Alloys in fpd /dental education courses

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Alloys used in fixed
prosthodontics
ALLOYS USED IN
FIXED PROSTHODONTICS
INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

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Metal
• A crystalline material that consists of positively
charged ions in an ordered, closely packed
arrangement and bonded with a cloud of electrons.
This type of bond, called a metallic bond, is
responsible for many of the properties of metals-
electrical and thermal conductivity, metallic luster,
and (usually) high strength

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Characteristic Properties Of Metals
• Hard
• Lustrous
• Dense
• Good conductors of heat and electricity
• Opaque
• Malleable and ductile
• give electro positive ions in solution

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Occurrence
• Metals occur either as pure elements or in
compounds with other elements .
Example; Gold (Au)
Silver (Ag) pure element
Copper Obtained as Cu2S, CuS
Iron Obtained as Fe2O3 compounds

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Classification Of Metals
• Pure Metal or Mixture of Metals –
Alloys
• Base Metal or Noble Metal
• Cast metal or wrought metal

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Another Classification Of Metals
• Light Metal – e.g., Al.
• Heavy Metal – e.g., Fe.
• High Melting Metal – e.g., Co, Cr.
• Low Melting Metal – e.g., Sn.
• High Ductile and Malleable metal –
e.g., Au.

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Microscopic Structure Of Metals
• Most metals have crystalline structure in
solid state which are held together by
metallic bonds.
• Metals also exist in liquid state eg: Hg, in
which crystalline alignment is lost and the
atoms move freely in mass of liquid metal.

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Solidification Of Pure Metal
• Pure metal has a melting point-known as
Fusion Temperature, and has specific heat.
• To melt a crystalline substance (metal)
some what more heat energy is required to
convert it from solid to liquid.
• This extra heat is stored away within the
atoms in the form of latent heat of fusion.

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Mechanism
• When the solid metal changes into liquid, its
crystalline pattern disappears, and the atoms are
randomly distributed in the mass of liquid and
they have more energy and are therefore move
about freely.
• In the reverse process of changing into solid,
temperature of the melt goes gradually (cooling);
atoms make an attempt to reform the crystalline
arrangement.

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Mechanism of Crystallization
• A pure metal may crystallize in a tree-branch
pattern to form what is called a NUCLEUS
• The initial nuclei are small in size and few in
number known as EMBRYO, which do not
stabilize in the melt and soon disappear.
• As the temperature of the metal gradually goes
down, a stable NUCLEUS is formed.

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• Such nucleus formations are called
DENDRITES.
• The metal is therefore made of
thousands of tiny crystals, such a
metal is called polycrystalline and
each crystal in the structure is called
a GRAIN.

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GRAIN SIZE
•The grain size can be altered by heating.
• When the metal is heated and rapidly quenched, small grains are
formed
• when they are allowed to cool slowly, large grains are formed
• The more fine the grain structure, the more uniform and better
are the properties.

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Crystal Space Lattice
• The formed crystals in a metal
are arranged in a orderly pattern
– layer by layer in regular
stacks.
• The crystals of a metal is in the
form of a space lattice.
• The type of space lattice varies
from metal to metal.

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Crystal Space Lattice

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Alloys
• Combination of two or more metals which are
generally mutually soluble in the liquid condition.
• A metallic material formed by the intimate
blending of 2 or more metals some times a non-
metal be added.
• A substance composed of 2 or more elements at
least one of which is a metal.

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Methods of alloying
• Melting
– together the base metal (main) and the alloying
element, mixing them thoroughly, and allowing
the mixture to cool and solidify. This is a
common method.
• Sintering or by powder metallurgy
– Metals are powdered, mixed and pressed to the
desired shape and then heated but not melted
till the powders unite to form a solid mass.

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Objectives of alloying
1. To increase hardness and strength.
2. To lower the melting point.
3. To increase fluidity of liquid metal.
4. To increase resistance to tarnish and corrosion.
5. To make casting or working on the metal easy.
6. To change the microscopic structure of the
metal.
7. To change the color of the metal.
8. To provide special electrical and magnetic
properties.

