Solidification and Microstructure Of Cast Dental Alloys

SOLIDIFICATION
AND MICROSTRUCTURE OF
CAST DENTAL ALLOYS
Dr.B.MUTHUKUMAR, MDS
HOD AND PROFESSOR
DEPARTMENT OF PROSTHODONTICS
GUIDED BY
Dr.K.MURUGESAN., MDS, PROFESSOR
Dr.PETER JOHN., MDS, READER
Dr.S.K.JAGDISH., MDS, SR. LECTURER
PRESENTER
Dr.P.VIVEK SHANKAR
PG FIRST YEAR
DEPARTMENT OF PROSTHODONTICS
SRM DENTAL COLLEGE AND HOSPITAL, RAMAPURAM

OVERVIEW
 Introduction
 Definition - Metals
 Metallic bonds
 Alloys – Definition, Classification
 Solidification
 Cooling Curve
 Microstructure
 Nucleus formation
 Solidification modes and effect on properties
 Crystallization
 Dendrite formation
 Grain size
 Grain boundaries
 Recrystallisation and grain growth
 Summary
 References

INTRODUCTION
In dentistry, metals represent one of the three major classes of materials used
for the reconstruction of damaged or missing oral tissues. Although metals
are readily distinguished from ceramics and polymers.
O’Brien WJ. Dental Materials and their selection, 3rd edition; Quintessence 2002, p-1.

METAL
• DEFINITION
• The Metals Handbook (1992) defines a metal as “an opaque
lustrous chemical substance that is a good conductor of heat and
electricity and, when polished, is a good reflector of light.”
• GPT 9
“any strong and relatively ductile substance that provides
electropositive ions to a corrosive environment and that can be
polished to a high luster ; characterized by metallic atomic
bonding”
Metals Handbook, Desk Edition. Metals Park, OH, American Society of Metals, 1992.

Periodic Chart of the Elements. (From Burtis, CA, and Ashwood, ER: Tietz Fundamentals of Clinical Chemistry, 5 ed.
Philadelphia, WB Saunders, 2001.)

METALLIC BONDS
• Chemical bonding due to the electrostatic attractive force
between conduction electrons and positively
charged metal ions.
• It may be described as the sharing of free electrons among
a lattice of positively charged ions (cations)
Ferracane JL. Materials in dentistry, Principles and applications 2nd edition; Lippicolts Williams & Wilkins 2001 ,p 18

Strength
Malleability
Ductility
Thermal and
electrical
conductivity
Opacity
Luster.
conductors
of both
thermal
energy and
electricity
Free electrons

• These characteristics are not found in ceramic
and polymeric materials in which the atomic
bonding occurs through a combination of the
covalent and ionic modes.

ALLOYS
• DEFINITION GPT 9
• “a mixture of two or more metals or metalloids
that are mutually soluble in the molten state;
distinguished as binary, ternary, quaternary, etc.,
depending on the number of metals within the
mixture; alloying elements are added to alter the
hardness, strength, and toughness of a metallic
element, thus obtaining properties not found in a
pure metal”;

CLASSIFICATION
ADA CLASSIFICATION OF THE DENTAL
CASTING ALLOY
• High noble alloys “precious metals”
– at least 60% noble. 40% of which is gold. The
remaining 40% is base metal
• Noble alloys(semiprecious)
– at least 25% noble (no gold requirements). 75%
base metal
• Base metal alloys
– Less than 25% noble
Watcha J C Casting alloys Dent Clin N Am 2004 (48); 499–512.

CLASSIFICATION
BASED ON MAJOR ELEMENT

CLASSIFICATION
THREE PRINCIPAL ELEMENTS
• Gold(Au)-palladium(Pd)-silver(Ag)
• palladium(Pd)-silver(Ag)-tin(Sn)
• nickel(Ni)-chromium(Cr)-Be
• co-cr-Mo; Ti-Al-V; Fe-Ni-Cr.
Watcha J C Casting alloys Dent Clin N Am 2004 (48) 499–512

CLASSIFICATION
DOMINANT PHASE SYSTEM
Single Phase Eutectic Phase
Peritectic Phase Intermetallic Phase
McCabe JF, Walls AG; Applied Dental Materials 9th edition, Blackwell; p 56

