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Metallurgy
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1) Metal
Metal is the pure state are used much more
in dentistry than in most other arts or
industries. The pure metals that are
commonly used in dentistry are gold and
platinum, silver and copper titanium.
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Properties of metals:
1) Metals are elements that ionize positivelyin solutions.
2) They are solids at room temperature
(except Hg and gallium which are liquidsand H2 which is a gaseous metal).
Luster: due to reflection of light waves by the
free electrons ans most of them are silveryin color (except that, copper is red andgold is yellow)
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4) All metals conduct heat and electricity because
they have free electrons.
5) All metals have high strength, high hardness,and high melting temperature due to the metallic
bonding.
6) They are malleable (can be hammered into
sheets) and ductile (can be drawn into wires).7) Give a metallic ring when they are struck.
8) All metals have high density which is related to
the atomic weight and to the type of latticestructure that determines how closely the atoms
are packed.
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Shaping of metals:
1) Casting cast metal: this is performed by
melting the metal and shaping it in a
mould. In dentistry a molten metal is
poured into a mould made from a waxpattern embedded in an investment
material
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2) Cold working (wrought metals):
Metal can be hammered into sheets or
pulled through dies to form wires at roomtemp. most dental appliances are cast
structures, however orthodontic wires and
clasps of partial dentures are wroughtmetals.
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3) Sintering (powder metallurgy):
A metal powder can be pressed to produce
an object. The product of this method isweak as there is little adhesion between
the particles. The strength of the formed
object can be improved by pressing andheating it in a non oxidizing atmosphere
below the melting point of the metal to
agglomerate the particles and improveadhesion. Amalgam tablets are made by
sintering
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4) Electroforming :
The process of electrolysis is used to plate a
metal on a conducting surface e.g. silver
and copper plated dies.
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Cooling of molten metal
A
C
D
Temp B
F
Time
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If a metal is melted and then allowed to cool,and if its temperature during cooling, and if
its temperature during cooling is plotted asa function of the time, the following figureresults. As can be noted in the figure thetemperature decreases regularly from A to
B. An increase in temperature then occursto C at that time the temperature becomesconstant until the time indicated by D (C-D
is the horizontal or plateau portion of thecurve). After D the temperature decreaseto room temperature at E.
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The temperature T, as indicated by the horizontal
or plateau portion of the curve at C-D, is the
freezing or melting point.
N.B. 1) During this time C-D the metal is solidifyingand there is evolution of latent heat of fusion
which compensates for the heat loss.
2) The initial cooling to B is called super cooling
which is due to solidification and the release of
the latent heat of fusion.
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Mechanism of crystallization:
Solidification starts at special centers called nuclei ofcrystallization. Some of these nuclei may beimpurities which exist even in a pure metal. Growthof crystals form nuclei occurs in three dimensions
(up & down, anteroposteriorly and right to left) in theform of dendrites or branched structures (treelikebranches). Growth continues until contact is madewith adjacent growing crystals. Each nucleus givesrise to one crystal or grain. The grater the number of
nuclei present the faster the solidification will be, andthe smaller the size of each grain will be the tightlypacked crystals are called grains and theirboundaries are called grain boundaries
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The grain structure of the solidified
materialEach crystal of a metal is termed a grain. Each grain isgrown from a nucleus. Within each grain, theorientation of the crystal lattice is uniform. Adjacentgrains have different orientations, because the initial
nuclei acted independently from each other. In otherwords, each grain starts from a different nucleus ofcrystallization and each grain, therefore, has anorientation different from that of its neighbor. Thecrystals do not join at their meeting points because
their space lattices do not match space to space orrow to row. If they did match exactly as theyapproached each other, they would probably join toform a larger grain, or crystal.
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Examination of the grain structure:
The grains can be seen with a microscope andphotomicrograph can be made provided that themetal surface is properly prepared. The surfaceof the metal is flattened, polished, and thenetched i.e. treated with chemical agents, whichattack the grain boundaries of the metal morethan the grains themselves. This is becauseatoms at the grain boundaries are more reactive,
since they are not surrounded symmetrically byother atoms, as are the ones in the center ofgrain.