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CLASSIFICATION OF ALLOYS
1. Acc to Uses
» All metal inlays
» Crown and bridges
» Metal ceramic restorations
» Removable partial dentures
2. Major element present….
» Ferrous alloys-iron
» Gold and silver alloys
» Babbit metals-tin and lead
» Nickel alloys

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3. Nobility (ADA 1984)…..
 High noble
 Noble
 Base metal
 Au-Pd-Ag
 Pd-Ag-Sn
 Co-Cr-Mo
 Ti-Al-V
4. Principle three elements….

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5. Based on Yield strength & elongation…
 Soft
 Medium
 Hard
 Extra hard
Isomorpous
Eutectic
Peritectic
Intermetallic compound
6. Based on dominant phase….

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7. Based on method of fabrication….
 Cast metal
 Wrought metal
8.Based on the number of metals….
 Binary
 Ternary
 Quaternary

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SOLID SOLUTIONS OR ISOMORPHOUS
STATE OR SINGLE PHASE
• Solid solution is nothing but solution in the solid
state.
• It consists of a solute and a solvent.
• Solvent is that metal whose space lattice persists
and solute is the other metal.
E.g. Au – Ag
Au – Cu
Au – Pt
Au – Pd
Ag – Pd

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The solid solution can be either :
1.SUBSTITUTIONAL SOLID SOLUTION
– Regular or Ordered
– Random or Disordered

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2.INTERSTITIAL SOLID SOLUTION

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Conditions Favoring Solid-Solubility
• Atom size - if the atom sizes of the mixing metal are
same, it will produce solid solution type of alloy.
• Valency - metals of the same valency will produce solid-
solution alloy.
• Crystal structure- Only metals with the same type of
crystal lattice can form a series of solid solutions.
• Chemical affinity - must be less to produce solid-
solution alloy.

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A B
metal solution
Freezing
temp
Freezing
range
T
E
M
P
E
R
A
T
U
R
E
Time in minutes
Cooling curve of a solid solution

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PHASE DIAGRAM OF A SOLID
SOLUTION ALLOY

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PROPERTIES OF A SOLID
SOLUTION ALLOY
The solid solution possesses:
• Increased hardness
• Increased strength
• Increased proportional limit
• Decreased ductility
• Decreased resistance to corrosion due to coring
• Melting range rather than a point

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EUTECTIC ALLOYS
• The eutectic alloy is one in which the components
exhibit complete solubility in the liquid state but
limited solid solubility
• The term eutectic means lowest melting point.
• In silver copper system the temperature of silver
is around 960.5°C and that of copper is 1083° C.
But that of the eutectic composition is 779.4° C.

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• a mixture of salt and ice
• These in contrast to other alloys do not have a
solidification range ; instead they have a
solidification point.
• It can be written as :
LIQUID = α SOLID SOLUTION + ß
SOLID SOLUTION

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PHASE DIAGRAM OF A
EUTECTIC ALLOY

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PROPERTIES OF EUTECTIC ALLOYS
• 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.

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PERITECTIC ALLOYS
• Peritectic is a phase where there is limited
solid solubility.
• They are not of much use in dentistry
except for silver tin system.
• This type of reaction occurs when there is a
big differences in the melting points of the
components.

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Phase diagram of peritectic alloy

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Inter metallic Compounds
• Inter metallic compounds are those when the
metals are soluble in the liquid state but unite and
form a chemical compound on solidifying.
• E.g.
– Ag3 – Sn,
– Sn7 – Hg8
• They are called inter metallic compounds because
the alloy is formed by a chemical reaction between
a metal and metal.

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Heat Treatment
• Heat treatment (not melting) of metals in the solid
state is called SOLID STATE REACTIONS.
• This is a method to cause diffusion of atoms of the
alloy by heating a solid metal to a certain
temperature and for certain period of time.
• This will result in the changes in the microscopic
structure and physical properties.

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• Important criteria in this process are:
1.Composition of alloy
2.Temperature to which it is heated
3.Time of heating
4.Method of cooling - cooling slowly
in the air or quenching rapidly in
cold water.

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Purpose of Heat Treatment
• Shaping and working on the appliance in the
laboratory is made easy when the alloy is soft.
This is the first stage and is 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.

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Types of Heat Treatment
 Softening Heat treatment
 Hardening Heat treatment
 Solution Heat treatment
 Age Hardening

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Softening heat treatment
• Also known as ANNEALING. This is done for
structures which are cold worked.
• 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 pro-portional limit but increase in ductility. In
other words the metal becomes soft. This is also
known as HOMOGENIZATION TREATMENT.

