3. CONTENTS
• Introduction
• General characteristics of metals
• Casting materials & alloys
Definition
Classification of casting materials
Desirable properties
• Components of alloys
Noble metals
Base metals
• Solidification & crystallization of metals & Alloys
• Phase diagrams & dental alloys
• Types Of alloys & significance
4. CONTENTS
• Effects of mechanical treatment
• Effects of heat treatment
• Alloy strengthening mechanisms
• Mechanism for corrosion resistance
• Casting gold alloys
• Metal ceramic alloys
• Base metal alloys
• Titanium alloys
• Comparison between noble metal & base metal alloys
• References
5. INTRODUCTION
Metallurgy
Mosby’s dental dictionary. Definition of metallurgy. Restorative Dental Materials. 11th Ed. Missouri:elsivier; 2004. p3904
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
Physical and chemical behavior of metals & alloys
• Bulk
• Atomic levelEffect of mechanical
treatment
Effect of heat
treatment
Study of metals and alloys
The study of metals and their
properties, including
separating metals from their
ores, the making and
compounding of alloys, and
the technology and science of
working and heat treating
metals to alter their physical
charachteristics.
6. INTRODUCTION
Metals & alloys
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-117
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
CASTING ALLOYS
FINAL
STRUCTURE
FURTHER
MANIPULATED
C
A
S
T
WROUGHT
ALLOY
7. INTRODUCTION
Significance of Metallurgy in dentistry
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
Diverse metals & alloys – characteristic behavior
Properties of metals - guide in fabrication of cast
structures
COMPOSITION OF CASTING
MATERIAL
CLINICAL USE &
ENVIRONMENT
BUR HARDNESS
IMPORTANT
CORROSION
RESISTANCE
LESS
IMPORTANT
CROWN BIOCOMPATIBILITY
CORROSION
RESISTANCE
8. Properties
• Lustre
• Opacity
• Density
• High boiling points & melting points
• Solids at room temperature
• Thermal conductivity
• Electrical conductivity
• Ductility & malleability
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-117
GENERAL CHARACTERISTICS OF METALS
9. GENERAL CHARACTERISTICS OF METALS
Properties - valence Elelectron configuration
Metallic bond
Results from ↑spatial extension of valence e- when an aggregate of
metal atoms is brought close together.
+ve
charged
ionic
core
+ve
charged
ionic
core
+ve
charged
ionic
core
+ve
charged
ionic
core
+ve
charged
ionic
core
+ve
charged
ionic
core
Unbound / free
valance e- forms
cloud or gas
equilibrium
between
electrostatic force s
results in inter
atomic space
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
10. • The term alloy system refers to all possible
compositions of an alloy
10
0% 100%
0%100%
SILVER
COPPER
CASTING ALLOYS
DEFINITION
• A substance with metallic properties that is
composed of two or more chemical elements, at
least one of which is a metal
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
11. CLASSIFICATION OF CASTING ALLOYS
BASED ON NOBILITY
HIGH NOBLE ALLOYS
PREDOMINANTLY BASE
METAL ALLOYS
NOBLE ALLOYS
Contains > 40 wt% Au and > 60 wt% of the noble
metal elements (Au + Ir + Os + Pd + Pt + Rh + Ru)
Contains > 25 wt % of the noble metal elements
Contains < 25 wt % of the noble metal elements
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
12. CLASSIFICATION OF CASTING ALLOYS
Gladwin M, Bagby M. Materials for fixed indirect Restorations and Prostheses. Clinical aspects of dental materials. China: Lippincott
Williams & Wilkins. 2009. p 131-145.
Revised ADA Classification, 2003
HIGH NOBLE ALLOYS
PREDOMINANTLY BASE
METAL ALLOYS
NOBLE ALLOYS
Contains > 40 wt% Au and > 60 wt% of the noble
metal elements (Au + Ir + Os + Pd + Pt + Rh + Ru)
Contains > 25 wt % of the noble metal elements
Contains < 25 wt % of the noble metal elements
TITANIUM & TITANIUM
ALLOYS
Titanium ≥ 85 wt%
13. CLASSIFICATION OF CASTING ALLOYS
BASED ON USE
CROWN & BRIDGE
ALLOYS
RPD ALLOYS
METAL CERAMIC ALLOYS
Gladwin M, Bagby M. Materials for fixed indirect Restorations and Prostheses. Clinical aspects of dental materials. China: Lippincott
Williams & Wilkins. 2009. p 131-145.
