Alloy Crystallization

8/22/2019 Alloy Crystallization

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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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Alloy Crystallization