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Hardening heat treatment
• 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.

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• The result of this is increase in strength,
hardness and proportional limit but
reduction in ductility.
• Also known as ORDER HARDENING or
PRECIPITATION HARDENING.

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Solution Heat Treatment Or
Solution-Hardening
• When the alloy is heat soaked, any
precipitations formed during earlier heat
treatment, will now once again become
soluble in the solvent metal.

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Age Hardening
• 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.

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Different Metals Used…..
Gold (Au)
• provides a high level of corrosion
and tarnish resistance
• increases an alloy's melting range
slightly.
• improves workability, burnish
ability, and raises the density .
• However, gold imparts a very
pleasing yellow color to an alloy
(if present in sufficient quantity).

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Palladium
• increase the strength, hardness (with copper),
corrosion and tarnish resistance of gold-based alloys.
• will elevate an alloy's melting range and improve its
sag resistance.
• has a very strong whitening effect
• possesses a high affinity for hydrogen, oxygen, and
carbon.
• lowers the density of the gold-based alloys slightly but
has little similar effect on silver-based metals.

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Platinum
• Platinum increases the strength, melting
range, and hardness of gold-based alloys
while improving their corrosion, tarnish,
and sag resistance.
• It whitens an alloy and increases the
density of non gold-based metals because
of its high density.

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Iridium
• serves as a grain refiner for gold- and
palladium-based alloys
• improve the mechanical properties as well
as the tarnish resistance.

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Ruthenium
• Ruthenium acts as a grain refiner for gold-
and palladium- based alloys
• Improve their mechanical properties and
tarnish resistance (like iridium).

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Silver
• lowers the melting range, improves fluidity,
and helps to control the coefficient of thermal
expansion in gold- and palladium-based
alloys
• Silver-containing porcelain alloys have been
known to induce discoloration (yellow,
brown, or green) with some porcelains.

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• Silver possesses a high affinity for oxygen
absorption, which can lead to casting porosity
• However, small amounts of zinc or indium added
to gold and silver-based alloys help to control
silver's absorption of oxygen.
• Silver will also corrode and tarnish in the presence
of sulfur. Although silver is a precious element, it
is not universally regarded as noble in the oral
cavity .

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Aluminium
• lowers the melting range of nickel-based alloys.
• Act as a hardening agent and influences oxide
formation.
• With the cobalt - chromium alloys used for metal
ceramic restorations, aluminum is one of the
elements that is "etched" from the alloy's surface
to create micromechanical reten-tion for resin-
bonded retainers (Maryland Bridges).

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Beryllium
• Like aluminum, beryllium lowers the melting range
of nickel-based alloys, improves castability,
improves polishability, is a hardener, and helps to
control oxide formation.
• The etching of nickel-chromium-beryllium alloys
removes a Ni-Be phase to create the micro retention
so important to the etched metal resin-bonded
retainer.
• Questions have been raised as to potential health
risks to both technicians and patients associated with
beryllium-containing alloys .

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Boron
• Boron is a deoxidizer.
• For nickel-based alloys, it is a hardening agent
and an element that reduces the surface tension
of the molten alloy to improve castability.
• The nickel-chromium beryllium-free alloys that
contain boron will pool on melting, as opposed
to the Ni-Cr-Be alloys that do not pool.
• Boron reduces ductility and increases hardness.

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Chromium
• Chromium is a solid solution
hardening agent that contributes to
corrosion resistance by its passivating
nature in nickel- and cobalt-based
alloys.

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Cobalt
• Cobalt is an alternative to the nickel-
based alloys, but the cobalt-based
metals are more difficult to process.
• Cobalt is included in some high-
palladium alloys to increase the alloy's
coefficient of thermal expansion and to
act as a strengthener

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Copper
• Copper serves as a hardening and
strengthening agent, can lower the melting
range of an alloy, and interacts with platinum,
palladium, silver, and gold to provide a heat-
treating capability in gold-, silver-, and
palladium-based alloys.
• Copper helps to form an oxide for porcelain
bonding, lowers the density slightly, and can
enhance passivity in the high palladium-copper
alloys.

