal alloy: an alloy predominantly
composed of elements considered precious, i.e.,
gold, the six metals of the platinum group
(platinum, osmium, iridium, palladium, ruthenium,
and rhodium), and silver
 Term precious stems from the trading of these
metals on the commodities market
136

 Biocompatibility
 Tarnish and Corrosion resistance
 Compatible thermal properties
 Castability
 Aesthetics
 Economical
138

Biocompatibility
 Tolerate oral fluids
 Do not release harmful products
139

 Corrosion resistance
Physical dissolution of material - corrosion
Too noble to react
Metallic elements form adherent passivating
surface film
eg:- Cr in Ni – Cr and Ti
140

Tarnish Resistance
Thin film of surface deposit/
interaction layer on metal surface - tarnish
Allergenic Components in Casting Alloys
Morally and legally to minimize risk
Aesthetics
optimal balance among properties
141

 Compatible thermal properties
 Compensation for solidification
Compensation for casting shrinkage from
solidus temperature to room temperature
achieved either through-
computer- generated oversized dies
 controlled mold expansion
 Alloys - closely matching thermal expansion
coefficients to be compatible with
porcelains
142

 Castability
 Alloy should flow freely into the most intricate
regions of the investment mold
 Measured by percent completion of a cast mesh
screen pattern or other castability patterns
143

 Strength requirements
 Alloys for bridgework require higher strength
than alloys for single crowns.
 Alloys for metal-ceramic prostheses are finished
in thin sections and require sufficient stiffness
144

 Porcelain bonding
 Sound chemical bond to ceramic veneering
materials, - thin adherent oxide, preferably one
that is light in color
 Aesthetic
145

 Economic considerations
 Cost of fabricating prostheses must be adjusted
periodically to reflect the fluctuating prices of
casting metals - high noble and noble metal
alloys.
146

 Alloys used for metal ceramic restoration can
be used for all metal prosthesis
 But Alloys for all metal restorations should
not be used for metal ceramic restoration
 Reasons :-
1) May not form thin, stable oxide
layers to promote atomic bonding to
porcelain.
2) Melting range too low to resist sag
deformation or melting at porcelain firing
temperatures.
3) Thermal contraction co-efficients
not close to porcelain.
147

 Soft, rich yellow color and a strong metallic luster
 Most malleable and ductile
 0.2% lead – brittle
 Soluble in aqua regia (combination of nitric and
hydrochloric acid )
 Alloyed with copper, silver, platinum – increases
hardness , durability and elasticity
148

Density
19.3
g/cm3
Melting
point
1063oc
Boiling
point of
2970oc
KHN 25
CTE of
14.2×10-
6/°c
Lowest in strength and surface hardness
High level of corrosion and tarnish resistance
 High melting point, low C.O.T.E value and very good conductivity
Improves workability, burnish ability, raises the density
Calcium improves its mechanical properties
Cohesive , welded at room temp.
149

Gold content:
Traditionally the gold content of dental casting alloys
have been referred to in terms of:
Carat:
 The term carat refers only to the gold content of the
alloy; a carat represents a 1⁄24 part of the whole. Thus
24 carat indicates pure gold. The carat of an alloy is
designated by a small letter k, for example, 18k or 22k
gold.
Fineness:
 Fineness also refers only to the gold content, and
represents the number of parts of gold in each 1000
parts of alloy. Thus 24k gold is the same as 100% gold
or 1000 fineness (i.e., 1000 fine) or an 18k gold would
be designated as 750 fine.
150

 Bluish white metal
 Hardness similar to copper
 Higher melting point ( 1772°C) than porcelain
 Coefficient of thermal expansion close to porcelain
 Lighten the color of yellow gold based alloys
 Common constituent in precision prosthetic
attachments
•High density 21.45 g/cm3
•High melting point 1772oC
•Boiling point of 4530 oC
•Low CTE 8.910-6/oC
151

 Malleable, ductile; white metal.
 Stronger and harder than gold, softer than copper.
 Absorbs oxygen in molten state-difficult to cast
 Forms series of solid solutions with palladium and gold
density 10.4gms/cm3
melting point 961oC
boiling point 2216 oC
CTE 19.710-6/oC
152

 Lowers the melting range
 Low corrosion resistance
 In gold-based alloys, silver is effective in
neutralizing the reddish color of copper.
 Silver also hardens the gold-based alloys via a solid-
solution hardening mechanism.
 Increases CTE in gold- and palladium-based alloys
 Foods containing sulfur compounds cause severe
tarnish on silver, and for this reason silver is not
considered a noble metal in dentistry.
 Pure silver is not used in dental restorations because
of the black sulfide that forms on the metal in the
mouth.
153

