2.Metals and Alloys for Prosthodontics - Copy.ppt

presentatin on metal s and alloys used in prosthodontics

3. Materials Science & Engineering Dept.
Research Experience for Undergraduates

4. INTRODUCTION
DEFINITION
METALLURGY -TYPES
GENERAL CHARACTERISTICS OF
METALS
STRUCTURE AND PROPERTIES OF
METALS
CONTENTS

5. CRYSTALLINE STRUCTURE
NUCLEI
DENDRITES
GRAIN
GRAIN BOUNDARIES
EQUIAXED GRAIN STRUCTURE
LATTICE TYPES
DISLOCATIONS
SLIP PLANES

6. QUENCHING
REFINED GRAIN STRUCTURE
SEEDING
COLD WORKING
DUCTILITY
MALLEABILITY
COLD WORKING
FIBROUS STRUCTURE
WORK HARDENING
RECRYSTALLIZATION TEMPERATURE
GRAIN GROWTH
ANNEALING
STRESS RELIEF ANNEALING

7. STRUCTURE AND PROPERTIES
OF ALLOYS
ALLOY
ALLOY SYSTEM
METALLOID
AMALGAM
BINARY
TERNARY
QUATERNARY

8. SOLID SOLUTION –PHASE
SUBSTITUTIONAL SOLID SOLUTION
INTERSTITIAL SOLID SOLUTION
SOLUTION HARDENING
(COOLING CURVE) TIME-TEMPERATURE CURVE
CORED STRUCTURE
CORING
PHASE DIAGRAM
LIQUIDUS LINE
SOLIDUS LINE
HOMOGENISATION
INTERMETALLIC COMPOUND
EUTECTIC FORMATION
PERITECTIC ALLOYS

9. CLASSIFICATION OF METALS
AND ALLOY SYSTEMS
DENTAL CASTING ALLOYS -
DESIRABLE PROPERTIES
AND USES

10. NICKEL-CHROMIUM ALLOYS
COBALT-CHROMIUM ALOYS (STELLITES)
TITANIUM AND TITANIUM ALLOYS
ALLOYS FOR METAL CERAMIC
RESTORATIONS
PARTIAL DENTURE ALLOYS

11. WROUGHT ALLOYS
AND
GOLD ALLOYS
CARBON STEELS
STAINLESS STEEL
COBALT-CHROMIUM (ELGILOY) NICKEL ALLOYS
NICKEL-TITANIUM ALLOYS ( NITINOL)
β-TITANIUM ALLOYS
GOLD ALLOYS
SILVER-PALLADIUM ALLOYS
DENTAL IMPLANT MATERIALS

12. BIOCOMPATIBILITY
CONCLUSION
REFERENCES

15. GPT 8 (2005) defines “METAL” as any
strongrelatively ductile substance that
provides electropositive ionsto a
corrosive environment and that can be
polished to a high lustre. Characterized
by metallic atomic bonding.

16. The metals handbook(1992) defines a “metal as
an opaque lustrous chemical substance that is a
good conductor of heatand electricity and,
when polished, is a good reflector of light”

17. In dentistry, metalsrepresent one of the four
major classes of materials used for the
reconstruction of decayed, damaged or
missing teeth.

18. The science and art of the extractionof metals
from their ores together with the refinement of
thesemetalsand their adaption to various uses.
METALLURGY

19. The extensive useof metals and their
combination during recent years has made
specializationin this field. This specialization
has resulted in the development of several
branchesof metallurgy , some of which are
closely associated with chemistry, physics and
mechanics.

20. Understanding of metallurgy and the
characteristic behaviorof various metals, or
combination of metals to form alloys, is
highly desirablein the study of restorative
materialsfor several reasons like :
There are numerous metalswhich are used in
various restorative operations.

21. A knowledge of the characteristic behavior
of metals is essential for an understanding of
the qualityof the restoration fabricated from
metals.
The properties that the metal or alloy will
display are quiet reproducible and serve as
guide in the studyof the many related issues
to the fabrication of dental restorations.

22. METALLURGY
CHEMICAL
PHYSICAL
MECHANICAL

23. Chemical metallurgy deals principally with
the production and refinementof metals.
Sometimes it is described as “process”
metallurgy since it considers the processing
of oresfor the productionof metals.
CHEMICAL METALLURGY

24. PHYSICAL METALLURGY
Physical metallurgy is newer scienceand
deals with the structureof possible
alteration in structure as well as the
characteristic physical propertiesof metals.
In some respects physical metallurgy and
metallographyare closely related.

25. Metallographyis primarily the microscopic
examination of the internal structureof
metals. This metallographic examination
gives some indication of the physical
behaviorwhich the metal can be expected to
exhibit.

26. MECHANICAL METALLURGY
It includes various processes in the fabrication
of a structuresuch as the casting, rollingor
drawingoperations.
In restorative materials, physical metallurgy
combined with metallographyand the
mechanicalphase of metallurgy are of greatest
importance.

27. FERROUS METALLURGY
It is the metallurgy of iron and steel.
In dentistryit is important in connection with
the manufacture and use of steel instruments
and equipments as well as stainless steel
appliances.
FURTHER SUBDIVISIONS

28. NON-FERROUS METALLURGY
It is the metallurgy of all metal andalloys
other thaniron and steel.
E.g.: Gold alloys, platinum alloys, Cr -Coor
stellite alloys, as well as bronze, aluminum,
and low fusing alloys etc.,

29. General characteristics of metals
A metal is any element that ionizes positivelyin
solution.
Metals have certain typical andcharacteristic
propertiesthat tend to identify and distinguish
them from the nonmetallic elements, such as
lustre, opacity, density, thermal and electrical
conductivity.

30. Extreme ductility andmalleabilityare often
desirable in metals used in dentistry and
these are found to predominate in pure
metalsrather than in alloys.