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Gallium
• Gallium is added to silver-free porcelain
alloys to compensate for the decreased
coefficient of thermal expansion created
by the removal of silver. (Concerns over
silver's potential to discolor dental
porcelain have greatly limited its use in
systems other than palladium-silver )

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Indium
• It is a less volatile oxide-scavenging agent (to
protect molten alloy)
• lowers the alloy's melting range and density
• added to non gold-based alloys to form an
oxide layer for porcelain bonding.
• enhance tarnish resistance in high silver
containing alloys.

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Iron
• Iron is added to some gold-based
porcelain systems for hardening and
oxide production.
• Iron is included in a few base metal
alloys as well.

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Manganese
• Manganese is an oxide scavenger and
a hardening agent in nickel- and
cobalt-based alloys.

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Molybdenum
• Molybdenum improves corrosion
resistance
• influences oxide production
• is helpful in adjusting the coefficient
of thermal expansion of nickel-based
alloys.

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Nickel
• Nickel has been selected as a base for porcelain
alloys because its coefficient of thermal expansion
approximates that of gold and it provides resistance
to corrosion.
• Unfortunately, nickel is a sensitizer and a known
carcinogen.
• Estimates of nickel sensitivity among women in
the United States range from 9% to 31.9% and
from 0.8% to 20.7% among men .

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Tin
• Tin is a hardening agent that acts to lower
the melting range of an alloy.
• It also assists in oxide production for
porcelain bonding in gold and palladium-
based alloys.
• Tin is one of the key trace elements for
oxidation of the palladium-silver alloys.

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Titanium
• Like aluminum and beryllium, titanium is
added to lower the melting range and
improve castability.
• Also acts as a hardener and influences oxide
formation at high temperatures.

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Zinc
• lowers the melting range of an alloy
• acts as a deoxidizer
• Zinc improves the castability of an alloy
and contributes to hardness when
combined with palladium.

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Historical Perspective Of Dental
Casting Alloys
History of dental casting alloys has been
influenced by three major factors.
1. The technological changes of dental
prosthesis
2. Metallurgical advancement
3. Price changes of noble metals since 1968

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major events in the history of dental
casting alloys
• Introduction of lost wax technique 1907
• Replacement of Co-Cr for Au in removable partial dentures 1933
• Development of resin veneers for Au alloys 1950
• Introduction of the porcelain fused to metal technique 1959
• Palladium based alloys as alternatives to Au alloys 1968
• Ni based alloys as alternatives to Au alloys 1971
• Introduction of all ceramic technologies 1980

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Desirable Properties Of Casting
Alloys
1. Bio-compatibility
2. Ease of melting & casting
3. Ease of brazing, soldering & polishing
4. Little solidification shrinkage
5. Minimal reactivity with mould material
6. Good wear resistance

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7. High strength
8. Sag resistance
9. Tarnish & corrosion resistance
10.Economic considerations
11. Lab cost

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Metal Type All-metal
prostheses
Metal ceramic
prostheses
Partial denture
frameworks
High Noble (HN) Au-Ag-Pd
Au-Pd-Cu-Ag
HN metal ceramic
alloys
Pure Au (99.7%)
Au-Pt-Pd
Au-Pd-Ag
(5-12 wt % Ag)
Au-Pd-Ag (>12 wt%
Ag)
Au-Pd
Au-Ag-Cu-Pd
Noble (N) Ag-Pd-Au-Cu
Ag-Pd
Noble metal
ceramic alloys
Pd-Au
Pd-Au-Ag
Pd-Ag
Pd-Cu-Ga
Pd-Ga-Ag
Predominantly Base
metal (PB)
CP Ti
Ti-Al-V
Ni-Cr-Mo-Be
Ni-Cr-Mo
Co-Cr-Mo
Co-Cr-W
Cu-Al
CP Ti
Ti-Al-V
Ni-Cr-Mo-Be
Ni-Cr-Mo
Co-Cr-Mo
Co-Cr-W
CP Ti
Ti-Al-V
Ni-Cr-Mo-Be
Ni-Cr-Mo
Co-Cr-Mo
Co-Cr-W
Classification of casting metals for full metal and metal ceramic prosthesis and partial dentures