14. CASTING MATERIALS
DESIRABLE PROPERTIES
PURE ELEMENTS
ALLOYS
COMBINED
Biocompatibility
Ease of melting
Ease of casting
Ease of polishing
Little solidification
shrinkage
Minimal reactivity with the
mold material
Good wear resistance
High strength
Excellent corrosion
resistance
Porcelain Bonding
15. NOBLE METALS
GOLD
Soft and ductile
A yellow “Gold” hue
• Density of 19.3 gms/cm3
• Melting point of 1063°C,
• Boiling point of 2970 °C
• CTE of 14.2×10-6/°C
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
Excellent corrosion resistance
16. NOBLE METALS
PLATINUM
Bluish white metal- tough ductile
& malleable
Increases strength and corrosion
resistance
It helps to reduce the grain size
• Density of 21.45 gms/cm3
• Melting point of 1769°C
• Boiling point of 4530 °C
• CTE 8.9×10-6/°C
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
17. NOBLE METALS
PALLADIUM
Hardens as well as whitens the
alloy
Raises the fusion temperature
temperature & provides tarnish
resistance
Absorbs large quantities of H₂
when heated
• Density of 12.02 gms/cm3
• Melting point of 1552°C
• Boiling point of 3980 °C
• CTE 11.8×10-6/°C
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
18. NOBLE METALS
IRIDIUM & RUTHENIUM
Added in very small quantities
about 100 to 150 ppm
Help to decrease the grain size
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
19. BASE METALS
CHROMIUM,COBALT, NICKEL, IRON, COPPER,
Reactive with environment
Invaluable components of dental casting alloys
Influences on physical properties
Strengthening effect
McCabe J F, Walls A W G. Base metal casting alloys. Applied dental materials. 9th ed. Hong kong; Blackwell Publishing Ltd: 2008. p,71-
79
20. BASE METALS
SILVER
Malleable, ductile white metal
Increases strength and hardness
In large amounts, it reduces
tarnish resistance- combines with
S,Cl₂, P
• Density 10.4gms/cm3
• Melting point of 961°C,
• Boiling point of 2216 °C
• CTE is 19.7 ×10-6/°C
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
21. NOBLE METALS
COBALT
Imparts hardness, strength and
rigidity
• Density of 8.85 gms/cm3
• Melting point of 1495°C
• Boiling point of 2900 °C
• CTE 13.8×10-6/°C
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
22. NOBLE METALS
NICKEL
Decreases
• Strength
• Hardness
• Modulus of elasticity
• Fusion temperature
• Density of 8.9 gms/cm3
• Melting point of 1453°C
• Boiling point of 2730 °C
• CTE 13.3×10-6/°C
Increases ductility
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
23. BASE METALS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
CHROMIUM
Passivating effect – corrosion
resistance
• Density of 7.19 gms/cm3
• Melting point of 1875°C
• Boiling point of 2665 °C
• CTE 6.2×10-6/°C
Increases the melting point &
content not to exceed 30 %
Acts in solid solution hardening &
Precipitation hardening
24. BASE METALS
COPPER
Principal hardener
• Density of 7.19 gms/cm3
• Melting point of 1083°C
• Boiling point of 2595 °C
• CTE 16.5×10-6/°C
Reduces the density of gold
alloys
Reduces resistance to tarnish and
corrosion of gold alloy
maximum content NOT to
exceed 16%.
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
25. BASE METALS
ZINC
Scavenger for oxygen
MELTING SILVER ALLOY
WITHOUT ZINC
O₂
O₂
SOLIDIFICATION OF SILVER ALLOY
WITHOUT ZINC
POROSITIES
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
26. BASE METALS
GALLIUM
Grayish metal
• Density of 5.91 gms/cm3
• Melting point of 29.8°C
• Boiling point of 2204 °C
Oxides are important to bonding
of ceramic to metal
27. NOBLE METALS
GOLD
Soft and ductile
A yellow “Gold” hue
• Density of 19.3 gms/cm3
• Melting point of 1063°C,
• Boiling point of 2970 °C
• CTE of 14.2×10-6/°C
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
Excellent corrosion resistance
28. SOLIDIFICATION & CRYSTALIZATION OF METALS
SOLIDIFICATION
FREEZING POINT OR
SOLIDIFICATION
TEMPERATURE OF PURE
METALFUSION TEMPERATURE
SUPERCOOLING
B’-B – CRYSTALIZATION
BEGINS FOR PURE
METALS
RELEASE OF THE LATENT HEAT OF
FUSION CAUSES THE
TEMPERATURE TO RISE TO TF
LIQUID – HIGH
ENERGY STATE
SOLID – LOW
ENERGY STATE
CRYSTALIZATION
COMPLETE
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
29. SOLIDIFICATION & CRYSTALIZATION OF METALS
SOLIDIFICATION
FREEZING POINT OR