31. STRUCTURE AND PROPERTIES OF
METALS
Metals usually have crystalline structuresin the solid
state.

32. In 1665, Robert Hooke(1635 -1703) simulated
the characteristic shapes of crystals by stacking
musket balls in piles.
A SPACE LATTICEcan be defined as
any arrangement of atoms in space such that
every atom is situated similarlyto every other
atom. It is also called a crystal.

33. There are14possible lattice types or forms, but
many of the metals used in dentistry belong to the
cubic systemarrangement.
Simple cubic space lattice
Single cells of cubic space lattice
Simple cubic
Face-centered cubic
Body-centered cubic
Models

35. Other simple lattice types of dental interest.
A) Rhombohedralb) Orthorhombic
c) Monoclinic d) Triclinic e) Tetragonal
f) Simple hexagonal
g) Close packed hexagonal h) rhombic.

37. When a molten metal or alloy is cooled, the
solidification processis one of crystallization
and is initiated at specific sites called nuclei. The
nuclei are formed from impurities within the
molten mass of metal.

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

40. Crystal growth continue until all the material has
solidified and all the dendritic crystalsare in
contact.

41. Each crystal is known as a grainand the area
between two grains in contact is the grain
boundary

42. After crystallization, the grains, have
approximately the same dimensionsin each
direction, measured from thecentral nucleus.
They are not perfectly spherical or cubic
however, nor do they conformto any other
geometric shape. They are said to have an
equiaxed grain structure.

43. A change from an equiaxed structureto one in
which the grains have a more elongated, fibrous
structurecan cause important changes in
mechanical properties.

44. The arrangement adopted by any one
crystal depends on specific factors such as
atomic radiusand charge distributionson
the atoms. Although there is a tendency
towards a perfect crystal structure,
occasional defects occur.

46. Such defects are normally referred to as
dislocations and their occurrence has an
effect on the ductility of the metal or alloy.

47. When the material is placed under a sufficiently
high stress the 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 planeand the stress
required to initiate movement is the elastic limit.

49. Grain boundariesform a natural barrierto the
movement ofdislocations. The concentration of
grain boundaries increases as the grain size
decreases. Metals with finer grain structure are
generally harder and have higher values of
elastic limit than those with coarser grain
structure.Hence it can be seen that material
properties can be controlled to some extent by
controlling the grain size.

50. A fine grain structurecan be achieved by rapid
cooling of the molten metalor alloy following
casting. This process, often referred to as
quenching, ensures that many nuclei of
crystallization are formed, resulting in a large
numberof relatively small grains.

52. Slow coolingcauses relatively few nucleito be
formed which results in a larger grain size.
Some metals and alloys are said to have a refined
grain structure.This is normally a fine grain
structure which is achieved by seeding the
molten metal with an additive metalwhich forms
nuclei crystallization.

53. For an applied tensile forcethe maximum degree
of extensionis a measure the ductilityof the
metal or alloy.
For an applied compressive forcethe maximum
degree of compressionis a measure of
malleability.
These changes occur when the stress is greater
than the elastic limit and at relatively low
temperatures.
COLD WORKING

54. Such cold working not only produces a change
in microstructure, with dislocations becoming
concentrated at grain boundaries, but also a
change in grain shape.
The grains are no longer equiaxedbut take up a
more fibrous.

56. Cold workingis sometimes referred to as work
hardeningdue to the effect on mechanical
properties. When mechanical work is carried out
on a metal or alloy at a more elevated
temperatureit is possible for the object to change
shape withoutany alteration in grain shape or
mechanical properties.

57. The temperature below which work hardening is
possibleis termed the recrystallization
temperature.
If the material is maintained above the
recrystallization temperature for sufficient time,
diffusion of atomsacross grain boundaries may
occur, leading to grain growth.
It is clear that grain growth should be avoidedif
the properties are not to be adverselyaffected.

58. It is process of heatinga metal to reverse the
effectsassociated 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.
Annealing

59. Recovery: is considered the stage at
which the coldwork properties begin to
disappearbefore any significant visible
changes are observed under the
microscope.

60. Recrystallization :
when a severely cold worked metal is
annealed, recrystallization occurs after the
recovery stage. The old grains disappear
completely and are placed by a new set of
strain free grains.

61. Grain growth:
The crystallized structure has a certain
average grain size, depending on the
number of nuclei .The more severethe cold
working, the greater the numberof such
nuclei. Thus, the grain size for completely
recrystallized material can range from
rather fineto fairly coarse.

62. Cold working may cause the formation of
internal stresseswithin a metal object. If these
stresses are gradually relieved they may cause
distortionwhich could lead to loss of fitof, for
example, an orthodontic appliance.
For certain metals and alloys the internal stresses
can be wholly orpartly eliminatedby using a
low temperature heat treatmentreferred to as
stress relief annealing.

63. This heat treatment is carried out well belowthe
recrystallization temperatureand has no
deleterious effecton mechanical propertiessince
the original grain structure is maintained.

64. STRUCTURE AND PROPERTIES OF
ALLOYS
An alloyis a mixture of two or more metals.
Mixtures of twometals are termed Binary alloys,
mixtures of three metalsare Ternary alloys
similarly mixture of four metalsis termed as
Quaternary alloysetc.
The term alloy systemrefers to all possible
compositions of an alloy. For examplethe silver-
copper systemrefers to all alloys with
compositions ranging between 100% silverand
100% copper.

65. In the molten state metals usually show
mutual solubility, one within another.
When the molten mixture is cooledto
below the melting pointthe following
things can occur.
The component metals may remain soluble
in each otherforming a solid solution.

66. The solid solution may take one of three
forms. It may be a random solid solution
in which the component metal atoms
occupy random sitesin a common crystal
lattice.