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13001215base metalCo-Cr
13101250base metalNi-Cr (Cr>20 wt %)
13901330base metalNi-Cr (Cr<20 wt %)
12701160base metalNi-Cr-Be (Cr<20 wt %)
1270990NobleAg-Pd
10451185NoblePd-Ag
12301145NoblePd-Cu
1270880NobleAu-Cu-Ag-Pd
960905High nobleAu-Cu-Ag-Pd
12601160High nobleAu-Pd
11401060High NobleAu-Pt
Liquidus temperature
(°C)
Solidus temperature
(°C)
ADA classificationAlloy type
Solidus and liquidus temperature of the commonly used classes of alloys :

76Balance0-150-1074-880-6
Noble
(Ag-based)
Metal
Ceramic
Balance-1788
High Noble
(Au-based)
Metal
Ceramic
Balance-3060-
Noble
(Ag-based)
Metal
Ceramic
Balance--3852
High Noble
(Au-based)
Metal
Ceramic
Balance14452515
Noble
(Ag-based)
IV
Balance1425456
High Noble
(Au-based)
IV
Balance-7025-
Noble
(Ag-based)
III
Balance839646
High Noble
(Ag-based)
III
Balance9113.575
High Noble
(Au-based)
III
Balance714177
High Noble
(Au-based)
II
Balance6100.583
High Noble
(Au-based)
I
Ga, In, and ZnCuAgPdAu
Elemental composition (wt%)
ClassificationAlloy type
Typical compositions of Casting Alloys for Full-Metal, Resin-Veneered and Metal- Ceramic Prostheses

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ALLOYS FOR ALL METAL
RESTORATION

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GOLD AND GOLD BASED ALLOYS
Type Au% Ag% Cu% Pt/Pd% Zn%
I (soft) 85 11 3 - 1
II(Medium) 75 12 10 2 1
III (Hard) 70 14 10 5 1
IV (Extra
hard)
65 13 15 6 1

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uses
1) Type I : for low stress inlays .
2) Type 2 : are the most widely used metals for inlays.
3) Type 3 : are used when there is less support from tooth
structure and when the opposing stress are high like for
crowns, bridges.
4) Type 4 : are used exclusively for construction of components
of partial dentures and for this reason are referred to as partial
denture casting alloys.

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Karat
Karat value represents the number of
parts by weight of gold per 24 parts of gold.
Fineness
Fineness indicate the number of part
per thousand parts of gold.

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IV
Corrosion
resistance
Proportional
limitType
I
II
III
Hardness Strength Ductility
INCREASES
DOWNWARDS
DECREASES
DOWNWARDS
Comparative properties;

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Mechanical Property Requirements in
ANSI/ADA Specification No.5 for Dental Casting
Alloys (1997)
310450-300Type 4
-12--240Type 3
-12-240180Type 2
-18-18080Type 1
Minimum
(%)
Minimum
(%)
Minimum
(MPa)
Maximum
(MPa)
Minimum
(MPa)
HardenedAnnealedHardenedAnnealed
ElongationYield strength (0.2% offset)
Alloy type

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The classification based on the
color of the alloy
1. Yellow gold – Those with more than 60% Au
2. Low gold or economy gold – With 42-55% Au, also has
yellow color
3. White gold – are those with gold more than 50%, but
palladium gives the white color
4. Silver palladium with or without gold but mainly silver
– Has white color
5. Palladium silver with mainly palladium gives white
color.

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Japanese gold
Also known as technique alloy used for
training students in casting technology -
has yellow color.
 composition
Cu - 53%
Zn - 37%
Al - 7%
Others - 3%

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Grain refined alloys
• The grain refined alloys are those that contain
iridium or ruthenium in 100-150 parts per million.
• The advantages of the refined alloys are :
High yield strength
High elongation
Homogenous casting
More resistance to corrosion

86
HEAT TREATMENT OF GOLD ALLOYS
 Softening heat treatment :
• the alloy is heated in an electric furnace at a
temperature of above 700°C for 10 min and then
quenched rapidly in water.
• The ductility and the corrosion resistance increase
whereas the strength, hardness and the proportional
limit decrease.
 Homogenization heat treatment :
• This is done when platinum and palladium are
present, to remove coring. This involves heating to
700°C for ten minutes, followed by quenching.