SOLIDIFICATION
TEMPERATURE OF PURE
METALFUSION TEMPERATURE
SUPERCOOLING
B’-B – CRYSTALIZATION
BEGINS FOR PURE
METALS
RELEASE OF THE LATENT HEAT OF
FUSION CAUSES THE
TEMPERATURE TO RISE TO TF
LIQUID – HIGH
ENERGY STATE
SOLID – LOW
ENERGY STATE
CRYSTALIZATION
COMPLETE
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
30. SOLIDIFICATION & CRYSTALIZATION OF ALLOYS
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
NUCLEI
MOLTEN ALLOY
C
O
O
L
I
N
G
Nucleation
Ir
GRAIN REFINERS
High MP
For even Nucleation
C
O
O
L
I
N
G
GRAINS
Grow into
ENLARGE
GRAIN
BOUNDARIES
DENDRITES
31. SOLIDIFICATION & CRYSTALIZATION OF ALLOYS
GRAIN SIZE
COOLING RATE
ALLOY COMPOSITION
PRESENCE OF GRAIN
REFINERS
ALLOY STRENGTH
WORKABILITY
SUCEPTIBILITY TO
CORROSION
Affects
Depends on
Faster cooling rate
Noble metals
Ir, Ru,
SMALL
GRAINED
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
32. SOLIDIFICATION & CRYSTALIZATION OF ALLOYS
SIGNIFICANCE
Typically solidify with a
dendritic micro-structure
Solidify with an Equiaxed
polycrystalline microstructure
Equiaxed – grains uniform in
size & shape
BASE METAL CASTING
ALLOYS
NOBLE METAL CASTING
ALLOYS
Fine grainedLarge grained
SMALLER THE GRAIN SIZE
MORE DUCTILE
AND STRONGER
PRODUCES A
MORE
HOMOGENOUS
CASTING
IMPROVES THE
TARNISH
RESISTANCE
elongated morphology of
dendrites
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-117
33. PHASE DIAGRAMS & DENTAL ALLOYS
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
PHASE A state of matter distinct from matter around it
ICE WATER
ARRANGEMENT OF ATOMS
SINGLE PHASE
Homogenous composition
MULTI-PHASE
Areas with different
composition
BINARY PHASE DIAGRAMS
TERTIARY PHASE DIAGRAMS
34. EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS
Anusavice K J. Equilibrium phases in cast alloys. Philips Science of dental materials. 11th ed. Missouri: Elsevier;
COOLING CURVE EXPERIMENTS PERFORMED ON A SERIES OF
ALLOYS FROM THE A-B SYSTEM
ISOTHERMAL
FREEZING
NO SINGLE FREEZING TEMPERATURE
SOLIDIFY OVER A RANGE
COOLING CURVE EXPERIMENTS
PERFORMED ON A SERIES OF
ALLOYS FROM THE A-B SYSTEM
EQUILIBRIUM PHASE DIAGRAM
LIQUIDUS
TEMPERATURE – FIRST
SOLID FORMS
SOLIDUS
TEMPERATURE – LAST
SOLID FORMS
LIQUIDUS
SOLIDUS
35. EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS
65% PALLADIUM & 35 %SILVER
COMPOSITION (65WT% Pd)
Alloy is in liquid state
DASHED LINE PO
POINT R
Temperature - 1400°c
77%
Pd
POINT S
57%
Pd - Y
71%
Pd - W
POINT T
52%
Pd - T
Anusavice K J. Equilibrium phases in cast alloys. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.119-141
1370
1340
36. EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-4724
SIGNIFICANCE DISTANCE BETWEEN LIQUIDUS & SOLIDUS LINES
SMALL – Ag-Au System
LARGE – Au-Pt System
Varies for Au-Cu Sytem
NARROW LIQUIDUS SOLIDUS RANGE
ALLOY IN LIQUID STATE FOR SHORTER
LESS SUCEPTIBLE TO OXIDATION
&CONTAMINATION
Alloy remain partially
molten for longer period
TEMPERATURE OF LIQUIDUS LINE
Ranges from 962-1064
Ranges from 1064-1772
LIQUIDUS LINE AT LOWER
TEMPERATURE
Heating easier
Fewer side reactions
Shrinkage less problematic
37. ALLOYS
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
TYPES
LIQUID METAL A LIQUID METAL BLIQUID ALLOY ABSOLID ALLOY AB
METALS MIXED IN MOLTEN STATE
OUTCOMES BASED ON SOLUBILITY OF METALS IN EACH OTHER
SOLID SOLUTION - soluble
EUTECTIC – not soluble
INTERMETALLIC COMPOUND – form specific compound
FOLLOWED BY COOLING
38. SOLID SOLUTIONS
GOLD COPPER SYSTEM
Miscible – in any combination – Properties resemble metals forming
alloy
Au atoms occupy some positions of FCC, Cu atoms occupy some other -
random
At a certain composition, Au atoms occupy some positions of FCC, Cu atoms
occupy some other – specific pattern- ordered solutions
HIGHER STRENGTH & HARDNESS
LOWER DUCTILITY
Melting ranges – Lower :smooth transition between two melting points
HIGHER CORROSION RESISTANCE
Presence of atoms of unequal
size – more difficult for atomic
planes to slide
Single phase – more homogenous
Presence of different phases- interact
electrochemically
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
39. SOLID SOLUTIONS
Anusavice K J. Equilibrium phases in cast alloys. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.119-141.