68. The solid solution may take one of three
forms. It may be a random solid solution
in which the component metal atoms
occupy random sitesin a common crystal
lattice.

69. Another possibility is the formation of an
ordered solid solutionin which component metal
atoms occupy specific siteswithin a common
crystal lattice.

70. The solid solution may take one of three
forms. It may be a random solid solution
in which the component metal atoms
occupy random sitesin a common crystal
lattice.

71. The third type of solid solution is the interstitial
solid solutionin which, for binary alloys, the
primary lattice sites are occupied by one metal
atomand the atoms of the second component do
not occupy lattice sites but lie within the
interstices of the lattice. This is normally found
where the atomic radiusof one component is
much smallerthan that of the other.

74. Solid solutions are generally harder, strongerand
have higher valuesof 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, is thought to be due to the fact that
atoms of different atomic radii within the same
lattice form a mechanical resistance to the
movement of dislocations along slip planes.

75. Metals and alloys are sometimes characterized
using cooling curves. The material is heated till
molten then allowed to cool and a plot of
temperature against timeis recorded.
Super cooling
Heterogeneous Nucleation

77. Each alloy grain can be envisaged as having a
concentration of gradient metals; the higher
melting metal being concentrated close to the
nucleus and the lower melting metal close to the
grain boundaries. The material is said to have a
cored structure.
Such coringmay influence corrosion resistance
since electrolytic cellsmay be set up on the
surface of the alloy between areas of different
alloy composition.

78. Since coring may markedly reduce the corrosion
resistance of some alloys, a heat treatmentis
some times used to eliminate the cored structure.
Such a heat treatment is termed a homogenization
heat treatment.
This involves heating the alloyto a temperature
just below the solidus temperaturefor a few
minutes to allow diffusion of atoms and the
establishment of homogeneous structure. The
alloy is then normally quenchedin order to
prevent grain growthfrom occurring.
E.g., Au-Ag system.

79. If the temperatures T1and T2are obtained over
a range of compositions for an alloy systemand
their values plotted against percentage
composition, a useful graph emerges.

81. This is illustrated for a hypothetical solid solutionalloy
of metals A and B. The melting points of the pure metals
are indicated by the temperatures TmAand TmB. The
upper and lower temperature limits of the crystallization
range, T1and T2 are shown for four alloys ranging in
composition from 80% A–20% Bto20% A-80% B.

83. The phase diagramis completed by joining
together all the T1 pointsand all the T2 points,
together with the melting points of the pure
metals, TmAand TmB.
At temperatures in the region above the top line,
known as the liquidus line, the alloy is totally
liquid. At temperatures in the region below the
bottom line, known as the Solidus line,the alloy
is totally solid.

85. At temperatures in the region between the solidus
and liquidus lines the alloy consists of a mixture
of solid and liquid. The composition of the solid
and liquid phases at any temperature between T1
and T2can be predicted with the aid of the phase
diagram.

86. When two metals are completely misciblein
liquid state, they are capable of forming any
alloy. When such a combination is cooled, one of
the three possibilities may take place :
a)Solid solution
b)Intermetallic compound
c)Eutectic formation

87. Intermetallic compounds
Chemicals with chemical affinityfor each other
can form intermetallic compounds.
E.g., Ag3Sncan be formed between silver and
tin, which is an essential constituentof
DENTAL AMALGAM ALLOYS .

88. Eutectic mixture
They occur when the metals are miscible in the
liquid statebut separate in the solid state. The
two metals will be precipitated as very fine
layers of one metal over the other one: such a
combination as is called an eutectic mixture.
E.g.,72 %silver, 28 %copper.

89. Phase diagramfor a Binary system where there
Is complete solid insolubility.
CEF
CDEGF
‘E’

90. A material of this composition is called a
“Eutectic alloy”
Important features:
Hardand Brittle
Lowest meltingalloy of the system -solders
Poor corrosion resistance
Time-temperature curvefor this alloy has a
“Horizontal plateau” (like that of a pure metal)

91. Peritectic alloys
Limited solubilityof two metals can lead to a
transformation referred as “Peritectic transformation”
E.g., Ag-Sn
(Basis for the original Dental Amalgam alloy, is a
Peritectic system)
Invariant reactionoccurs at particular temperature and
composition.

92. CLASSIFICATION
OF
METALS AND ALLOY SYSTEMS

93. Metals can be broadlyclassified according to
composition as
NOBLE METALS
The term nobleidentifies elements in terms of their
chemical stabilityi.e., they resist oxidationand are
impervious to acids.
Gold, Platinum, Palladium, Rhodium, Ruthenium,
Iridium, Osmium, and Silver are the eight noble
metals.
In the oral cavity Silver is more reactiveand
therefore is not consideredas a noble metal.

94. PRECIOUS METALS
The term “precious” merely indicates whether a
metal has intrinsic value, the noble metals (all eight)
are also precious metals and are defined as such by
major metallurgical societiesand the federal
government agencieslike National institute of
science and technology.
All noble metals are preciousbut all precious metals
are not noble.
Silver is usually the major ingredient in most alloys
considered as precious.

95. SEMIPRECIOUS METALS
There is no accepted compositionthat differentiates
“precious from semiprecious” therefore, this term is
usuallyavoided.

96. BASE METALS
These are Ignobleelements. These remain
invaluable componentsof dental casting alloys
because of their influence on physical properties,
control of the amountand type of oxidation, or
for their strengthening effects.
e.g., Chromium, Cobalt, Nickel, Iron, Copper
etc.

97. The bureau of standards established gold casting alloys
type i through type ivaccording to function, with
increasing hardnessfrom type i to iv (1927)

98. In 1984, ADAproposed a simple classification for
Dental casting alloys

99. Alloy types by description

101. Removable partial denture alloys
Although type IV noble metal alloys may be
used, majority of the removable partial frame
works are made from base metal alloys.
E.g., Cobalt-chromium,
Nickel-chromium.