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Grain size :
There is an inverse relation between grain
size and strength i.e. the smaller the
grains are the stronger and the harder the
cast structure is. The size of the grainsdepends upon the number of nuclei at the
time of solidification. If the nuclei are
equally spaced, grains will beapproximately equal in size.
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The solidification proceeds from the nuclei in
all directions at the same time in the form
of sphere. When these spheres meet, theyare flattened along various surfaces.
However, the tendency for each grain to
remain spherical still exists, and the grain
tends to have the same diameter in all
dimensions. Such a grain is said to be
equiaxed (not elongation). Dental castings
generally tend to exhibit an equiaxed grainstucture.
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Factors affecting grain size
1) Rapid cooling produces more nuclei of
crystallization, thus more grains in a given
volume, and therefore each grain is
smaller.
2) Impurities or additives act as nucleating
agents hence, refining (decreasing) the
grain size .
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Factors affecting the grain size and
shape:
1) Rate of cooling:
Slow cooling results in the formation of a coarse
grain structure, whereas rapid cooling gives a
fine grain structure because it produces morenuclei of crystallization. Rapid cooling of a
molten metal is obtained in the following cases
(a) when a mould of high thermal conductivity is
used, (b) if the casting is small, and (c) if metal isheated just above its melting temperature.
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2) Nucleating agents:
Either impurities or additives can act as
nucleating agents, hence refining the grainstructure.
3) Cold working:
Drawing a cast metal into a wire transformsthe grain structure into a fibrous structure,
with high strength, high hardness but less
ductility (brittle), also internal stresses areinduced in the structure.
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4) Stress relief anneal (recovery):
The process of releasing internal stresses
by heating is called annealing. It is a lowtemperature which has little effect on thefibrous structure. A relief of the internalstresses will only occur.
5) Recrystallization:
Further heating of a cold worked materialcan change its elongated fibrous structure,
into fine grain structure of improvedproperties. The metal is said to have beenrecrystallized.
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6) Grain growth:
If a metal is over heated, or heated for a
longer time during recrystallization, grain
growth occurs with a very high ductility
and very low strength and hardness. This
must be avoided if high strength and
hardness are desired.
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Crystal imperfection
Real crystal structure usually contains a
variety of defects. Defect (point, line or
plane) in crystals have a considerable
effect on the properties of the metal oralloy.
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a) Point defects:
1- impurities: these can cause distortion of
the crystal lattice. Impurities mayinterstitial or substitution.
2- vacancies: these can allow atoms to
move in the crystal (solid state diffusion).
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b) Line defects (dislocation):
Dislocation is the movement of a row of
atoms along each other in the lattice. Thisdislocation moves across the crystal, as
show in A deforming it in a series of single
steps, and the dislocation finally movesout of the crystal.
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All the techniques used for improving the strengthof metal depend on the stop of the motion ofdislocations. Treatment, which will be discussed
later, including alloying, precipitation hardening,grain refining, and cold working, can stopdislocation movement. For example, metal aregrain refined to produce finer grain sizes. When
a dislocation moves through a grain-refinedmetal it will encounter more grain boundariesthan with a material with coarse grains.Dislocations become stuck on grain boundaries,
thereby preventing further dislocation motionand strengthening the metal occurs.C) plane defect: as grain boundaries.
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Deformation of metals:
At stresses below the proportional limit, the atomsin the crystal lattice are displaced in amount yet,when the stress is relieved, they can return totheir original positions (stretching of the bonds).
However, once the proportional limit isexceeded, a permanent deformation takes placeand the structure does not return to its originaldimensions when the load is released(dislocation) eventually, this displacement
becomes so great that the atoms are separatedcompletely and a fracture results (loss ofcohesion).
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Practical consideration
1) Cooling a molten metal should be done rapidlyto get a fine grain structure, if strength andhardness are important.
2) Cold working increases hardness and strength.However, this reduces ductility, so the materialbecomes more brittle. It becomes liable tofracture if further cold working is carried out,because the potential for further slip is lost.
3)cold worked structures should be annealed torelief stresses and thus increasing ductility.
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