metals and alloys 2 Flashcards
what are the advantages of alloys over metals?
improved mechanical properties
-EL, FS, hardness
corrosion resistance
lower melting point
what are dental uses of alloys?
steel- burs, instruments gold- inlays, crowns, bridges, RPDs, wires CoCr- partial dentures nickel-titanium- wires nickel chromium- crowns, bridges, wires amalgam -restorative
what is a phase?
physically distinct homogeneous structure
what is a solution?
homogenous mixture at an atomic scale
describe the solubility of molten metals?
any 2 metals are soluble when molten
what state can 2 metals be on crystallisation?
insoluble- no common lattice- 2 phases
intermetallic compound with specific chemical formulation
soluble= form solid solution, common lattice
how many phases if grains of metal A only?
one phase
how many phases if individual grains of metal A & metal B in lattice?
2 phases
as grains are insoluble
how many phases if metals A & B in homogeneous mix?
one phase
co-existing in same lattice
what are the 2 types of solid solution?
substitutional
interstitial
what is a substitutional solution?
atoms of one metal replace the other metal in the crystal lattice/grain
what are the 2 types of substitutional solution?
random
ordered
describe a random substitutional solution
metal atoms similar in size, valency, crystal structure
can substitute randomly
in lattice arrangement with no order- random
describe an ordered substitutional solution
metal atoms in regular lattice arrangement
similar valency, crystal structure, size
ordered- predict next atom in lattice
what is an interstitial solution?
atoms markedly different in size
smaller atoms located in spaces in lattice/grain structure of larger atom
-small atoms have no specific location
-squeezed into arrangement of ordered larger atoms
what happens at the plateau of cooling metal?
crystallisation happens
when crystallisation ends temp will resume cooling
how does crystallisation happen when cooling an alloy?
temp reaches set temp
continues cooling at slower pace
first few atoms cooled sufficiently to form individual nuclei of crystallisation-grows
-2 metals w/ dif melting points so temp range not point
what does the liquidus line on a phase diagram represent?
temp where different alloy compositions begin to crystallise- above line = liquid
what does the solidus line on a phase diagram represent?
temp at which dif alloy compositions have completely crystallised
what does slow cooling of molten alloy achieve?
metal atoms diffuse through lattice
ensure grain composition is homogeneous
results in large grains
-only small grains can be used in cold working
what is coring?
rapid cooling
ensure small grains
results in 4 layers of different concentration
what are the conditions for coring?
fast cooling of liquid state
liquidus and solidus must be separated and determines extend of coring
what is a negative consequence of coring?
may reduce corrosion resistance of the solid form of alloy
what is homogenising anneal?
once solid cored alloy formed
reheat to allow atoms to diffuse and so cause grain composition to become homogeneous
-has to stay below recrystallisation temp otherwise grains altered
what is solution hardening?
alloys forming a solid solution and consisting of metals of dif atomic size have a distorted grain structure
impedes dislocation movement so improved mechanical properties
-EL, UTS, hardness
describe slip in an alloy with metal atoms samesize
stress/force from LHS applied
defect undergoes slip and moves along lattice plane
needs to get around adjacent metal atom
defect slides along along lattice plane until reaches grain boundary
little force needed- rolls over atoms
describe slip in an alloy with a defect
metals dif size
distorts regular lattice array
for defect to undergo slip, needs to get round adjacent atom-climbs
settles between small & large atoms
more energy needed to overcome next atom etc. until reaches grain boundary
why is requiring more stress to move dislocations an advantage?
makes alloy more fracture resistant (strong) than metals
what is order hardening?
alloys forming ordered solid solution have distorted grain structure
impedes dislocation movement so improved mechanical properties
what are the eutectic alloys?
metals soluble in liquid state
metals insoluble in solid state
each metal forms physically distinct grains
what are the properties of eutectic alloys?
lower melting point
hard but brittle
poor corrosion resistance
what is eutectic composition?
where liquidus and solidus coincide
-crystallisation process occurs at single temp
where grains of individual metals formed simultaneously
what is non-eutectic composition?
excess metal crystallises first
then liquid reaches eutectic composition
both metals crystallise forming separate grains
when does precipitation hardening occur?
in partially soluble alloys on annealing, a supersaturated alloy undergoes precipitation hardening
what is the result of precipitation hardening?
stronger & surface harder