102. DENTAL CASTING ALLOYS
The history of dental casting alloys has been
influenced by threemajor factors.
a)The technological changes of dental prosthesis.
b)Metallurgic advancements
c)Price changes of noble metals since1968.
In 1932, the dental materials group at national
bureau of standards surveyed thealloysbeing used
and roughly classified them type I-IV.

103. Uses
1)Fabrication of inlay, onlays
2)Fabrication of crowns, conventional all metal
bridges, metal-ceramic bridges, resin bonded
bridges.
3)Endodontic posts.
4)Removable partial denture frameworks.

104. Desirable properties
1) Biocompatibility.
2) Ease of melting.
3) Ease of casting, brazing and polishing.
4) Less solidification shrinkage.
5) Minimal reactivity with the mould material.
6) Good wear resistance.
7) High strength and sag resistance.
8) Excellent tarnish and corrosion resistance.

105. Nickel-chromium and Cobalt-chromium
Alloys
Dental applications:
1)Partial denture framework: Co-Cr, Ni-Cr
2)Porcelain -metal restorations: Co-Cr, Ni-Cr
3)Crowns and bridges: Ni-Cr

106. During the years since the Co-Cr casting alloys
became available for cast removable partial
denture constructions, they have continued to
increase in popularity.

107. Function of various alloying elements:
•Chromium is responsible for the tarnish resistance
and stainless properties of these alloys.
•When chromium content of alloy is over 30%, the
alloy is difficult to cast. With this percentage of
chromium, the alloy also forms a brittle phase, known
as sigma phase. Therefore cast base metal dental
alloys should notcontain more than 28-29%of
chromium.

108. •Cobaltincreases the elastic modulus, strength and
hardness of alloy more than does nickel.
•One of the effective ways of increasing their hardness is
by altering carbon content.
0.2% increasechanges the properties such that
alloy would no longer be used in dentistry.
[Too brittle]
0.2% decreasewill reduceyield and ultimate
tensile and yield strengths.

109. •Aluminum in nickel containing alloys increasesthe
ultimate tensile and yield strengths.
Microstructure
Microstructure of any substance is the basic
parameter that controls the properties. In other words,
a change in the physical propertiesof material is a
strong indication that there must have been some
alteration in its microstructure.

110. The microstructure of Co-Cr alloys in the cast
condition is in homogeneous, consisting of an
austenitic matrixcomposed of a solid solution of
cobalt and chromium in a cased dendritic
structure. The dendritic regions are cobalt-rich,
where as the interdendritic regions can be a
quaternary mixture.

111. Three main disadvantagesin employing these alloys
(Co-Cr)
Clasps made of such alloys break in service; some
break after relatively short time.
Due to relatively high hardnessand low elongation
properties of these alloys someminor but necessary
adjustmentsneeded at the time of delivery are
difficult and also will consumethe chair time of
dentist.
Due to their high degree of hardness, the teeth
contacting the metal becomes worneasily.

112. Morris 1975stated that Co-Cr alloysare harder
than iron base alloys .
In 1979, he stated that heat treatmentdecreases
strengthof the alloy compared to Au-Pd alloys.
J.C. Wataha et.al, 1992stated that preparative
proceduressuch as steam sterilization,
irradiation, plasma treatment and acid treatment
altered the surfaceof alloys.

113. In 1974, A C Rowestated that adding Tantalum (13%)
to a Co-Cr-Ni alloy the properties like ultimate tensile
strength, yield strength are increased by 12-13%.
Tantalum reduces dislocations, a well ordered
structureis formed. Tantalum is a stabilizer. Example
for stabilizers are carbon, molybdenum, tungsten.
Hamid Mohammadand Kamal asgar1973, indicated
that a cobalt made from 40% Co, 30% Ni, 30% Cr
strengthenedby precipitation of coherent Intermetallic
compounds of Tantalum.

114. They also have criteriato select an additional element
1) Corrosion resistance.
2) Resistance to oxidation during alloying.
3) Efficiency as a nucleating agent during solidification.
4) Efficiency as a solid solution hardener.
5) Fineness of precipitate.
6) Coherency.

115. Titanium And Titanium Alloys:
Titanium’s resistance to electrochemical
degradation; the benign biological responsethat it
elicits; its relatively low weight; and its low
density, low modulus, and high strengthmake
titanium based materials attractive for use in
dentistry.

116. Ti forms a very stable oxidelayer with a thickness
on the order of angstroms and it repassivatesin a
time on the order of nanoseconds. This oxide
formation is the basis for the corrosion resistance
and biocompatibility of Ti.
Commercially Pure Titanium(CpTi) is used for
fabricating dental implants, and more recently, as
crowns, partial and complete dentures, and
orthodontic wires.
Wrought alloys of Ti and V and of Ti and Mo are
used for orthodontic wires.

117. Commercially pure Ti is available in 4 gradeswhich
vary according to the Oxygen(0.18-0.40 wt%) and iron
(0.2-0.5 wt%) contents.
At room temperature CpTi has a HCPcrystal lattice,
which is denoted as the alpha phase. On beating, an
allotropic phase transformation occurs, at 883°c, a BCC
phase, which is denoted as the beta(β)phaseforms.
A component with a predominantly beta phaseis
strongerbut more brittlethan a component with an alpha
phase microstructure,

118. Titanium alloys
Pure titanium is of two types –
Grade I
Grade II.
Alloying elementsare added to stabilizeeither the
αor βphase by changing βto αtransportation
temperature.

119. For example, in Ti 6 Al-4V, aluminumis an α
stabilizer, which expands the αphase field by
increasing the (α+ β) to βtransformation temperature.
Vanadium, as well as copper andpalladiumare β
stabilizers, which expand the ‘β’-phase field by
decreasing (α+ β) transformation temperature.

