metals and alloys 2
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what is an alloy
A combination (or mixture) of two or more metals Or Metal(s) with a metalloid (silicone, carbon)
what are the advantages of alloys
• Improved properties ○ Mechanical (EL, UTS, hardness) ○ Corrosion resistance ○ Lower melting point than an individual metal
what are the dental uses of alloys
• Steel ○ Burs ○ Instruments • Amalgam ○ Filling material ○ Special case • Gold alloy ○ Inlays ○ Crowns ○ Bridges ○ Partial dentures ○ Wires • Nickel chromium ○ Crowns ○ Bridges Wires
what does phase mean
Phase = physically distinct homogenous structure (can have more than one component
what is a solution
homogenous mixture at an atomic scale 2 metals co-exist in a lattice structure
if a lattice structure only has one type of metal in it how many phases are there
one phase
if a lattice structure has individual grains of metal A and metal B, how many phases are there
2 phases 2 grains, 2 metals but they do not co-exist in the same grain
if a lattice structure has 2 metals in a homogenous mixture how many phases are there
one phase
when are metals normally soluble
when molten
what are the 3 options for metals on crystallisation
On crystallisation, metals may: A) Be insoluble, no common lattice - 2 phases B) Form metallic compound with specific chemical formulation (eg Ag3Sn) - This is the case with amalgam C) Be soluble and form a solid solution, ie form common lattice • 3 types of solid solution • 2 metals are soluble in one another • 2 types of metals co-exist in the same lattice arrangement
what are the types of solid solution
substitutional > atoms of one metal replace the other metal in the crystal lattice interstitial > atoms markedly different in size
what are the types of substitutional solid solution
> random • metal atoms similar in size valency, crystal structure • can’t tell what the next atom in a row will be > ordered • metal atoms in regular lattice arrangement • can predict what the next atom in a row will be because there is an order to the lattice arrangement
what does interstitial solid solution consist of
atoms of different sizes smaller atoms are located in spaces in lattice / grain structure or larger atoms cannot predict where the small atom will be just know it wont be in the lattice structure position
how does the cooling curve of an alloy differ from the cooling curve of a metal
metal: crystallises at one temperature alloy: crystallises over a temperature range
what is TL on the alloy cooling curve
the temperature where crystallisation of the alloy begins
what is TS on the alloy cooling curve
the temperature when the crystallisation process is complete
what happens to soluble metals during the alloy cooling curve
solid solution formed (homogenous mixture of metals in each grain)
what happens to insoluble metals during the alloy cooling curve
grains of individual metals formed
what is a phase diagram
shows temperature Vs alloy composition
what is complete solid solubility
can have any combination of the metals different compositions of the 2 metals works
what is the liquidus
Line representing the temperature which different alloy compositions begin to crystallise
what is the solidus
Line representing the temperature which different alloy compositions have completely crystallised
what does slow cooling of molten alloy result in
> allows metal atoms to diffuse through lattice ensure grain composition is homogenous results in large grains
what does rapid cooling result in
> small grains = good mechanical properties > concentration gradient = more likely to corrode > causes coring - composition varies throughout the grain > prevents atoms diffusing through lattice > Generates many small grains which impede dislocation movement, improving its mechanical properties