 White metal darker than platinum
 Density little more than half that of Pt and Au
 Absorbs hydrogen gas when heated
 Not used in pure state in dentistry
 Whitens yellow gold based alloys.
• density 12.02gms/cm3
• melting point 1552oC
• boiling point 3980 oC
• lower CTE 11.810-6/oC when
compared to gold.
154

 Grain refiners
 Improves mechanical properties and uniformity of
properties within alloy
 Extremely high melting point of Ir - 2410°C and Ru -
2310°C – serve as nucleating centers
 Osmium(Os) has a very high melting point, and is
very expensive, hence not used in dentistry.
155

 Constitute majority of mass of many
noble alloys
 Useful in understanding the behavior
of more complex alloys
 Six important combinations
 Au –Cu Pd - Ag
 Pd –Cu Au - Pd
 Au – Ag Au - Pt
160

 Alloy composition and temperature
 Differences between liquidus and solidus
line small for Ag – Au system
larger for Au - Pt system
 Desirable to have narrow liquidus – solidus
range – Potential for coring less
161

 With slow cooling the crystallization - diffusion
and a random distribution of atoms - with no
coring.
 Rapid cooling - denies the alloy the energy and
mobility required for diffusion - cored structure
is ‘locked in’
 Reducing the cooling rate - self-defeating –
Results in alloy with large grain size - inferior
mechanical properties
162

 Heat treatment - to eliminate the cored
structure - homogenization heat treatment.
 Heating the alloy to a temperature just below
the solidus temperature for a few minutes -
allow diffusion of atoms
 The alloy is then normally quenched - prevent
grain growth from occurring.
 Eg : Gold-silver system. – Pt or Pd are present -
homogenization heat treatment
 Heating to 700ºC for 10 minutes, then quenching
163

 Hardening of noble alloys
 Hardening heat treatments are not beneficial for
the types 1 and 2 alloys - insufficient quantities of
copper and silver.
 Solid solution & ordered solution hardening
 Precipitation
 Grain refiners such as Ir, Rh, and Ru
164

 Solid solutions - stronger and harder than either
component pure metal.
 Presence of atoms of unequal size - difficult for
atomic planes to slide by each other.
 Ordered solutions - Further strengthen a solid -
pattern of dissimilar sizes throughout the alloy's
crystal structure
165

 Au-Cu system ,heated to molten state-cooled
slowly –mass solidify at 8800 C - Solid solution
 Cools slowly to 424 o C –ordered solution
166

 Precipitation hardening - By heating some cast
alloys carefully, a second phase - appear in the
body of the alloy.
 Blocks the movement of dislocations - increasing
strength and hardness.
 The effectiveness greater - if precipitate is still
part of the normal crystal lattice. - coherent
precipitation.
 Overheating may reduce alloy
properties - second phase grow
outside of the original lattice structure.
167

 Grain refiners - Ir, Rh, and Ru
 Fine grained - grain sizes below 70 pm in
diameter.
 0.005% or 50 ppm of iridium and ruthenium
 Tensile strength and elongation are improved
significantly (30%)
 Hardness and yield strength - less effect
169

 Cold working an alloy will significantly
strengthen it.
 But - less ductile
170

Treatment of noble and high noble alloys
 Type lll and type lV gold alloys can be hardened
and softened.
 Softening heat treatment/homogenizing-Solution
heat treatment.
 Hardening heat treatment-Age hardening.
171

 Increases ductility
 Reduces tensile strength ,proportional limit and
hardness
Method:
 Casting placed in electric furnace
 10 minutes,700°Cquenched in water resulting
disordered solid solution
 Indicated-alloys that are to be ground, shaped or
otherwise cold worked either in or out of mouth.
172

 Increases strength, proportional limit, and hardness,
but decreases ductility
 If positioning of two elements become ordered-
ordered solution
 Copper present in gold alloy helps in this process.
Method:
 Soaking/ageing casting-15 to 30 minutes before water
quenching 200°C to 450°C
 Ideally, before age hardening it should first be
subjected to softening heat treatment
173

 The hardening heat treatment is indicated for
metallic partial dentures, saddles, FDPs -
Rigidity of the prosthesis is needed.
 For small structures, such as inlays, a hardening
treatment is not usually required.
 Age hardening reduces the ductility of gold alloy
174