120. Ti-6Al-4V
Most widelyused.
At room temperature, Ti-6 Al-4V is a two phase α+β
alloy.
At approximately 975 °C an allotropic phase
transformation takes place, transforming the
microstructure to a single phase BCC βalloy.
Mostly used for surgical implants.

121. Based on attributes, extensive knowledge, and clinical
success of wrought Ti implants, interest was developed
in cast titanium for dental applications.
Thetwo most important factors in casting Titanium
based materials are the high melting pointand
chemical reactivity.
Cast Titanium:

122. Ti readily reacts with gaseous elements such as
hydrogen, oxygen andnitrogenparticularly at high
temperatures. So any manipulation of Ti at elevated
temperaturesmust be performed in a well-controlled
vaccum, Without a well controlled vaccum, Ti surfaces
will be contaminatedwith an oxygen enriched and
hardened surface layer, which can be as thick as
100 µm.
surface layers of this thickness reduce strengthand
ductility and promote crackingbecause of embrittling
effectof oxygen.

123. Because of the high affinityTitanium has for
hydrogen, oxygen and nitrogen, standard crucibles and
investment materialscannotbe used.
Investment materialsmust have oxides that are more
stable than the very stable Ti oxideand must also be
able to withstand a temperature sufficient to melt
titanium. if this is not the case, then diffusion of
oxygeninto the molten is likely to occur.

124. Investment materials such as phosphate bonded
silica and phosphate investment materials with
added trace elementsachieve this goal. It has
been shown that with magnesium oxide-based
investments, internal porosity results.

125. Because of the low density of titanium, it is difficult to
cast. In the last 10 to 15 yrs, advanced casting
techniques, which combine centrifugation, vaccum
pressure and gravity casing, and new investment
materials are used.
Properties of Alloyed Titanium
1)Lower melting points compared to pure Ti, but same
as as Ni-Cr or Co-Cr alloys.
2)Mechanical properties of cast CPTi are similar to
those of type III and IV gold alloys.

126. Other alloysTi-15V, Ti-20Cu, Ti-30pd,
Ti-Co, Ti-Cu.
Disadvantages(for dental purpose)
a) High melting point.
b) High reactivity.
c) Low casting efficiency.
d) Inadequate expansion of investment.
e) Casting porosity.

127. f) Difficulty in finishing this metal.
g) Difficult to weld, solder.
h) Expensive equipment.

128. Aluminum Bronze alloy
Traditionally bronze is copper-rich copper tin.
Compositionof ADA approved alloy of this group has
81-88% copper
7-11% wt aluminum
2-4% nickel
1-4% iron.
Disadvantage:
Copper reacts with sulfur to form copper sulfide,
which tarnishesthe surface of this alloy.

129. METAL CERAMIC RESTORATIONS:
The chief objectionto the use of dental porcelain
as a restorative material is its low tensileand shear
strength. Thiscan be minimized by bonding
porcelain directly to a cast alloy substructuremade
to fit the prepared tooth. If a strong bond is
attained between the porcelain veneers and the
metal, the porcelain veneer is reinforced.

130. The original metal ceramic alloys contained 88%
goldand were much too soft for stress-bearing
restorations. As there was no evidence of a
chemical bondbetween these alloys and dental
porcelain, then mechanical retentionand undercuts
were used to prevent detachment of the ceramic
veneer. By adding lessthan 1% of oxide elements
such as iron, indium and tin to this high-gold
content alloy, the porcelain metal bond strength
was improved by three folds.

131. Classification of alloys
used for metal ceramic restorations:
High noble
Au-Pt-Pd
Au-Pd-Ag
Au-Pd
Noble
Pd-Au
Pd-Au-Ag
Pd-Ag
Base metal
Pure Ti, Ni-Cr-Mo-Be, Ti-Al-V, Ni-Cr-Mo

132. Inspite of vastly different chemical compositions,
all alloys share at least three common features:
They have the potential to bond to dental
porcelain.
They possess co-efficient of thermal contraction
compatible with those of dental porcelains.
Their solidus temperature is sufficiently high to
permit the application of low-fusing porcelains.

133. The following high noble alloys are used
Gold based metal ceramic alloys
These have a gold content ranging up to 88%with
varying amounts of Pd, Pt and small amounts of
base metals. Alloys of this type are restricted to
Three unit spans, anterior cantileveror crowns.

134. Gold-Palladium Silver alloys
The gold based alloys contain between 39% and 77%
goldup to 35% palladium, and silverlevels as high as
22%.
Thesilverincreases the thermal contraction co-efficient
but it also has a tendency to discolorsome porcelains.

135. Gold-Palladium alloys
They have 44-55% of gold and 35-45% of Pd.
Used with porcelainshaving low co-efficient of
thermal contractiontoavoidthe development of
axial and circumferential tensile stressesin
porcelain during the cooling part of the porcelain
firing cycle.
More economical than high gold alloys.

136. NOBLE ALLOYS
These are Pd basedalloys.
These alloys were introduced in late 197O’s
The disadvantage was they had a tendency to
discolorthe porcelain during firing
This greenish-yellow discoloration, popularity
termed an “GREENING” is due to the silver
vapour that escapes from the surface of these
alloys during firing of the porcelain.

137. The silver vapour diffuses as ionic silverinto the
porcelain, and is reduced form colloidal metallic
silverin the surface of porcelain.
Some of the high palladium alloys develop a layer of
dark oxideon their surface during cooling from
the degassing cycle, and this layer has proven
difficult to mask by the opaque porcelain.

138. Compositionof Pd-Ag alloys fall within a narrow
range 53% to 61% palladium and 28% 40%
silver, Tin orindiumor both are usually added to
increase alloy hardnessand to promote oxide
formationfor adequate bonding of porcelain.