FACTORS DETERMINING EXTENT OF SOLID
SOLUBILITY OF METALS
ATOMIC SIZE SIZE DIFFER BY LESS THAN 15%
VALENCE
METALS OF SAME VALENCE MORE
LIKELY TO FORM SOLID SOULTIONS
CHEMICAL AFFINITY
HIGH DEGREE OF CHEMICAL
AFFINITY – INTERMETALLIC
COMPOUND
CRYSTAL STRUCTURE SAME TYPE OF CRYSTAL STRUCTURE
40. EUTECTIC ALLOYS
SILVER COPPER SYSTEM
Solidus lower than pure
Ag OR Cu779.4
960.5
1083
28.1 % Cu 71.9 % Ag
ABEGD - solidus
AED – liquidus
Meet at mid range
Pure eutectic - melting
point
Eutectic composition
Ag-Cu System important
in high copper dental
amalgam
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
41. INTERMETALLIC COMPOUNDS
SILVER TIN SYSTEM
Two metals react to form a specific compound
Ag₃Sn important intermetallic
compound in dental amalgam
Ag₃Sn Intermetallic compound in Ag-Sn System, seen at 26.8wt% of Sn
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
42. EFFECT OF MECHANICAL TREATMENT
COLD / WORK HARDENING
CAST INGOT SUBJECTED
TO
ROLLING,
SWAGING,
WIRE
DRAWING
MECHANICAL
DEFORMATION
WROUGHT
STRUCTURE
CRYSTALLINE
/DENDRITIC
STRUCTURE
GRAINS
BROKEN,
ENTANGLED,
ELONGATED
FIBROUS
STRUCTURE
SUPERIOR MECHANICAL
PROPERTIES
EMBRITTLE ALLOYS – LESS
DUCTILE
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
43. EFFECT OF HEAT TREATMENT
RECRYSTALLIZATION & GRAIN GROWTH
COLD WORKED
METALS & ALLOYS
HEATING / ANNEALING
FIBROUS STRUCTURE
GRADUALLY LOST
GRAIN/CRYSTALLINE
STRUCTURES
REAPPEARSRECRYSTALLIZATION/
GRAIN GROWTH
D
E
P
E
N
D
S
TEMPERATURE &
DURATION OF
HEATING
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
44. ALLOY STRENGTHENING MECHANISMS
Solid solutions
Precipitation hardening
Grain refiners
Cold working
HEATING ALLOYS
SECOND PHASE
APPEARS IN
BODY OF ALLOY
BLOCKS THE
MOVEMENT OF
DISLOCATIONS
STRENGTH &
HARDNESS
INCREASED
Au-Pt alloy Fe
FePt₃
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
45. MECHANISMS FOR CORROSION RESISTANCE
Polishing metal restorations
Passivation
Increasing noble metal content
Avoiding dissimilar metal
restorations
THIN ADHERENT
OXIDE
PREVENTS
DIFFUSION OF O₂
Fe - C Cr
Stainless steel
Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
46. GOLD ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
ADA SPECIFICATION No. 5
High Gold Alloys - >70% Type I
Type II
Type III
Type IV
Low Gold Alloys - >10wt%, < 45-50%
White Gold Alloys- Silver palladium alloys
INLAY GOLD ALLOY
CROWN & BRIDGE
ALLOY
47. CASTING GOLD ALLOYS
TYPE I - SOFT
HIGHLY DUCTILE
BURNISHED EASILY
Areas of low occlusal stress designed for simple
inlays such as used in class I, III & V cavities
PROPERTIES
Hardness VHN (50 – 90)
Tensile Strength 276 MPa
Yield Strength 180 MPa
Linear Casting Shrinkage 1.56%
Elongation or ductility 18- 46%
COMPOSITION
Au Ag Cu Pt Pd Zn&Ga
83% 10% 6% - 0.5% balance
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-4724
48. CASTING GOLD ALLOYS
TYPE II- MEDIUM
DUCTILE
HIGHER YEILD
STRENGTH
Inlay or onlay restorations subject to moderate
stress, thick three quarter crowns, pontics and
full crowns
PROPERTIES
Hardness VHN (90-120)
Tensile Strength 345 MPa
Yield Strength 300 MPa
Linear Casting Shrinkage 1.37%
Elongation or ductility 10 - 40.5%
COMPOSITION
Au Ag Cu Pt Pd Zn&Ga
77% 14% 7% - 1% balance
HARDER
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-4724
49. CASTING GOLD ALLOYS
TYPE III- HARD
NOT DUCTILE
AGE HARDENED
Inlays subject to high stress and for crown and
bridge
PROPERTIES
Hardness VHN (120-150)
Tensile Strength 360 MPa
Yield Strength 331MPa
Linear Casting Shrinkage 1.42 %
Elongation or ductility 5- 39.4 %
COMPOSITION
Au Ag Cu Pt Pd Zn&Ga
79% 11% 5% - 3.5% balance
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-4724
50. CASTING GOLD ALLOYS
TYPE IV- EXTRA HARD
NOT DUCTILE
AGE HARDENED
Very high stress, crowns and long span
bridges
PROPERTIES
Hardness VHN (150-200)
Tensile Strength 462 MPa
Yield Strength 703 MPa
Linear Casting Shrinkage 2.03 %
Elongation or ductility 3 - 17 %
COMPOSITION
Au Ag Cu Pt Pd Zn&Ga
56% 25% 14% - 4% balance
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-4724
51. CASTING GOLD ALLOYS
HEAT TREATMENTS
SOFTENING HEAT
TREATMENT
HEATING FOR 10 MINS AT 700 C THEN
QUENCHING
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-4724
AGE HARDENING
HEAT TREATMENT
HEATING CASTING TO ABOVE 450°C AND THEN
ALLOWING IT TO COOL SLOWLY UNTIL TEMP
DROPPED TO 200ᵒC FOLLOWED BY
QUENCHING
52. METAL CERAMIC ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
REQUIREMENTS
They should be able to bond with porcelain.
Coefficient of thermal expansion
Its melting temperature should be higher than the
porcelain firing temperature. It should be able to resist
creep or sag.
It should not stain or discolor porcelain.