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 Stress relief anneal :
– This is done when any adjustments are done to the
appliance to remove the stresses. This involves
heating in a low temperature to remove the stresses
for a given period of time.
 Hardening heat treatment :
– This is done for type III and type IV alloys which
contain sufficient amount of copper. This is due to
solid state transformations. The casting is heated to
above 450° C and allowed to cool slowly until
200°C, then quenching. This takes about 20 min.

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LOW GOLD CONTENT ALLOYS :
 SILVER - PALLADIUM ALLOYS
• Contains predominantly silver
• Palladium upto 25%
• May or may not contain gold and copper
• Traces of zinc and indium
 High casting temp
 Low density
 Good tarnish and corrosion resistance
 Properties similar to type III and type IV gold.
 ALUMINIUM BRONZE ALLOYS
• Cu 81-88 wt%
• Al 7-11 wt%
• Ni 2-4 wt%
• Fe 1-4 wt%

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Alloys for Metal Ceramic
Restoration
Definition
Partial crown, full crown or fixed partial
denture made with a metal substrate to
which porcelain is bonded for esthetic
enhancement via an intermediate metal
oxide layer

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Property Requirements of metal ceramic alloys :
• Fusion temperature should be 100°C greater than the
fusion temperature of porcelain.
• Contact angle between the ceramic and metal should be
less than 60°
• Should form oxides on the surface for bonding to
porcelain. -Tin, Indium and Iron are added.
• should have compatible co-efficient of thermal
expansion(0.5 x 10-6
/°C). - Palladium
• Adequate stiffness and strength
• High sag resistance
• Lab workability and Casting Accuracy – in order to
provide clinically acceptable castings

93
High Noble Alloys
Gold-Platinum-Palladium Alloys
Composition :
• Gold: 75%-88%
• Platinum: up to 8%
• Palladium: up to 11 %
• Silver: up to 5% (if present)
• Trace elements like indium, iron, and tin
for porcelain bonding. (If the palladium
content exceeds that of platinum, then the
alloys should be classified as Au-Pd-Pt.)

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Advantages
• Excellent castability
• Excellent porcelain bonding
• Easy to adjust and finish
• Excellent corrosion and tarnish resistance
• Burnishable
Disadvantages
• High cost
• Poor sag resistance
• Low hardness (greater wear)
• High density (fewer castings per ounce)

95
Gold-Palladium-Silver Alloys
Introduced in 1970 as will ceram w
 composition -
• Gold: 39%-53%
• Palladium:25%-35%
• Silver: 12%-22%
• Tin ,Indium & Ruthenium
 Advantages
• Less expensive than Au-Pt-Pd alloys
• Improved rigidity and sag resistance
 Disadvantages
• High coefficient of thermal expansion
• Tarnish and corrosion resistant

96
Gold-Palladium alloys
 Composition
• Gold – 44% -55%
• Palladium – 35% - 45%
• Gallium up to 5%
• Iridium and tin up to 8% - 12%
 Advantages
• Excellent castability
• Good bond strength & sag resistance
• Tarnish and corrosion resistant
• Lower density
 Disadvantages
• High cost
• Not thermally compatible with high expansion dental
porcelains

97
Palladium-Silver alloys
 Composition
• Palladium – 55% - 60%
• Silver – 28% - 30%
• Indium and tin
AdvantagesAdvantages
• Good castabilityGood castability
• Good porcelain bondingGood porcelain bonding
• Excellent sag resistanceExcellent sag resistance
• Low HardnessLow Hardness
• BurnishabilityBurnishability
• Good tarnish and corrosionGood tarnish and corrosion
•Low densityLow density
• Low costLow cost
• Moderate nobility levelModerate nobility level
• Suitable for long-span fixedSuitable for long-span fixed
partial denturespartial dentures
Noble Metal

98
Disadvantages
•High coefficient of thermalHigh coefficient of thermal
expansionexpansion
•Discoloration (yellow, brown,Discoloration (yellow, brown,
or green) may occur with someor green) may occur with some
dental porcelainsdental porcelains
•Some castability problemsSome castability problems
•Pd and Ag prone to absorbPd and Ag prone to absorb
gasesgases
•Require regular purging ofRequire regular purging of
the porcelain furnacethe porcelain furnace
•May form internal oxidesMay form internal oxides
•Should not be cast in aShould not be cast in a
carbon cruciblecarbon crucible
•Non carbon phosphateNon carbon phosphate
bonded investmentsbonded investments
recommendedrecommended