139. Palladium-Copper alloys
Comparable in cost to Pd-Ag alloys.
Susceptible to creepdeformation at elevated firing
temperatures, so attentionis given when these alloys
are used for long span FPD’s with small connectors.
Composition: 74-80% Palladium, 2-15% copper.
Porcelain discolorationdue to copper is not a major
problem.
These have 1145 Mpa of yield strength and hardness
values equal tobase metal alloys.
These have a poor potential for burnishingwhen the
marginal areas are thin

140. Palladium-Cobalt alloys
Comparable in cost to Pd-Ag alloys.
Often advertised as gold free, nickel free, beryllium
free.
These have a fine grain size to minimize hot tearing
during the solidification process.
It is the most sag-resistance of all noble alloys.
Composition: 78-88% of Pd and 4-10% of Co.

141. Palladium-Gallium-Silver and Pa-Gallium-Silver
Gold alloys
These are most recentalloys.
These have a slightly lighter coloredoxide than
the Pd-Cu or Pd-Co alloys and they are thermally
compatible with lower expansion porcelains.
Silver content is low(5%) and is inadequate to
cause porcelain greening.
Are compatiblewith lower expansion porcelains
such as vita porcelain.

142. Physical properties of high noble and noble alloys:
Should have a high melting rangeso that the metal
is solid well above the porcelain sintering
temperature to minimize distortionof casting
during porcelain application.
Must have considerably low fusing temperature.
Goodcorrosion resistance.
Highmodulus of elasticity.

143. Base metal alloys
Compared with ADA certified type IVgold alloys.
Cobalt based alloys, Nickel based alloys, and Pure
titanium have the following advantages.
1) Low cost
2) Low density
3) Greater stiffness
4) Higher hardness
5) High resistance to tarnish and corrosion.

144. Composition
Co-Cr53-67% of cobalt
25-32% of chromium
02-06 wt % molybdenum.
Ni-Cr61-81 wt % Nickel
11-27% chromium
02-05 wt of molybdenum.
Chromiumprovides passivationand
corrosion resistance.

145. Properties:
1)Higher hardness and stiffness.
2)More sag resistant at elevated temperatures.
3)It is improbable than significant occlusal wear of
these alloys occur. Therefore, particular attention
must be directed toward perfecting occlusal
equilibration.
4)It deforms only less than 25 µmwhen porcelain is
fired over it.

146. Metals for partial denture alloy
These are classified as:
High noble
Au-Ag-Cu-Pd
Noble
Ag-Pd-Au-Cu
Ag-Pd
Base Metal
Pure Ti, Ti-Al-V,
Ni-Cr-Mo-Be, Ni-Cr-Mo, Co-Cr-Mo.

147. Properties required
High tarnish -corrosion resistance
Should be easily castable
Good modulus of elasticity, which is a measure of
stiffness and rigidity. It helps in determining
thickness of various portions of framework.
Should have high strength and hardness.
Ductility should be higher which represents a
measure of amount of plastic deformation that a
denture framework can withstand before it
fractures.

148. WROUGHT BASE METAL AND GOLD ALLOYS:
When a casting is plastically deformedin any
manner, it is called wrought metal.
Wrought base metal alloys are used in dentistry,
mainly as wiresfor orthodonticsand as clasp arms
for removable partial dentures.
The alloys include:
Stainless steel : iron-chromium-nickel alloy
Co-Cr-Ni
Ni-Ti
β-Titanium alloys.

149. CARBON STEELS:
Steels are iron based alloysthat usually contain less
than 1.2% carbon.
The different classes of steels are based on three
possible lattice arrangementsof iron.

151. STAINLESSSTEEL
When 12-30% Cr is added to steel, the alloy is called asStainless steel

152. Ferritic stainless steel:
Often designated as American Iron and Steel
institute (AISI) series 400 stainless steels.
Good corrosion resistance.
Is nothardenable by heat treatment.
Limited application in dentistry.

153. Martensitic stainless steel:
Share the AISI 400designation.
Have high strength and hardness, so used for
surgical andcutting instruments.
Poorcorrosion resistance.

154. Austenitic stainless steel:
Most corrosion resistantof all.
AISI 302is basic type, containing 18% or 8% Ni
and 0.15% carbon.
Type 304has 0.08% of carbon.
Both are designated as 18-8 stainless steel
Type 316L (0.03% carbon) is ordinarily employed
for implants.

155. Generally austenite stainless steel is preferable to
ferritic because of the following characteristics.
1) Greater ductilityand ability to undergo cold work
without fracturing.
2) Substantial strengtheningduring cold working.
3) Greater ease of welding.
4) Ability to fairly readily overcome sensitization.
5) Less critical grain growth.
6) Comparative easein forming.

156. CORROSION RESISTANCE:
The 18-8 stainless steel may loseits resistance to
corrosionif it is heated between 400°C and 900°C.
The reason for a decrease in corrosion is the
precipitation of chromium carbideat the grain
boundaries at high temperature. The small, rapidly
diffusing carbon atoms migrate to grain
boundaries from all parts of the crystal to combine
with the large, slowly diffusing chromium atoms
at the periphery of the grain, where energy is
highest.

157. When chromium combines with the carbonin this
manner, its passivating qualities are lost, and, as a
consequence, corrosion resistanceof the steel is
reduced.
Because that portion of grain adjacent to grain
boundary is generally depleted to produce
chromium carbide, intergranular corrosionoccurs,
and a partial disintegrationof metal may result
with general weakening of structure.

158. STABILIZATION:
By adding Titanium(approximately 6 times of
carbon) precipitationof chromium carbide can be
inhibitedfor a short period at temperatures
ordinarily encountered in soldering procedures.

