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 ofone or two tubes of gold or Iridium as a supportfor individual teeth or crown.

In 1898 R. E Payne at the National DentalAssociation meeting gave the first clinicaldemonstration by placing the silver capsule in theextracted tooth socket.

In 1896 B. F. Philbrook, attempted to make soft,fusible metal inlays by a lost wax process, hefitted several white metal inlays and one goldinlay.

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 drawingoperations.

In restorative materials, physical metallurgycombined with metallography and the mechanicalphase 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 arrangementof atoms in space such that every atom issituated similarly to every other atom.

• Types - Length of each of three unit cell edges(called the axes) and the angles between theedges.

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

Slip planes

32

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 dendriticstructure

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 threeor 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

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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

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Biocompatibility

Tarnish and Corrosion resistance

Compatible thermal properties

Castability

Aesthetics

Economical

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Biocompatibility

Tolerate oral fluids

Do not release harmful products

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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

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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

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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

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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

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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

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Porcelain bonding

Sound chemical bond to ceramic veneering

materials, - thin adherent oxide, preferably one

that is light in color

Aesthetic

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Economic considerations

Cost of fabricating prostheses must be adjusted

periodically to reflect the fluctuating prices of

casting metals - high noble and noble metal

alloys.

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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.

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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

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Density 19.3 g/cm3

Melting point

1063oc

Boiling point of 2970oc

KHN 25CTE 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.

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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.

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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 8.910-6/oC151

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 19.710-6/oC

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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.

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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 11.810-6/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.

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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

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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

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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

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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

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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

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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

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Au-Cu system ,heated to molten state-cooled

slowly –mass solidify at 8800 C - Solid solution

Cools slowly to 424 o C –ordered solution

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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.

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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

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Cold working an alloy will significantly

strengthen it.

But - less ductile

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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.

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Increases ductility

Reduces tensile strength ,proportional limit and

hardness

Method:

Casting placed in electric furnace

10 minutes,700°Cquenched in water resulting

disordered solid solution

Indicated-alloys that are to be ground, shaped or

otherwise cold worked either in or out of mouth.

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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

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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

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Functional Mechanical Properties of Casting alloys

Elastic modulus

Yield strength

Ductility

Hardness

Fatigue Resistance

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Classification of Noble PFM alloys

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Au Based Pd Based

Au-Pt-Pd (21 K) Pd-Ag

Au- Pd (13 K) Pd-Cu

Au-Pd-Ag (13 K) Pd-Co

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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

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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

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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

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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).

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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%.

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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.

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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%

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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

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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

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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)

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(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

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(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

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(v) Reduce porcelain discoloration by metal

coating agents :-

(a) gold film fired on metal substrate

(b) ceramic conditioner

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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.

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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.

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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

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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.

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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

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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

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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.

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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.

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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

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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

Precious metal alloys

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