WHY STUDY SOLIDIFICATION?
– It affects properties of material
– Most metals are made through casting
– For process and quality control during casting
– For controlling phases in material

MELTING POINT
VS
MELTING RANGE

PHASE DIAGRAM
• Phase diagrams are "maps" of the phases that
occur when metals are mixed together.
• Binary phase diagram
• Ternary phase diagram

PHASE DIAGRAM
Craig RG, Powers JM, editors. Restorative dental materials. 11th edition. St. Louis: Mosby; 2002. p. 172

• Line ACB - Liquidus.
• Line ADB – Solidus.
• At any composition between these extremes,
the melting range is defined as the
temperature difference between the liquidus
(ACB) and solidus (ADB).
Craig RG, Powers JM, editors. Restorative dental materials. 11th edition. St. Louis: Mosby; 2002. p. 172

SOLIDIFICATION OF METALS
• Solidification is the process of material transforming
from liquid to solid state.

During solidification cast form
develops cohesion and acquires
structural characteristics.
As soon as the metal is molten the
process of solidification starts

The fusion temperature of metals and alloys and their
solidification behaviour are important to us.
Typically an exact wax or plastic replica of the prosthesis form is
prepared initially.
Anusavice KJ. Phillips’ science of dental materials. 11th ed. Philadelphia: WBSaunders; 2002. p. 112
Prosthodontic considerations

Using highly accurate dental investment ,an expanded mold is
prepared from the pattern ,into which molten alloy is cast under
pressure.

When the alloy solidifies it shrinks and the
original pattern is reproduced as cast metal structure
Anusavice KJ. Phillips’ science of dental materials. 11th ed. Philadelphia: WBSaunders; 2002. p. 112

SOLIDIFICATION DEFECTS
SHRINKAGE
• Most materials contract
or shrink during
solidification and
cooling.
• Shrinkage can sometimes
cause cracking to occur
in component as it
solidifies.
GAS POROSITY
• Many metals dissolves a
large quantity of gas
when they are liquid.
• However when metal
solidify they retain only a
small part of the gas. But
these form bubbles
trapped in the solid metal
producing gas porosity.

 The mode of solidification affects properties of casting and acquires
metallurgic structures which is determined during solidification.
METALLIC STRUCTURE
Grain size,shape and orientation
Distribution of alloying elements
Underlying crystal structure and
its imperfections

STEPS
• STEPS IN SOLIDIFICATION :
– Liquid state
– Nucleation
– Crystallization.
– Grain growth

COOLING CURVE
Plotting of temperature during cooling as a function of time as a graph.
McCabe JF, Walls AG Applied Dental Materials 9th edition, Blackwell 2003

INTERPRETATION
• straight or plateau portion of the curveTf
• freezing point/solidification temperature of pure
metal/fusion temperatureBC
• It is defined as the number of calories of heat
liberated from 1 gram of a substance when it
transforms from the liquid state to the solid state.
Latent heat of
solidification
Anusavice KJ. Phillips’ science of dental materials. 11th ed. Philadelphia: WB Saunders; 2002. p. 112

• -
• It is important to emphasize that supercooling
of pure metals only occurs in clean and inert
containers.
Anusavice KJ. Phillips’science of dental materials. 11th ed. Philadelphia: WB Saunders; 2002. p. 112
• The initial cooling of the
liquid metal from Tf to
point B’.
Supercooling

NUCLEATION
• Nucleation : Formation of a centre around which further
crystallization takes place is called nuclei and process of its
formation is called nucleation.
– It largely depends on critical radius of nucleation
– Nuclei with smaller than rc are less likely to form crystal
and are called embryos. They may get dissolve in liquid
again.
– Generally nucleation is combination of both types.
Craig RG, Powers JM, editors. Restorative dental materials. 11th edition. St. Louis: Mosby; 2002. p.
163–84.

TYPES
• HOMOGENEOUS NUCLEATION
– Nucleation takes place throughout
material simultaneously
– Nucleation sites are uniform
throughout material
– Takes place due to under
cooling/slow cooling.
– Most of solidification takes place
through it.
Craig RG, Powers JM, editors. Restorative dental materials. 11th edition. St. Louis: Mosby; 2002. p. 163–84.