159. Soldering for stainless steel:
Silver soldersare used as their soldering
temperature islow. These are alloys of Ag, Cu,
and Znto which Sn, Inmay be added to lower
fusion temperatureand improve solderability.

160. COBALT-CHROMIUM-NICKEl ALLOYS:
Co-Cr-Ni alloys are used successfully in orthodontic
appliances.
These alloys were originally developed for use as
watch springs (Elgiloy).
COMPOSITION:
A representative composition by massis Co-40%,
Cr-20%, Ni-15%, Mo-70%, Mn-2%, C-0.16%,
Be-0.04%, Fe-15. 8%.

161. PROPERTIES:
Excellent resistanceto tarnish & corrosion.
Yield strength, hardness, tensile strength are
approximately equal to 18-8 stainless steel.
Ductility is greaterthan 18-8 stainless steel.
More responsiveto low temperature heat
treatment.

162. NICKEL-TITANIUM ALLOYS
Called as NITINOL
It has a large working range because of low
stiffnessin combination with moderately high
strength.

163. COMPOSITION
Ni-Ti alloysused in dentistrycontain
approximately 54% Ni, 44% Ti and 2% or less
cobalt.
This alloy can exist in various crystallographic
forms. At high temperatures, a BCC lattice
austentite phase occurs, on coolinga CH
Martensitic phase occurs.

164. These characteristics of the austentite to
martensite phase transition results in two unique
featuresof potential clinical relevance:
Shape memoryand Super elasticity.
Memory effect is achieved by first establishing a
shape at temperatures near 482°C.

165. If the appliance such as an orthodontic arch wire, is
then cooled and formed into a second shape and
heated through a lower transition temperature, the
wire will return into its original shape.
Inducing the austentite to martensite transition by
stress can produce super elasticity, a phenomenon
that is employed with some nickel-titanium
orthodontic wires and some endodontic files.

166. β-Titanium alloys
Like stainless steel and Nitinol, pure titanium has
different crystallographic formsat high and low
temperatures.
At temperatures lower than 885°C, the hexagonal
close-packed (HCP) or α-crystal lattice is stable,
where as at higher temperature, the metal re-
arranges into a BCC or β-crystal lattice.
An alloy with the composition of Titanium-11%,
molybdenum-6%, Zirconium-4%, tin is produced
in wrought wire formfor orthodontic applications.

167. Properties:
1)Low elastic modulus.
2)High ratio of yield strength to elastic modulus
produces orthodontic appliances that can sustain
large elastic activations.
3)Highly cold worked.
4)Excellent corrosion resistance and environmental
stability.

168. GOLD ALLOYS :
Gold wires are occasionally employedin the
construction of removable partial denture clasps
but used in fabricating orthodontic appliances, and
as retention pins for restorations.

169. COMPOSITION:
Many gold wires resemble the type IV gold casting
alloys in composition, but typically they contain less
gold.
Two types of gold wiresare recognized in ADA.
Specification No.7 (1984).
Type I-High noble or noble metal alloys, they must
contain at least 75%of gold and platinum group
metals.
Type II-High noble or noble metal alloys, that must
contain at least 65%of some noble metals.

170. GENERAL EFFECTS OF THE CONSTITUENTS:
Pt-Pd ensurethat wire does not meltor recrystallize
during soldering procedures.
Ensurea fine grain structure.
Cu-contributes to ability of alloy to age harden,
Ni-strengthener, reduces ductility.
Zn-scavenger.

171. MECHANICAL PROPERTIES OF NOBLE ALLOY
WIRES:
A wire of a given composition is generally superior
in mechanical properties to a casting of the same
composition.

172. Because:
Casting contains unavoidable porosity, which has
a weakening effect.
When cast ingot is drawn into wire, the small
pores and surface projections may be collapsed,
and welding may occur so that defects disappear.
Any defects of this type that are not eliminated
will weakenthe wire.
Because of Fibrous microstructure.

173. Silver-palladium alloys:
White in color
Predominantly silver in composition but have
substantial mounts of palladium, that provide nobility
and promote the silver resistance.
May or may not have copper and a small amount of
gold.
Disadvantages
Poor castability
Greater potential for tarnish and corrosion.

174. DENTAL IMPLANT MATERIALS:
Most commonly, metals andalloysare used.
Initially surgical grade stainless steeland Co-Cr
alloyswere used because of their acceptable
physical propertiesand relatively good corrosion
resistanceand biocompatibility.

175. STAINLESS STEEL: (S-S)
Surgical stainless steel is an iron-carbon (0.05%)
alloy with approximately 18% chromiumto impart
corrosion resistance and 8% nickelto stabilize the
austentite structure.
The alloy is most frequently usedin a wroughtand
heat-treated condition.
It has increased strengthand ductility; thus it is
resistant to fracture.

176. Co-Cr-Mo alloy
These are most often used in a castor cast and
annealed condition.
Composition63% of Co, 30% of Cr, 5% Mo
and small concentrations of C, Mn, Ni.
Molybdenumserves to stabilizethe structure, and
carbonas hardener.
These have outstanding resistance to corrosion.
These are least ductile.

177. TITANIUM
AND
Ti-Al-V ALLOY

179. Commercially pure Ti (CPTi) has become one of
the material of choicebecause of its predictable
interaction with the biological environment.
Titanium is a highly reactive material, it oxidizes
on contact with air or normal tissue fluids. This
reactivity is favourable for implant devices
because it minimizes biocorrosion.
An oxide layer10A°thick forms on the cut
surface of pure Ti within a millisecond. Thus, any
scratch or nick in the oxide coating is essentially
self healing.

180. Composition of alloyed Ti
Ti-90wt%
Al-6wt%
Va-4wt%

181. Properties:
High strength : weight ratio.
Modulus of elasticityapproximately one halfof
that of stainless steel or Cr-Co alloys.
Few titanium substructures are plasma-sprayedor
coated with athin layer of calcium phosphate
ceramic.

