metals and alloys 2 Flashcards
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
what does coring result in
grains structures of different concentrations
what are coring conditions
• Fast cooling of liquid state
• Liquidus and solidus must be separated (see phase diagram) and determines extent of coring
○ Eg Au-Pt
• May reduce corrosion resistance of the solid form of the alloy
how is coring resolved
homogenising anneal
what is homogenising anneal
• Once solid cored alloy formed then reheat to allow atoms to diffuse and so cause grain composition to become homogenous
• Reheat alloy grains (but not to melting temperature) and this allows the atoms to move around and eliminate the core structure
○ Crucial process
• Note: keep below recrystallisation temperature, otherwise grains are altered
what is solution hardening
Alloys forming a solid solution and consisting of metals of different atomic size have a distorted grain structure
Which impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
explain dislocation movement in a metal lattice
defect rolls over the atoms in the lattice plane until it settles at the grain boundary
little energy / force is needed for the defect to move along the slip plane
explain dislocation movement in a solid solution
defect does not roll over the lattice plane, it has to climb over each atom before it will settle by the grain boundary
instead it falls into the larger space existing between large and small atom
more energy / force is needed for the defect to overcome the different sized atoms and move along lattice to the grain boundary
as greater stress is needed to move dislocations in solid solutions - alloys are more fracture resistant / stronger than metals
explain order hardening
Alloys forming an ordered solid solution (atoms distributed at specific lattice sites) have a distorted grain structure (eg Au-Cu)
Which impedes dislocation movement and so improves mechanical properties (EL, UTS, hardness)
what are the properties of eutectic alloys
• Metals are soluble in liquid state
• Metals insoluble in solid state (so 2 phases)
Ie each metal forms physically distinct grains
2 metals not soluble in one another form physically distinct grains
what is eutectic compostion
• Where liquidus and solidus coincide
○ Ie where crystallisation process occurs at a single temperature
• Where grains of individual metals formed simultaneously
what is binary eutectic
complete solid insolubility
becomes melting point of the alloy?
what are the advantages and disadvantages of eutectic alloys
Lowest melting point - at eutectic composition: used for solder
Hard but brittle
Poor corrosion resistance
what is non-eutectic composition
- Excess metal crystallises first
- Then liquid reaches eutectic composition
- And both metals crystallise (forming separate grains)
what happens in a partially soluble alloy
has a solubility limit line which indicates the range of compositions of metal a and metal B (corresponding to the horizontal section of the solidus [H1 to H2]) are not possible
has alpha and beta solid solutions
extremes - the alloy is usually very rich in one metal or the other
what is precipitation hardening
heat treatment process that helps make metals stronger. The process does this by producing uniformly dispersed particles within a metal’s grain structure that help hinder motion and thereby strengthen it
happens in partial soluble alloys
on annealing a super saturated alloy will undergo precipitation hardening
what has better mechanical properties - alloys or metals?
alloys - better fracture strength, ridgidity, elastic limit, surface hardness - due to > solution > order > precipitation hardening
how should cored structures be dealt with
removed by annealing