• HETEROGENEOUS NUCLEATION
– Nucleation takes place randomly due
to supercooling.
– Takes place at mould melt interface
surface of melt and impurities.
In this way,
 imperfections in the mould walls
Particles of dust
Other impurities in molten metal can produce heterogenous nucleation.
Craig RG, Powers JM, editors. Restorative dental materials. 11th edition. St. Louis: Mosby;
2002. p. 163–84.

FREE ENERGY FORMATION
Free energy of formation of a nucleus as a function of its radius. Fv is the volume free energy
of the embryo, Fs is the free surface energy, and R is the resultant free energy

Surface
free
energy
(FS)
Volume
free
energy
(FV)
Overall
free
energy of
embryos
• increases as the square of
the embryo radius.
SURFACE FREE
ENERGY (FS)
(Positive)
• varies as the third power of
the spherical embryo radius.
VOLUME FREE
ENERGY Change (FV)
(Negative)

at small values of
embryo radius, FS is
dominant and the overall
free energy for the
formation of the embryo
is positive (energetically
unfavorable).
At larger values of the
embryo radius, FV
becomes dominant and
the overall free energy of
the embryo is negative
(energetically favorable)

• critical nucleus size, designated as ro corresponds
to the maximum point in the total free energy of
the embryos as a function of radius.
• For an embryo of radius (ro), the overall free
energy (R) decreases with the addition of another
atom and continues to decrease as the embryo
grows.

It follows that the greater the amount of
supercooling, or equivalently, the greater the
rate of temperature reduction below Tf, the
smaller is the critical radius ro, because the
value of FV for a given embryo size becomes
increasingly negative. (The value of FS per
unit area is not greatly affected by the
amount of supercooling.)

CRYSTALLISATION
Pure metal may crystallise in three branch pattern
from nucleus such formations are called dendrites.
Noort RV. Introduction to Dental Materials 2nd edition, 2002; p – 25.

It usually occurs during solidification of alloys
because of constitutional super cooling.
The dentritic structure can be seen after etching and
polishing of alloy.
Extensions or elevated areas
(termed protuberances) form spontaneously on the
advancing front of the solidifying
metal and grow into regions of negative temperature
gradient
CRYSTALLISATION

• The latent heat released by the solidifying metal also
lowers the amount of supercooling at the liquid-solid
interface, hindering growth in regions adjacent to the
protuberances and resulting in separated, highly
elongated crystals.

• Dendritic
microstructure
Base
Metal
Alloys
• Equiaxed grain
structure
Noble
Metal
Alloys

In crystallisation
growth starts
from centre of
nuclei and
crystals grow
towards each
other.
When two or
more crystals
collide their
growth is
stopped.
Finally the entire
space is filled
with crystals.

• Brantley WA, Cai Z, Vermilyea SG, Papazoglou E et al: Effects of
solidification conditions and heat treatment on the microstructure and Vickers
hardness of Pd-Cu-Ga dental alloys. Cells Mater 6:127, 1996
This article shows that the incidence of hot tears in a
high–palladium alloy with a dendritic as-cast
microstructure is much greater when the alloy is
quenched rather than benchcooled.
The path of crack propagation is between adjacent
interdendritic regions.

GRAINS
Grains are important for the physical properties of
alloys because grain size influences other clinical
properties.
When a molten alloy cools to the solid state, crystals
form around tiny nuclei (clusters
of atoms).
Wataha JC. Alloys for prosthodontic restorations. J Prosthet Dent 2002;87:351–83.

As the temperature drops, these crystals
grow until the crystal boundaries meet each other in the
solid state. At this point, each crystal is called a grain and
the boundaries between crystals are grain boundaries.
Wataha JC. Alloys for prosthodontic restorations. J Prosthet Dent 2002;87:351–83.
GRAIN BOUNDARIES
The size of the grains
depends on the cooling
rate, alloy composition,
presence of grain refiners,
and other factors.

GRAIN BOUNDARIES
• Grain boundaries form a natural barrier to the
movement of dislocations.
• The concentration of grain boundaries increases as
the grain size decreases.
• Metals with finer grain structure are generally harder
and have higher values of yield stress than those with
coarser grain structure.