Functional Mechanical Properties of Casting alloys
 Elastic modulus
 Yield strength
 Ductility
 Hardness
 Fatigue Resistance
175

 Classification of Noble PFM alloys
176
Au Based Pd Based
Au-Pt-Pd (21 K) Pd-Ag
Au- Pd (13 K) Pd-Cu
Au-Pd-Ag (13 K) Pd-Co

177
Noble alloys
Gold-copper-silver-palladium
Palladium-copper-gallium
Palladium-silver and silver-palladium
High noble alloys
 Gold-Platinum alloy
 Gold-Palladium alloy
 Gold-copper-silver-palladium alloys

1. Noble metal content
2. Hardness
3. Yield strength
4. Elongation
5. Fusion temperature
6. Porcelain-Metal Compatibility
7. Color stability
8. Biocompatibility
178

 Minimum of 60% noble metals (any combination
of gold, palladium and silver) with a minimum of
40% by weight of gold.
 Tin, indium and/or iron oxide layer formation
chemical bond for the porcelain
180

 Developed alternative to palladium alloys
For full cast as well as metal-ceramic restorations.
 More prone to sagging, they should be limited to short
span bridges.
 A typical composition is Gold 85%;
 Platinum12%;
 Zinc 1%;
 Silver (in few brands
181
Large two-phase region

 Used for full cast /metal-ceramic restorations.
 Palladium - high melting temperature
- impart a white or gray color
- improves sag resistance
 These alloys usually contain indium, tin or gallium
to promote an oxide layer.
A typical composition
 Gold 52%;
 Palladium 38%;
 Indium 8.5%;
 Silver (in some brands).
182

 Have low melting temperature
 Not used for metal-ceramic applications.
 Greening of porcelain – due to silver
 Copper tends to cause sagging during porcelain
processing.
A typical composition is
 Gold 72%,
 Copper 10%;
 Silver 14%;
 Palladium 3%.
183

 Contain at least 25% by weight of noble metal
(gold, palladium or silver)
 Have relatively high-strength, durability, hardness,
ductility.
 They may be yellow or white in color.
185

 More copper and silver
 Have a fairly low melting temperature
 More prone to sagging during application of porcelain.
 Used mostly for full cast restorations rather than PFM
applications.
 A typical formula is: gold 45%
 Copper 15%
 Silver 25%
 Palladium 5%
186

 Introduced in 1983
 Very rigid excellent full cast or PFM restorations.
 Contain copper  prone to sagging
during porcelain firing.
 Gallium  reduces the melting
temperature
A typical composition is
 Palladium 79%
 Copper 7%;
 Gallium 6%
 Hardness is comparable to base metal alloy but are not
burnishable
187
2 superlattice transformations

 Higher palladium alloys - PFM frameworks.
 Higher silver alloys - susceptible to corrosion
- greening of porcelain
 High resistance to sagging
 very rigid - good for long spans
188

 More castable (more fluid in the molten state)
 Easier to solder and easier to work with than
the base metal alloys.
 Typical composition for Palladium- silver alloy:
 Palladium 61%; silver 24%; Tin (in some)
Silver-palladium alloy:
 Silver 66%; Palladium 23%; gold (in some
formulation)
189

(i) Greening : colloidal dispersion of silver
atoms entering porcelain - from vapor
transport or surface diffusion - green ,
yellow – green, yellow – orange ,orange and
brown hues
(ii) (ii) More near cervical region – marginal
metal , localized silver concentration
190

(iii) Certain porcelain resistant to silver
discoloration - silver ionization by porcelains
with high oxygen potential
(iv) Greening when porcelains fired on silver –
free alloys- vaporization of silver from walls
of contaminated furnaces
191

(v) Reduce porcelain discoloration by metal
coating agents :-
(a) gold film fired on metal substrate
(b) ceramic conditioner
192

193
Compositions of casting alloys
determine their color.
• Palladium content >10 wt%, -
white
• Copper -reddish color
• Silver lightens either the red
or yellow color of alloys.