53. METAL CERAMIC ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Oldest metal ceramic alloys
Composition:
Gold – 75% to 88%
Palladium – Upto 11%
Platinum – Upto 8%
Silver – 5%
Trace elements like Indium,
Iron and Tin
for porcelain bonding
Gold-Platinum -Palladium
Biocompatibility
Ease of melting
Ease of casting
Ease of polishing
Sag reistance
COTE
Good wear resistance
Hardness
Resistance to discolouration
Porcelain Bonding
Economical
54. METAL CERAMIC ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Composition:
Gold – 39% to 53%
Silver – 12% to 22%
Palladium – 25% to 35%
trace amount of oxidizable
elements are added for porcelain
bonding
Gold-Palladium-Silver Biocompatibility
Ease of melting
Ease of casting
Ease of polishing
Sag resistance
COTE
Good wear resistance
Hardness
Resistance to discoloration
Porcelain Bonding
Economical
55. METAL CERAMIC ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Composition:
Palladium – 55% to 60%
Silver – 25% to 30%
Indium and Tin
Palladium – 50% to 55%
Silver – 35% to 40%
Tin (Little or no Indium
trace amount of oxidizable
elements are added for porcelain
bonding
Palladium-Silver Biocompatibility
Ease of melting
Ease of casting
Ease of polishing
Sag resistance
Thermal compatibility
Good wear resistance
Hardness
Resistance to discoloration
Porcelain Bonding
Economical
56. METAL CERAMIC ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Composition:
Palladium – 78% to 88%
Cobalt – 4% to 10%
trace amount of oxidizable
elements are added for porcelain
bonding
Palladium-Cobalt alloy Biocompatibility
Ease of melting
Ease of casting
Ease of polishing
Sag resistance
Thermal compatibility
Good wear resistance
Hardness
Resistance to discoloration
Porcelain Bonding
Economical
58. CONTENTS
• Base metal alloys
• Titanium alloys
• Comparison between noble metal & base metal alloys
• References
59. NOBLE METALS
Craig R G, Powers J M. Noble dental alloys and solders. Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472
BASE METALS
60. SOLIDIFICATION & CRYSTALIZATION OF METALS
SOLIDIFICATION
Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
61. Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS
62. Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
TYPES OF ALLOYS
SOLID SOLUTIONS
EUTECTIC
INTERMETALLIC
COMPOUNDS
63. Anusavice K J. Solidification and Microstructure of metals. Philips Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
TYPES OF ALLOYS
GOLD ALLOYS
METAL CERAMIC
ALLOYS
BASE METAL
ALLOYS
64. BASE METAL ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Nickel-chromium
Be FREE ALLOYS
Be CONTAINING
ALLOYS
Composition:
Nickel – 62% to 77%
Chromium – 11% to 22%
Boron ,iron, molybdenum,
Niobium or columbium and
tantalum (trace elements).
Composition:
Nickel – 62% to 82%
Chromium – 11% to 20%
Beryllium – 2.0%
aluminum, carbon, gallium,
iron, manganese,
molybdenum, silicon, titanium
and /or vanadium are
present.
ECONOMICAL
ALLERGIC
POTENTIAL
CAN BE
ETCHED
CASTING IS
EASIER
65. BASE METAL ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Composition:
Cobalt - 55 to 65%
Chromium - 23 to 30%
Nickel - 0 to 20%
Molybdenum - 0 to 7%
Iron - 0 to 5%
Carbon - upto 0.4%
Tungsten, Manganese, Silicon and
Platinum in traces.
COBALT CHROMIUM ALLOY
HIGH STRENGTH
EXCELLENT CORROSION
RESISTANCE
POOR BURNISHABILITY
66. BASE METAL ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
67. TITANIUM ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Properties of titanium:
• Relatively light weight
• Low density (4.5 g/cm3)
• High strength (yield strength = 170-480 MPa;
• ultimate strength = 240-550 MPa)
• Passivity
• Low coefficient of thermal expansion (8.5 x 10–6/°C)
• Melting & boiling point of 1668°C & 3260°C
“MATERIAL OF CHOICE” IN
DENTISTRY
OXIDE FORMATION
PROPERTY
BIOCOMPATIBILITY
68. TITANIUM ALLOYS
Smith E A. A Manual on dental metallurgy and non-metallic Materials 6th Ed. London:J & A Churchill Ltd;1947.p3-4
Difficulties in casting Titanium :
-High melting point
-High reactivity > 600°C – H, O, N
-Difficulty in finishing
-Difficulty in welding
-Requires expensive equipments
LOW DENSITY
DIFFICULT TO CAST IN
CENTRIFUGAL CASTING
COMBINING
CENTRIFUGAL,
VACUUM, PRESSURE,
GRAVITY CASTING WITH
ELECTRIC ARC MELTING
TECHNOLOGY
69. Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
DENSITY
