alloys Flashcards
what are alloys
a combination of two or more metals
or a metal with a metalloid
Benefits of alloys
improved properties
o EL
o FS
o UTS
o Hardness
o More resistant to corrosion
o Lower melting point than individual metal
uses of alloys
steel
burs and instruments
gold alloy
inlays and crowns etc
cobalt chromium
dentures
nickel titanium
ortho wires
nickel chromium
crowns and bridge
amalgam
restoration
phase definition
Physically distinct homogenous structure
Can have more than one component
solution definition
Homogenous mixture at an atomic scale
pure metal has how many phases
1
an alloy with 2 insoluble metals has how many phases
2 phases
solid solution has how many phases
1
solid solution def
2 metals coexisting in a lattice network; both metals in a homogenous mixture
3 categories of alloys
- 2 phases (insoluble)
- intermetallic compound (partially soluble)
- 1 phase (solid solution)
solid solution has 3 types what r they
- substitutional random
- substitutional ordered
- interstitial
diff between cooling curve of metal and alloy?
pure metal Crystalises at one temperature
alloy Crystallizes over a range of temperatures (between TL and Ts)
liquidus def
line representing the temperature which different alloy composition begin to crystallise
solidus def
line representing the temperature which different alloy composition have completely crystallised
do you want fast or slow cooling
fast
benefits and disadvantages of fast cool
small grains good but coring occurs, can overcome with annealing
benefits and disadvantages of slow cool
homogenous good but large grains with poor properties
what determines the extent of coring?
the larger the difference between the liquidus and solidus line, means that there will be MORE CORING during rapid cooling
when does coring occur
fast cooling, and a difference between the liquidus and solidus state
why is coring bad
alloy becomes less resistant to corrosion
Benefits of annealing after coring
- makes the grains homogenous
- allows atoms to diffuse and rearrange
Benefits of small grains over large ones
- impede dislocation movement
- improve mechanical properties
why is there less dislocation movement in alloys compared to metals
- because solid solution has metals of different atomic size which means there is a distorted grain structure.
- distorted grain structure impedes dislocation movement and improves mechanical properties such as (EL UTS FS hardness)
-> defects have to “climb” over each atom and settle in between the metal atoms. it requires a lot of energy to climb over the next atom in the lattice plane. hence it takes more force to move the defect to the grain boundary. greater stress to move any dislocations, harder to distort, more fracture resistant and stronger than metal
what sort of alloy is Au-Cu
solid solution, completely soluble
what sort of alloy is Au-Pt
solid solution
what is eutectic alloy
an alloy with 2 metals that are completely insoluble so the metals exist in seprate grains
DO NOT FORM solid solution, that is for soluble metals
what sort of alloy is Ag-Cu
partially soluble
when does crystallisation occur for eutectic alloys
at a single temperature, where the liquidus line meets the solidus line, the grains of each metal form simultaneously
function of eutectic alloys
soldering alloys tgt since it has the lowest melting point of all alloys
properties of eutectic alloys
hard
brittle
poor corrosion resistance
How do you improve the mechanical properties of a partially soluble alloy
precipitation hardening = pushes the metals to the grain boundaries to make the alloy stronger and harder
what is the key point of partially soluble alloy graph with the solid solubility limits?
solid Solubility limit lines means alloys cannot form grains with the composition between H1 and H2 %
they can only form grains with composition below H1 or above H2
producing alpha or beta grains which are either silver rich or copper rich grains