GRAIN REFINERS
• In some alloys, fine particles of a high melting point
element such are Ir are added to encourage even nucleation
throughout the alloy. These particles, used in this manner,
are called grain refiners.
• Process of reducing the crystal (grain) size in a solid metal
by adding an element or compound to the molten metal and
cooling at a prescribed rate.
• Iridium, Ruthenium or Rhenium for noble metal alloys

GRAIN REFINERS
• For dental base metal casting alloys, in which nickel,
cobalt, iron and titanium are the principal elements,
the use of grain-refining elements has not been
reported.
• Fine-grained (equiaxed) alloys are generally more
desirable for dental applications because they have
more-uniform properties

Zhu J, Kamiya A, Yamada T, Shi W, Naganuma K,Influence of
boron addition on microstructure and mechanical properties of
dental cast titanium alloys, In Materials Science and
Engineering: A, Vol 3, 2003; 53-62.
It has been shown that a small amount of boron addition
induces a significant refinement of as-cast structure and
improvement of mechanical properties.. This is primarily
due to the role of borides precipitated at the prior β
boundary and refinement of the prior β grains. Cast Ti–B
alloys with a good combination of greater tensile ductility
and strength can be obtained with very low boron addition.

Under normal conditions, the grain
structure of alloys is not visible. Special
acid etching and magnification are
generally necessary to view grains.

Alloy microstructure is viewed by polishing the alloy
surface, then etching with an acid to bring out relevant
features.
A series of successively finer abrasives (typically aluminum
oxide or silicon carbide) are employed.
Initial grinding stages - abrasives embedded in polishing
papers
Later polishing stages - slurries of water and abrasive
powders.
Final polishing stage, an abrasive with a particle size of
0.05 μm is used, since the width of the resulting scratches
will be about an order of magnitude smaller than the
wavelength of visible light, and thus they will not be
visible by eye or in the optical microscope.

The chemical or electrolytic etching medium
preferentially removes atoms and creates grooves
at the grain boundaries, because these atoms are in
a less regular arrangement and have higher energy
compared with atoms in the interiors of grains.
As a consequence, the grain boundaries have a
darker appearance than the bulk grains in the
optical microscope because of light scattering by
these grooves

• Gupta S, Mehta AS. The effect of remelting various combinations of new
and used cobalt–chromium alloy on the mechanical properties and
microstructure of the alloy. Indian J Dent Res 2012;23:341-7.
.
Repeated remelting of base metal alloy for dental casting
without addition of new alloy can affect the mechanical
properties of the alloy. Microstructure analysis shows
deterioration upon remelting. However, the addition of
25% and 50% (by weight) of new alloy to the remelted
alloy can bring about improvement both in mechanical
properties and in microstructure

GRAIN SIZE
The linear intercept method can be used to estimate the grain size of an
alloy.
Random lines of known length, such as 10 cm, are placed on
a series of photomicrographs obtained at a standardized
magnification for the polished and etched alloy.

Number of grain boundaries intercepted per centimeter.
Total number of grain boundary intersection points
The total length of the lines used.
X Magnification
The reciprocal of this number is used as a
measure of the grain size, which is usually
expressed in microns

GRAIN SIZE
• A fine grain structure can be achieved by rapid cooling of the molten
metal or alloy following casting. This process, often referred to as
quenching, ensures that many nuclei of crystallization are formed,
resulting in a large number of relatively small grains.
• Slow cooling causes relatively few nuclei to be formed which results
in a larger grain size.
• Some metals and alloys are said to have a refined grain structure.
• Fine grain structure is achieved by seeding the molten material with
an additive metal which forms nuclei for crystallization.
Nielsen JP, and Tuccillo JJ: Grain size in cast gold alloys. J Dent Res 45:964, 1966

• The atoms within each grain are arranged in a regular three-
dimensional lattice. There are several possible arrangements
such as cubic, body-centred cubic and face-centred cubic

Optical microscopic image of polished
and etched palladium based alloy with
dentric as cast microstructure.
Carr AB,Brantely WA .New high palladium casting alloys part 1:overview and initial studies Int J
prosthodont 4:265,1991

SEM image of fracture surface of cast base metal
Alloy for removable partial denture frameworks
Showing crack propagation in dentric
microstructure

SEM image of etched and polished high palladium
alloy with an equiaxed fine grain.