MODULUS OF ELASTICITY ELONGATION
CASTING ENVIRONMENT
TARNISH & CORROSION
ULTIMATE STRENGTH
RANGE OF MELTING
TEMPERATURE
FINISHING & POLISHING
CASTABILITY-MOLDABILITY
SOLDERING
BIOCOMPATIBILITY
PORCELAIN BONDING
CASTING SGRINKAGE
70. DENSITY
HIGHEST DENSITY
LESS FORCE IN
CENTRIFUGAL CASTING
DENSITYALMOST HALF
OF HIGH NOBLE
LOWER DENSITY
ALLOWS MORE
RESTORATIONS PER
UNIT WEIGHT & LIGHTER
ACCELERATES FASTER –
FORMS COMPLETE
CASTING MORE EASILY
VERY THIN CROSS
SECTION DETAILS TO BE
AVOIDED
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
71. MELTING
RANGE
LOWEST MELTING
RANGE
CAST USING LOW HEAT
TECHNIQUE
HIGHEST MELTING
RANGE
GAS AIR FUEL
Ca SO₄ DIHYDRATE
BONDED INVESTMENTS
PHOSPHATE & SILICATE
BONDED INVESTMENTS
HIGH HEAT TECHNIQUE
ELECTRIC
RESISTANCE/INDUCTION
MELTING
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
73. ELONGATION
BURNISHABLE
EASIER TO ADJUST
MARGINS IN CASE OF
DISCREPANCY
NOT BURNISHABLE
MARGINS TO BE
PLACED IN REACHABLE
AND REPRODUCABLE
AREAS,
CLASP ADJUSTMENT
DIFFICULT
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
74. MODULUS OF
ELASTICITY
2-3 TIMES LESS THAN
BASE METAL ALLOYS
ABRASION RESISTANCE
LESS IN COMPARISON
HIGH MODULUS OF
ELASTICITY
GOOD ABRASION
RESISTANCE
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
75. PORCELAIN
BONDING
COMMONLY USED FOR
FABRICATION OF
METAL CERAMIC
RESTORATIONS
MOSTLY Ni Cr USED
FOR METAL CERAMIC
McCabe J F, Walls A W G. Gold and Alloys of Noble metals, Applied dental materials. 9th ed. Hong kong; Blackwell Publishing Ltd: 2008. p62-70,
McCabe J F, Walls A W G. Base metal casting alloys. Applied dental materials. 9th ed. Hong kong; Blackwell Publishing Ltd: 2008. p71-79
76. CASTING
ENVIRONMENT
LESS SENSITIVE TO
CASTING
ENVIRONMENT
ALLOY LESS SUCEPTIBLE
TO CONTAMINANTS
MORE CONTROLLED &
DEVOID OF
CONTAMINANTS
PALLADIUM – HIGH
AFFINITY FOR OXYGEN
– POROSITY
CONTAMINANTS ( C,O₂)
ADVERSELY CHANGE
PROPERTIES OF ALLOY
SILVER HAS HIGH
AFFINITY FOR OXYGEN
– POROSITY &
ROUGHNESS
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
78. FINISHING &
POLISHING
EASILY ACCOMPLISHED
LESS COMPLICATED
ARMAMENTARIUM
MORE TIME TO ATTAIN
BIOLOGIC SURFACES
REQUIRE HIGH SPEED
EQUIPMENT
MORE ABRASIVE TOOLS
ELECTROLYTIC POLISHING
REQUIRED FOR Co Cr
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
79. SOLDERING
FAIRLY PREDICTABLE &
EASY
GOLD SOLDERS FOR
HIGH NOBLE. OR
SILVER SOLDERS
TECHNIQUE SENSITIVE
REQUIRE INERT
ENVIRONMENT
SPECIFIC SOLDER FOR
EACH ALLOY
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
81. BIOCOMPATIBILITY
GENERALLY REGARDED
AS HAVING GOOD
BIOCOMPATIBILITY
PALLADIUM ALLERGY
CONTAIN
COMPONENTS
REGARDED AS TOXIC
OR KNOWN
ALLERGENS
Ni – KNOWN ALLERGEN
Be THOUGHT TO BE
CARCINOGENIC
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323.
82. CONCLUSION
NOBLE METAL ALLOYS SUPERIOR
TO BASE METAL ALLOYS
COST IMPORTANT FACTOR
Marzouk MA. Cast restorations. 1st ed. India: All India publishers and distributers; 2001p.307-323
83. REFERENCES
• Anusavice K J. Equilibrium phases in cast alloys. Philips Science of
dental materials. 11th ed. Missouri: Elsevier; 2003p.119-141.
• Anusavice K J. Solidification and Microstructure of metals. Philips
Science of dental materials. 11th ed. Missouri: Elsevier; 2003p.103-
117.
• Smith E A. A Manual on dental metallurgy and non-metallic Materials
6th Ed. London:J & A Churchill Ltd;1947.p3-4.
• Craig R G, Powers J M. Nature of Metals and Alloys. Restorative Dental
Materials. 11th Ed. Missouri: Mosby; 2002. p164-180.
• Craig R G, Powers J M. Noble dental alloys and solders. Restorative
Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-472.
• Craig R G, Powers J M. Cast and wrought base metal alloys.
Restorative Dental Materials. 11th Ed. Missouri: Mosby; 2002. p449-
472.
84. REFERENCES
• Marzouk MA. Cast restorations. 1st ed. India: All India
publishers and distributers; 2001p.307-323.
• Marzouk MA. The biologic form and choice of materials for cast
restorations. 1st ed. India: All India publishers and distributers;
2001p.341-344.
• Darvell BW. Metals I, Metal II, Casting alloys. 9th ed. New Delhi:
Woodhead publishing INDIA Private limited; 2009. p 280-305,
306- 322, 435-449.
• Gladwin M, Bagby M. Materials for fixed indirect Restorations
and Prostheses. Clinical aspects of dental materials. China:
Lippincott Williams & Wilkins. 2009. p 131-145.