99
Palladium-cobalt alloys
Composition
• Palladium: 78%-88%
• Cobalt: 4%-10%
Advantages
• Low cost
• good sag resistance
• Low density
• Good polishability
• easier to presolder than high Pd-Cu alloys
Disadvantage
• Produce a thick, dark oxide which may cause bluing of
porcelain
• Prone to gas absorption
• Little information on long-term clinical successwww.indiandentalacademy.com

100
High Palladium-Silver-Gold alloys
 Composition
• Silver: less than 1 %-7%
• Gold: 2%-6%
• Platinum: less than 1.0% (if present)
 Advantages
• Improved sag resistance
• Light-colored oxide layer
 Disadvantages
• A relatively new alloy group
• No data on long-term performance
• Like other palladium-based alloys are prone to gaseous
absorption
• Should not be cast in carbon crucibles

101
Palladium-Copper alloys
Composition
• Copper: 9%-15%
• Gold: 1 %-2% (if present)
• Platinum: 1 % (if present)
Disadvantages
• Produce dark, thick oxides May discolor (gray)
some dental porcelains
• Should not be cast in carbon crucibles
• Prone to gaseous absorption

102
Pd-Ga-Ag & Pd-Ga-Ag-Au Alloys
• These are the most resistant of the noble
alloys.
• These were introduced because of their
tendency to form lighter oxides than Pd-Cu
or Pd-Co.
• They are compatible with lower expansion
porcelains like vita porcelain.

103
PHYSICAL PROPERTIES OF HIGH
NOBLE AND NOBLE METAL ALLOYS
• All are biocompatible
• Good resistance to tarnish and corrosion
• Melting temperature of around 1000°C.
• Density of 15gm/cm3
.
• Hardness from soft to hard
• Elongation which is a measure of ductility of about 20-39%
• Linear coefficient of thermal expansion in the range of 14 – 18 x
10 -6
/°C.
• Yield strength in the range of 103 – 572 MPa.

104
BASE METAL ALLOYS
 According to the ADA the following
combinations are available :
1. Cobalt – chromium
2. Nickel – chromium
3. Nickel – chromium – beryllium
4. Nickel – cobalt – chromium
5. Titanium – aluminium – vanadium

105
Nickel-chromium-beryllium
alloys
 Composition
• Nickel: 62%-82%
• Chromium: 11 %-20%
• Beryllium: up to 2.0%
• Also includes:
aluminum
carbon
gallium
iron
manganese
molybdenum
silicon
Titanium
vanadiumwww.indiandentalacademy.com

106
Advantages
• Low cost
• Low density permits more casting per ounce
• High sag resistance
• Can produce thin castings
• Poor thermal conductor
• Can be etched

107
 Disadvantages
• Cannot use with nickel- sensitive patients
• Beryllium exposure may be potentially harmful to
technicians and patients
• Proper melting and casting is a learned skill
• Bond failure more common in the oxide layer
• High hardness (may wear opposing teeth)
• Difficult to solder
• Difficult to cut through cemented castings
The occupational health and safety administration
(OSHA) specifies that exposure to Beryllium dust
in air should be limited to a concentration of 2
ug /meter cube

108
Nickel-chromium beryllium-free alloys
 Composition
• Nickel: 62%-77%
• Chromium: 11 %-22%
• Boron , Iron, Molybdenum, Tantalum.
• May not cast as well asNi-Cr-Be alloys
• Produce more oxides than Ni-Cr-Be alloys

109
Cobalt-chromium alloys
vitallium (1928)
 TYPES
• Type I -fusion temperature greater than 2400F.
• Type II - fusion temperature less than 2400F.
Composition
• Cobalt: 53%-68%
• Chromium: 25%-34%
• Trace elements include molybdenum, ruthenium

110
Disadvantages
• More difficult to process than nickel-base
alloys
• High hardness (may wear the opposing
dentition)
• Oxidize more than both nickel-based alloys
• No information on long-term clinical studies