 Dental Laboratory Technicians
Exposed to high concentrations of
beryllium and nickel dust and beryllium
vapor.
Beryllium concentration in dental
alloys rarely exceed 2% by weight.
On melting of Ni – Cr – Be alloys the
beryllium vapor remain over an extend
period of time in absence of exhaust and
filtration system.
194

 The Occupational health and Safety
Administration (OSHA) limit beryllium
concentration to 2 µg/m3
Symptoms include contact dermatitis, coughing
, chest pain, general weakness to pulmonary
dysfunction.
195

 Potential patient hazard
 Intraoral exposure to nickel – Nickel
allergy
 Inhalation , ingestion and dermal contact
of nickel and nickel alloys
 Incidence – 5 to 10 times higher for
females
196

 National institute for occupational safety and
health (NIOSH ) recommend standard limit to
employee exposure to inorganic nickel to
workplace to 15mg/m3
 To minimize exposure , a high speed
evacuation system used.
 Patient informed of potential allergic effects.
 Medical history of the patient taken to rule
out nickel allergy.
197

 GOLD UCLA-TYPE ABUTMENTS
• 64% gold, 22% palladium
• Melting range 2400˚F-2500˚F (1320˚C-1370˚C)
• Gold alloy abutment screw retention increases the
preloading force there by assuring precision fit to
implant
Ceraone & Mirus cone abutments
• SEMI-BURNOUT CYLINDER
Non-oxidizing, high precious gold platinum alloy
with a plastic wax-up sleeve Cylinder base
198

 The study evaluated the cervical and internal fit of
complete metal crowns that were cast and recast
using palladium-silver alloy and 3 different marginal
configurations used were straight shoulder, 20-degree
bevel shoulder, and 45-degree chamfer.
 Results showed - The new alloy provided significantly
better adaptation than the recast alloy for both
marginal and internal discrepancy measurements.
Marginal designs did not shown any statistical
differences when the new metal was used
Lopes,S.Consani et al ,Influence of recasting palladium-silver alloy
on the fit of crowns with different marginal configurations
J Prosthet dent,2005;94,5:430-434
200

 The study was to evaluate the bond strength of 4
recently introduced noble alloys by using 2 techniques
for porcelain application
 For both conventional layering and press-on-metal
techniques, all 4 noble alloys had a mean metal-to-
ceramic bond strength that substantially exceeded the
25 MPa minimum in the ISO Standard 9693. The
results for Aries support the manufacturer’s
recommendation not to use the press-on-metal
technique for alloys that contain more than 10% silver.
201
Mofida R. Khmaj et al ;Comparison of the metal-to-ceramic bond strengths
of four noble alloys with press-on-metal and conventional porcelain layering
techniques (J Prosthet Dent 2014;112:1194-1200)

 The purpose of this in vitro study was to determine
whether heat treatment affects the metal ceramic bond
strength of 2 Pd-Ag alloys containing different trace
elements.
 Conclusions : Heating under reduced atmospheric
pressure effectively improved the bond strength of the
ceramic-to-Pd-Ag alloys.
202
Jie-yin Li et al ;Effect of heating palladium-silver alloys on ceramic bond
Strength (J Prosthet Dent 2015;114:715-724)

 The diversity of alloys available to the dental
practitioner has never been more extensive. We
now have the opportunity to select the alloys
based on the individual patient’s specific
biological, functional, and economic
requirements. There is no one alloy suitable for
all applications, because in metallurgy there is a
constant trade off in properties as changes in
formulations are made. To make optimal uses of
the choices available, and for ethical and medico
legal considerations, it is incumbent upon the
practitioner to be aware of the identity and
composition of the alloys prescribed
203

 The science and engineering of materials, 4th edition,
Donald R Askland Pp 139-469
 Anusavice, Phillips Science of Dental Materials, 12th edition,
2012, Elsevier publications, Florida, Pp 69-91, pp 367- 384
 John F. McCabe & Angus W.G. Walls, Applied Dental
Materials, 9th edition, 2008, Blackwell science publications,
United kingdom, Pp 62-70
 Craig G.R. Powers J.M., Restorative Dental Materials, 12th
edition,2006, Elsevier publications, USA, Pp 359-37
 O’Brien W.J., Dental materials and their selection, 3rd
edition, 2002, Quintessence publications, Canada, pp 65-
74, pp 192-200
 John J Manappallil -Basic Dental Materials –3rd edition
204

 Lopes,Consani S et al, Influence of recasting
palladium-silver alloy on the fit of crowns with different
marginal configurations, J Prosthet dent;94,5:430-434
 Mofida R. Khmaj et al ;Comparison of the metal-to-
ceramic bond strengths of four noble alloys with press-
on-metal and conventional porcelain layering
techniques (J Prosthet Dent 2014;112:1194-1200)
 Jie-yin Li et al ;Effect of heating palladium-silver alloys
on ceramic bond Strength (J Prosthet Dent
2015;114:715-724)
205

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