• Mosby’s dental dictionary. Definition of metallurgy. 2nd Ed.
Missouri:elsevier; 2004. p390.
WHAT EXACTLY IS METALLURGY…??? VERY OFTEN IT IS THOUGHT OF AS THE SCIENCE THAT DEALS WITH THE EXTRACTION OF METALS FROM THEIR ORES… BUT IS SEEING THINGS IN A VERY NARROW SENSE… IN FACT PUT VERY SIMPLY METALLURGY CAN BE SAID TO BE THE….. IT IS THE SCIENCE THAT EXPLAINS THE.. & IT ALSO DESCIRBES THE EFFECTS OF BOTH
In dentistry, metals represent one of the three major classes of materials used for REHABILITATION OF MISSING STRUCTURES,WHICHIS WHY METALLURGY GAINS A LOT IMPORTANCE , NOW THESE METALS & ALLOYS ARE CAST… FOR TWO DIFFERENT PURPOSES..IN THE FIRST INSTANVE THEY MAY SERVE AS THE FINAL STRUCTURE OR SECONDLY IT MAY BE FURTHER MANIPULATED TO FORM WIRES, BARS, SHEETS, IN WHICH CASE THEY ARE CALLED WROUGHT ALLOYS
DIVERSE METALS & ALLOYS ARE USED IN DENTISTRY… EACH OF WHICH MAY HAVE A UNIQUE & CHARACHTERISTIC BEHAVIOUR.. THIS BEHAVIOUR OF THE ALLOY IS HEAVILY DEPENDENT ON THE VERY COMPOSITION OF THE ALLOY… THE COMPISITION OF THE ALLOY MUST THUS BE TAKEN INTO CONSIDERATION DEPENDING ON THE CLINICAL USE & THE ENVIRON MENT IN WHICH IT IS USED FOR EGS…
Specific gravity, ringing sound when struck, high fracture tougness.. They are stronger & denser than most non metallic elements
Previously mentioned propertie .. of metals are in fact closely related to their valence electron configuration THE OUTERMOST E- s ..around the neutral atoms are loosely bound
The use of pure metals is limited in dentistry.. This is because pur metals tends to be soft or may corrode easily…. To optimize properties most metals used in dentistry are in fact alloys
BOTH THE HIGH MP EN COTE AT MAKES IT COMPATIBLE TO BE USEDWITH PORCELAIN
IN THEPRESENCE OF ZINC.. IT PREFERENTIALLY REACTS WITH THE OXYGEN FORMS A WHITE OXIDE OF LOW DENSITY
It is important to note that pure metalls have a specific bp mp. Let us now consider the solidification phenpomena that occur duruing thhe freezing of a pure metal… the graph on screen shows the temperature plotted as a function of time when a pure metal is melted end cooled to room temperature.
Note the temp decreases steadily from point a to pnt b..increase in temp then occurs from pt b b at b the temp remains constant until time indic by c isreached.. After this temp decreases steadily to room temp
It is important to note that pure metalls have a specific bp mp. Let us now consider the solidification phenpomena that occur duruing thhe freezing of a pure metal… the graph on screen shows the temperature plotted as a function of time when a pure metal is melted end cooled to room temperature.
Note the temp decreases steadily from point a to pnt b..increase in temp then occurs from pt b b at b the temp remains constant until time indic by c isreached.. After this temp decreases steadily to room temp
BYADDITION OF SMALL AMTS OF ELEMENTS SUCH AS BY VIRTUE OF HAVING A HIGH MP THEY REMAIN UNMOLTEN AND SERVE AS NUCLEATING CENTRES THROUGHOUT THE ALLOY
In a mixture of ice and water there are two phases because although ice en wat are same chemically thy each have distinct arrangement of atoms… ice has crystalline arrang whereas water has random atomic arrangemt… similarly aa solid dental alloy may also hav one phase if the composition of the alloy is essentially homogenous… if the alloys has areas where compositions are different it is called multiphase alloy… this distinction is imp wrt strength corrosion en other properties
Consider the graph on screen .. Temp plotted as a function of time… obtained when cooling curve expt are carried out on a series of alloys from the ab system… CURVES 1 & 6 ARE FOR PURE METALS SHOWS USUAL SITUATION OF ISOTHERMAL FREEZING BUT WITHOUT SUPERCOOLING.. CURVES 2TO 5 SHOW THAT SOLID SOLUTION ALLOYS DO NOT HAV A SINGLE FREEZING TEMP BUT SOLIDIFY OVER A TEMP RANGE .. THE REGION LABELLED L=S IS A TWO PHASE REGION COMPOSEDOF SOLID AND LIQUID….THESE COOLING CURVES CAN NOW BE USED TO DETERMINE ….. EQUILIBRIUM PHASE DIAGRAM… THE TEMP AT WHICH THE FIRST SOLID FORMS IS CALLED THE LIQUIDUS EN TEMP AT WHICH LAST SOLID FORMS IS CALLED SOLIDUS….WHEN THESE POINTS ARE CONNECTED WITH SMOOTH CURVES THE EQUILIB PHASE DIAG RESULTS… THE UPPER CURVE IS CALLEED LIQUIDUS EN LOWER CURVE CALLED SOLIDUS… SINCE THE ALLOYS ARE ENTIRELY SOLID FOR TEMP EN COMPOS BELOW THIS CURE