182. The rationalefor coating the implant with tricalcium
phosphate or hydroxyapatite, both rich in calcium
and phosphorus is to produce a bioactive surface
that promotes bone growthand induces a direct
bond between the implant and hard tissue.
The rationalefor plasma sprayedsurface is to
provide a roughened, biologically acceptable
surface forbone ingrowthto ensure anchoragein
the jaw.

183. OTHER METALS:
Gold, Palladium, Tantalum, Platinum and alloys of
these metals.
Recently Zirconium, Tungstenare used.

184. BIOCOMPATABILITY OF METALS:
Laboratory techniques performed with metals
may exposeus occasionally or routinely to
excessively high concentrations of Beryllium
andNickel dust and Beryllium vapour.

185. BERYLLIUM
Although the beryllium concentration in dental
alloys rarely exceeds 2 wt %the amount of
beryllium vaporreleased in to the breathing space
during melting of Ni-Cr-Be alloys may be
significantover an extended period.

186. The risk of Beryllium vapour exposure is greatest
for dental techniciansduring alloy melting
especially in the absence of an adequate exhaust
andfiltration system.
High levels of Beryllium have been measured
during finishing andpolishingwhen a local
exhaust system was not used. They were reduced
to levels considered safe when exhaust fanwas
used.

187. Exposure of beryllium may result in acute and
chronic forms of Beryllium disease
BERYLLIOSIS.
CLINICAL FEATURES:
Symptomsrange from coughing, chest pain and
general weakness to pulmonary dysfunction.
Contact dermatitis
Chemical pneumonitis

188. NICKEL:
It is a great concern to dental patients with a
known allergyto this element.
The cloud of controversy continues to hang
over the use of nickel in Dentistry.
Dermatitisresulting from contact with nickel
solutions was described as early as 1989.

189. Inhalation, ingestion and dermal contactof
nickel or nickel containing alloys are
common because nickel is found in
environmental sourcessuch as air, soil and
food as well as in synthetic objects such as
coins, kitchen utensils, and jewelry.

190. Nickel allergy was determined by PATCH TEST
(Luis-Blanco-Dalmau JPD 1982: 48; 99-101)
described a standard patch test consisting of 5%
Nickel sulfate solution or 5% Nickel sulfate
solution on a petrolatum base, in centre portion
of a square Band-Aid of good quality.

191. Band-Aids in position
One Band-Aid is removed. Observe for ++
Both the Band-Aids are removed for comparison
Magnified erythema,papules,and vesicles,+++

192. The patch is applied on medial aspect of upper arm,
which was cleaned with a alcohol swab, this is left
in place for 48 hrs undisturbed. The patient is
instructed not to moisten the arm or remove the
patch during this time. A Band-Aid without any
reagentis placed adjacent to the first acts a control.
After 48 hrs, the control Band-aid is removed. The
second Band-Aid is removed and the skin is
cleaned using alcohol or acetone, tests are read
after 20 min.

193. Signs for recording degrees of patch test reactions are :
0No reaction.
+Erythema.
++ Erythema, papules.
+++Erythema, papules, vesicles.
++++Marked edema with vesicles.

194. DIMETHYL GLYOXINE TEST :
FEIGL andSHOREstated that few drops of 1%
alcohol solution of dimethyl glyoxime, few drops
of ammonium hydroxideadded to a metallic
object, skin on solution will produce a strawberry
red insoluble saltin presence of nickel.

195. LAMSTER(1987), showed 2 cases
demonstrating Loss of alveolar bonearound Ni rich
nonprecious alloy and porcelain crown within 18
monthsof placement. Reason for this was thought
that the electrolysis of metalleading to corrosion
and bioviability of Nickel.

196. TIMOTHY. K. JONES(1986) stated that
incidence of Ni hypersensitivity was more in
women(l0 times more than men). The reason was
attributed to increased contactwith nickel plated
objects at home.

197. Dental implications of Nickel
hypersensitivity

198. JOHN. C. WATAHA(1998) stated that transient
exposureof casting alloys to an acidic oral
environment is likely to significantly increase
elemental releasefrom Ni alloys, but notfrom
high noble alloys.
J.GEIS-GERS(1993) -From point ofcorrosion
resistance Beryllium free Ni-Cr-Moalloys should
be preferred in clinical use.

199. Symptoms of sensitivityrange from urticaria :
Pruritis,
Xerostomia,
Eczema
Vesicular eruptions.

200. ReleaseofNi ions from dental alloysis high
enough to be clinically significant.
If so, potential alteration in endocrine functions,
changes in vital functionssuch as blood pressure,
pulse, temperaturemay be expected.
Ni containing alloyshas been linked to decrease
inlymphocytes in human.

201. CONCLUSION
Great variety of alloys currently available
can lead to uncertainty in choosing an
optimal alloyfor a given patient and
situation. In addition to working and
mechanical properties an important
consideration is given to corrosion
resistance.

202. So selection of a specific alloy should be
based on a balanced considerationof cost
and alloy propertiesrelevant to a particular
use of material.

203. REFERENCES
1. Science of dental materials-Anusavice, 11th Edn.
2. Restorative dental materials-Craig,10th Edn.
3. Dental biomaterials-E.C. Coombe.
4. Applied dental Materials-John F. Mc Cabe,7th Edn.
5. Dental materials, Properties & Manipulation-Robert
G. Craiget.al, 5th Edn.

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Journal of Prosthet Dent. 1987; 85: 1-5.
Journal of Prosthet Dent. 1986; 56: 507-509.
Journal of Prosthet Dent. 1982; 48: 99-101.

205. Dental materials. 1987; 3: 125-130.
Dental materials. 1993; 9: 177-181.
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