metals and alloys Flashcards
give some dental uses of metals and alloys?
partial denture framework (CoCr) crowns (stainless steel) denture base (stainless steel) orthodontist appliance (NiTi) restorations (amalgam)
an aggregate of atoms in a crystalline structure is a?
metal
a combination of metal atoms in a crystalline structure is an?
alloy
crystal growth of equal dimension in each direction is known as?
equi-axed grains
regions of a crystal where grains make contact are known as?
grain boundaries
a single crystal with atoms orientated in a given direction is a?
grain
why are small fine grains advantageous ?
high elastic limit increased UTS (ultimate tensile strength) and hardness
why are small fine grains not advantageous?
decreased ductility
does fast cooling give small fine grains or large course grains?
small fine grains
does slow cooling give small fine grains or large course grains?
large course
imperfections/defects in the crystal lattice are known as?
dislocations
which technique allows dislocations to be collected at the grain boundary?
cold work
how does cold work result in stronger/harder material?
results in dislocations being collected at the grain boundary
what techniques does cold working involve?
benign and rolling a metal at low temperature
what effect does cold working have on elastic limit (EL), ultimate tensile strength (UTS) and hardness?
increase all
what effect does cold working have on the ductility of a metal?
reduced ductility
cold working increases residual stress causing distortion of the metal over time, how is this relieved?
annealing process
briefly describe what the annealing process involves?
heating a metal/alloy so that greater thermal vibrations allow migration of atoms
name the 3 stages of annealing?
recovery
recrystallisation
grain growth
describe the purpose of the recovery stage of annealing?
eliminates residual stress caused from cold working allowing atoms to rearrange within grains
what happens during the recrystallisation stage of the annealing process?
- cold work material is subject to a temp above recrystallisation temp
- this causes new smaller grains, lowers EL, UTS and hardness while increasing ductility
- spoils the benefits of cold work allowing more cold work
what happens during the grain growth stage of the annealing process?
excessive temp rise causes large grains to replace smaller ones
a combination of 2 or more metals or metals within a metalloid is known as?
an alloy
list the advantages of an alloy compared to a metal?
improved mechanical properties (EL, UTS and hardness)
more corrosion resistant
lower melting pt than individual metal
give some dental uses of alloys?
steel burs and instruments
amalgam
gold alloy crowns/bridges
what is a phase in relation to an alloy?
a physically distinct homogenous structure
a homogenous mixture at an atomic state is known as a ?
solution
how many phases does a solid solution have?
1
name the 3 types of solid solution?
random substitutional solid solution
ordered substitutional solid solution
interstitial solid solution
smaller atoms located in lattice/grain stricture of larger atoms describes which type of solid solution?
interstitial
which crystallises at a single temperature, an alloy or a pure metal?
pure metal
which crystallises over a temperature range, an alloy or a pure metal?
alloy
the line representing the temperature by which an alloys composition begins to crystallise is known as?
liquidus
the line representing temp by which an alloys composition has completely crystallised is known as ?
solidus
which type of alloy cooling, slow or fast, results in a desirable homogenous grain composition but undesirable large grains ?
slow cooling
do we cool alloys slowly or rapidly?
rapidly
does rapid cooling of an alloy result in a homogenous grain structure?
no
a defect in an alloy caused by cooling too quickly is known as?
coring
how can coring of an alloy be reversed?
homogenous annealing
which requires little energy/force, dislocation of metal lattice or dislocation of alloy?
metal lattice
describe dislocation of a defect in an alloy?
defect climbs over each atom and settles in-between metal atoms until it reaches grain boundary, this requires a lot of energy
a mix of metals having melting pt lower than any of its components is known as?
eutectic alloy
what are the ideal properties for a partial denture alloy?
rigid
strong
hard
ductile
for the denture base, do you want high or low youngs modulus and why?
high to maintain shape in use
for the denture clasps, do you want high or low youngs modulus and why?
low to allow flexure over tooth