NOW CONSIDER COOLING OF 65 EN 35 AS INDIC BY LINE PO PERPEND TO BASE LINE… IF POINT ON PO CORRESPONDS TO THE TEMP OF 1500 IS CONSIDERED THE ALLOY ISCLEARLY IN LIQUIDSTATE… WWHEN TEMP DECREASES TO APPROX 1400 THE VERTICAL LINE PO TOUCHES THE LIQUIDUS CURVE AT POINT REN THE FIRST SOLID FORMS… HOWEVER COMPOSITION IS DIFFEENT FROM PARENT METAL 65 PD EN 35 SILVERTO DETERMINETHIS COMPOA LINE RM ISDRAWN PARALELL TO BASELINE UNTIL IT INTERSECTS SOLIDUS CURVE AT POINT M.NOW THOS POINT OF INTTERSECTION IS PROJECTED TO THE BASELINE TO DETERMINE THE COMPOSITION OF FIRST SOLID FORMED WHICH IS EQUAL TO..WHEN TEMP DECREASES TO POINT S.. COMPOS IS DETERMINED BY DRAWING LINE YW LOCATING ITS POINT OF INTERSECTION WITH SOLIDUS EN LIQUIDUS
IN THIS PHASE DIAGRAM WHAT STANDS OUT IS THAT THE LIQUIDUS EN SLIDUS MEET AT MID RANGE COMPOSITION.. THE SOLIDUS IS LOWER AT 779.4 THAN EITHER PURE AG OR CU… THIS LIQUID SOLIDUS CONFIGURATION IS CHARACTERISTIC OF A EUTECTIC ALLOY SYSTEM…..
Pd > 25%
HEATING FOR 10 MINS AT 700 C THEN COOLING
intermediate phases are changed to an ordered solid solution
It is important to note that pure metalls have a specific bp mp. Let us now consider the solidification phenpomena that occur duruing thhe freezing of a pure metal… the graph on screen shows the temperature plotted as a function of time when a pure metal is melted end cooled to room temperature.
Note the temp decreases steadily from point a to pnt b..increase in temp then occurs from pt b b at b the temp remains constant until time indic by c isreached.. After this temp decreases steadily to room temp
It is important to note that pure metalls have a specific bp mp. Let us now consider the solidification phenpomena that occur duruing thhe freezing of a pure metal… the graph on screen shows the temperature plotted as a function of time when a pure metal is melted end cooled to room temperature.
Note the temp decreases steadily from point a to pnt b..increase in temp then occurs from pt b b at b the temp remains constant until time indic by c isreached.. After this temp decreases steadily to room temp
It is important to note that pure metalls have a specific bp mp. Let us now consider the solidification phenpomena that occur duruing thhe freezing of a pure metal… the graph on screen shows the temperature plotted as a function of time when a pure metal is melted end cooled to room temperature.
Note the temp decreases steadily from point a to pnt b..increase in temp then occurs from pt b b at b the temp remains constant until time indic by c isreached.. After this temp decreases steadily to room temp
It is important to note that pure metalls have a specific bp mp. Let us now consider the solidification phenpomena that occur duruing thhe freezing of a pure metal… the graph on screen shows the temperature plotted as a function of time when a pure metal is melted end cooled to room temperature.
Note the temp decreases steadily from point a to pnt b..increase in temp then occurs from pt b b at b the temp remains constant until time indic by c isreached.. After this temp decreases steadily to room temp
SECONDARY FLARES, DELICATE INTERNAL BOXES, VERY SMALL CAST PINS
Noble metal alloys have been known to fail ,,, but even in case of failure it is not catastrophic
IS A MEASURE OF DUCTILITY, HOWEVER FOR CROWN & BRIDGE APPLICATIONS . A LOW VALUE OF ELONGATION IS NOT MUCH OF A CONCERN CAUSE PERMANENT DEFORMATION OF ALLOY IS NOT DESIRABLE
ADDITIONAL REQUIREMENTS – THERMAL STABILITY
BONDING TO CERAMIC
COMPATIBILTY TO CERAMIC
SUPPORT FOR CERAMIC
THERE IS PERSISTENT RISK OF SOLDER FAILURE EN CHANGE IN COMPOSITION OF PARENT METAL
MORE THAN 1 % NI.. 0.02% BE…. TI EN ALLOYS OF THIS METAL INCLUDING VANADIUM ARE KNOWN TO HAVE FAVOURABLE BIOCOMPATI… EN ARE LIKELY TO BECOME WIDELY USED FOR APPLICATIONS IN FUTURE..AT PRESENT RESTRICTED TO IMPLANTSTHERE IS PERSISTENT RISK OF SOLDER FAILURE EN CHANGE IN COMPOSITION OF PARENT METAL
Several years ago the choice of an alloy was simple.as therewas No other choice than four types of gold alloy. At present these choices are difficult to make DUE TO VARIOUS ALLOYS AVAILABLE…it is thus a dentists duty to understant the properties of the cast materials used and to prepare the tooth and manipulate the materials to get the.best out of them. It is our professional duty too inform patients of the advantages as well as the possible drawbacks of the materials that may be used. without doubt gold alloys are sup to.. however ultimately it all boils down to patient affordability .