Alloys Flashcards
what is an alloy?
a combination of two or more metals or a metal with a metalloid
what are the advantages of an alloy?
improved properties (mechanical, corrosion resistance, lower melting point)
what are the uses of alloys?
burs, instruments, inlays, crowns, partial dentures, wires, bridges, restorative materials
what does phase mean?
physically distinct homogenous structure
what is solution
homogenous mixture at an atomic scale
what does it mean if a metal has one phase?
it is consisted of only one type of metal
what does it mean if an alloy has two phases?
individual grains of metal A and B situated in lattice network
what does it mean if an alloy has one phase?
metals A and B are in a homogenous mixture
what happens to metals when they are crystallising?
they can be insoluble and exit as 2 phases, or they can form an intermetallic compound with a specific chemical formulation or be soluble and form a solid solution
how many types of solid solution are there?
3
what are the 3 types of solid solution
random substitutional, ordered substiutional and interstitial
what do metals do in a substitutional solution?
atoms of one metal replace the other metal in the crystal lattice/grain
what are metals in a random substituional solid solution like?
they are similar in size, valency, crystal structure
what is the lattice like with ordered substiutional solutions?
it is regular lattice arrangement with one row of metal A and another row of metal B
what are the atoms like in an interstitial lattice?
atoms are markedly different in size, smaller atoms are located in spaces of the lattice/grain structure of larger atoms
what is the difference in the cooling curve between alloys and metals?
with metals, there is no change in temperature during the crystallisation period, with alloys there is a change in temperature over the crystallisation period
if a metal is soluble what type of solution is formed?
solid solution (homogenous mixture of metals in each grain)
if a metal is insoluble what is formed?
grains of individual metals formed
in the phase diagram what is the liquidus?
line representing the temperature which different alloy compositions begin to crystallise
in the phase diagram what is the solidus?
line representing the temperature at which different alloy compositions have completely crystallised
what does slow cooling of molten alloys do?
allows metal atoms to diffuse through lattice and ensures grain composition is homogenous but has larger grains
what is coring
when the alloy is cooled in layers and it is rapid
what does rapid cooling of molten alloys do?
prevent atoms diffusing through lattice, cause coring, composition of metals varies throughout grain
what are the necessary coring conditions?
fast cooling of liquid state, liquidus and solidus must be separated and determines extent of coring
what can coring do to corrosion resistance?
reduce it
what does fast cooling do?
generates many small grains which impedes dislocation movement and improves mechanical properties, causes coring which is undesirable
what is homogenous annealing
reheating an alloy to allow atoms to diffuse and so cause grain composition to become homogenous
what sort of grain structure do alloys forming a solid solution and consisting of different atomic size have?
distorted which impedes dislocation movement and so improves mechanical properties
what happens in a metal lattice with a defect?
the defect goes up and around the other metal atoms and slides along until it reaches the grain boundary, little energy is needed for it to move along the plane
what happens in an alloy lattice with a defect?
the defect goes up and over each atom but this is repeated for each atom there is instead of the defect rolling over al of the atoms so more energy is needed and hence greater stress to move dislocations
what happens once an alloy is cooled to room temperature
it needs to be annealed to remove coring
with eutectic alloys what are the grains like?
physically distinct
what is the eutectic composition?
where liquidus and solidus coincide (where crystallisation process occurs at a single temperature) and where grains of individual metals are formed simultaneously
what are the advantages of eutectic alloys?
lowest melting point, hard, poor corrosion resistance
what occurs during crystallisation with non-eutectic composition
excess metal crystallises first, then liquid reaches eutectic composition and both metals crystallise
give an example of a solid solution
gold platinum
give an example of a partially solid solubility
silver copper
what is the phase diagram like for partially solid solubility?
there are dashed lines of H1 and H2 (solubility limit lines)
what do solubility limit lines mean?
that alloys of silver copper cannot form grains with composition between H1 and H2
what are the two solutions formed on a partially solid solubility phase diagram for AgCu
one solid solution which is mostly Ag and another solid solution which is mostly Cu
what does a solubility limit line indicate?
that a range of compositions of Ag and Cu are not possible
what happens to partially soluble alloys during annealing?
precipitation hardening
what does precipitation hardening do?
makes the alloy stronger and surface harder
why do alloys have better mechanical properties than metals?
due to solution, order and precipitation hardening