COLD WORKING
• When the metal is to be used for wires, bands, bars, or other
types of wrought structures, it is first cast into ingots that are
then subjected to rolling, swaging, or wire-drawing operations
that produce severe mechanical deformation of the metal.
• Such operations are described as hot or cold working of the
metal, depending on the temperature at which the operation is
performed.
McCabe JF, Walls AG Applied Dental Materials 9th edition, Blackwell ,p 55

• When this metal is subjected to cold-working operations, such
as drawing into a wire, the grains are broken down, entangled
in each other, and elongated to develop a fibrous structure or
appearance that is characteristic of wrought forms.
Craig RG, Powers JM Restorative Dental Materials, 11th edition Mosby 2002; p 176

Low-power light microscopic view of a wrought wire microstructure.
The grains visible have been broken apart and tangled among one another.
The entangled grains are lined up along the axis of the wire.
This type of microstructure is called a fibrous structure for obvious reasons.

ANNEALING
• Controlled heating and cooling process designed to produce
desired properties in a metal.
• The annealing process usually is intended to soften metals, to
increase their plastic deformation potential, to stabilize shape,
and to increase machinability

RECRYSTALLISATION AND GRAINGROWTH

• Recrystallization - Process of forming new stress-free crystals
in a work-hardened metal through a controlled heat-treatment
process

A, The fibrous microstructure is present and
arrows indicate residual stresses.
B, Minimal heat leaves the fibrous structure
intact but relieves the stresses. However, the
lattice remains distorted.
C, Annealing with more heat allows the lattice
deformation to be relieved. The fibrous
microstructure remains.
D and El Further heating causes a loss of the
fibrous structure and growth of the grains,
which increase in size with
increasing application of heat.

DENTAL APPLICATIONS
• Orthodontic wires
• Clasps for removable partial dentures
• Root canal files and reamers
• Crowns in pediatric dentistry
• Surgical instruments.

References
• O’Brien WJ. Dental Materials and their selection, 3rd edition; Quintessence
2002, .
• Craig RG, Powers JM Restorative Dental Materials, 11th edition Mosby 2002.
• McCabe JF, Walls AG Applied Dental Materials 9th edition, Blackwell 2008.
• Anusavice KJ. Phillips’ science of dental materials. 11th ed. Philadelphia: WB
Saunders; 2002.
• Metals Handbook, Desk Edition. Metals Park, OH, American Society of
Metals, 1992.
• Noort RV. Introduction to Dental Materials 2nd edition, 2002.

• Periodic Chart of the Elements. (From Burtis, CA, and
Ashwood, ER: Tietz Fundamentals of Clinical Chemistry, 5 ed.
Philadelphia, WB Saunders, 2001.)
• Carr AB,Brantely WA .New high palladium casting alloys part
1:overview and initial studies Int J prosthodont 4:265,1991.
• Nielsen JP, and Tuccillo JJ: Grain size in cast gold alloys. J
Dent Res 45:964, 1966
• Wataha JC. Alloys for prosthodontic restorations. J Prosthet
Dent 2002;87:351–83.

• Watcha J.C casting alloys Dent Clin N Am 48 (2004)
• Gupta S, Mehta AS. The effect of remelting various combinations of new and
used cobalt–chromium alloy on the mechanical properties and microstructure
of the alloy. Indian J Dent Res 2012;23:341-7.
• Ayyıldız S, Soylu EH, İde S, Kılıç S, Sipahi C, Pişkin B, Gökçe
HS. Annealing of Co-Cr dental alloy: effects on nanostructure and Rockwell
hardness. J Adv Prosthodont. 2013 Nov;5(4):471-478
• Zhu J, Kamiya A, Yamada T, Shi W, Naganuma K,Influence of boron addition
on microstructure and mechanical properties of dental cast titanium alloys, In
Materials Science and Engineering: A, Vol 3, 2003; 53-62

Solidification and Microstructure Of Cast Dental Alloys

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