111
COMPARISON OF PROPERTIES OF THE VARIOUS
TYPES OF BASE METAL ALLOYS
HighHighFairExcellent
Bond to
porcelain
ExtremelyModeratelyModerately HighMinimal
Technique
sensitivity
GoodExcellentExcellentPoor-excellentSag resistance
103207145-22090
Elastic Modulus
(GPa)
4.58.77.514Density (g/cm3
)
ExcellentFairExcellentExcellentBiocompatibility
CPTiNi-Cr-BeCo-CrHigh noble alloyProperty

112
COMPARISON OF THE PROPERTIES OF TYPE IV AND Co-Cr ALLOY :
Simple adequate
high
Complicated
adequate
reasonable
Heat treatment
Tarnish resistance
price
1.25 – 1.652.3Casting shrinkage
Co-Cr require electrical induction
or oxyacetylene
Lower than 1000As high as 1500Melting temperature
(o
C)
Both resist stresses without
deformation.
500700Proportional limit (MPa)
Co-Cr more rigid for the same
thickness
100220Modulus of elasticity
(GPa)
Co-Cr clasps may fracture if
adjustments are made.
15 (as cast)
8 (hardened)
2Ductility
More flexibleStiffStiffness
More difficult to polish but retains
polish during services.
250 (Softer than
enamel)
420 (Hard than
enamel)
Hardness (Vickers)
More difficult to produce defect
free castings for CO-Cr but
dentures are lighter.
158Density (gms / cu.cm)
Both acceptable750850Tensile Strength (Mpa)
IV CommentsType GoldCo-CrProperties

113
COMPARISON OF PROPERTIES OF TYPE III AND Ni-Cr ALLOY
Ni-Cr more difficult to polish but
retains polish during service.
Burnishing is possible but high forces
are required.
20 (as cast)
10 (hardened)
300 upto
30%
Hardness (Vickers
Ductility)
Higher modulus of Ni-Cr advantage
for larger restorations.
85220Modulus elasticity
(GPa)
Both high enough to prevent
distortions when used.
290230Tensile strength
(MPa)
Ni-Cr alloys require electrical
induction or oxyacetylene flame.
Both adequate
Lower than
1000
as high as
1350
Fusion temperature
(oC)
More difficult to produce defect free
castings for Ni-Cr alloys.
158Density
(gm/cu.cm)

114
Titanium alloys
• High biocompatibility
• According to the American Society for Testing
and Materials (ASTM), there are five unalloyed
grades of CP Ti (Grades 1-4, and Grade 7), based
on the concentration of
• oxygen (0.18 wt% to 0.40 wt%) and
• iron (0.2 wt% to 0.5 wt%).
• Other impurities include nitrogen (0.03 wt% to 0.05
wt%),
• carbon (0.1 m%), and hydrogen (0.015 wt%).

115
• Grade 1 CP Ti is the purest and softest form
• It has a moderately high tensile strength
• moderately high stiffness
• low density
• low thermal expansion coefficient.
• The elastic modulus of CP Ti is comparable to
that of tooth enamel and noble alloys, but it is
lower than that of other base metal alloys

116
• Casting of titanium alloys is difficult due to a high
casting temperature – 2000° c
• Rapid oxidation and reactions with investments
• Melting is done in specially designed furnaces
with an argon atmosphere
• Ti-6Al-4v has been used for PFM restorations
• Used with low expansions porcelains

117
SINTERED COMPOSITE
• These composites consist of sintered
high noble alloy sponge infiltrated
with an almost pure gold alloy.
• The result is a composite between the two gold
alloys that is not cast, but fired onto a refractory
die.
• The porcelain does not bond through an oxide
layer in these systems, but it bonds mechanically
to a micro rough surface.
• The advantages of this that any stress
concentration on the ceramic is relieved by the
excellent ductility of the metal

118
CONCLUSION
 Finally the guidelines for the selection
of an alloy for a restoration should
be based on :
1. A thorough understanding of the alloy
2. Avoid selecting an alloy based on its color
unless all other factors are equal
3. Know the complete composition of alloys, and
avoid elements that are allergic to the patient
4. Whenever possible use single phase alloys

119
5. Using clinically proven products
from quality manufacturers
6. Use alloy that have been tested for elemental
release and corrosion and have the lowest
possible release of elements.
7. Focus on long term clinical performance
8. Finally it is important for the dentist to
remember and take up the responsibility of being
responsible for the safety and efficacy of any
restoration.

120
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