Metals and Alloys 2
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what is an alloy?
an alloy is a combination of: - two or more metals OR - a metal with a metalloid
what are some advantages of alloys > metals?
• higher elastic limit stress • high fracture strength • more rigid • corrosion resistance • lower melting point than individual metal
what are some dental uses for alloys?
STEEL = burrs, instruments GOLD ALLOY = inlays, crowns, bridges, partial dentures, wires COBALT CHROMIUM = partial dentures AMALGAM = restorative material
what is meant by ‘phase’ in reference to alloys?
a physically distinct homogenous structure (can have more than one component)
what is meant by solution when in reference to alloys?
homogeneous mixture at an atomic scale
on crystallisation of alloys, the two metals may take what structures?
• be insoluble (exist as two phases) • form an inter metallic compound with a specific chemical formulation • be soluble & form a solid solution (form a common lattice)
what is a substantial solid solution? what are the classifications of this type?
atoms of one metal replace the other metal in the crystal/lattice grain: • Random • Ordered
what are the different types of solid solutions?
• substitutional 2. interstitial
how can the grain structure of a random substitutional solid solution be described?
unpredictable/no order - the metal atoms are randomly positioned within the grain
how can the grain of an ordered substitutional solid solution be described?
order is predictable
what is an interstitial solid solution?
solid solution in which the atoms are markedly different in size
how can the grain structure of an interstitial solid solution be described?
smaller atoms are located in spaces in the lattice/grain structure of larger atom
how does the cooling curve of a pure metal differ to that of an alloy?
pure metal = when crystallisation happens, the temperature steadies and doesn’t change (crystallises at one temp) alloy = crystallisation takes place while the temperature changes (over a temperature range)
what does the liquidise line on a phase diagram represent (in reference to alloys)?
line representing the temperatures which different alloy compositions begin to crystallise
what does the solidus line represent on a phase diagram (in reference to alloys)?
line representing the temperatures which different alloy compositions have completely crystallised
what occurs if an alloy is cooled slowly?
• allows metal grains to diffuse through lattices • ensures grain composition is homogeneous • BUT this causes large grains (undesirable)
what is coring in reference to alloys?
a defect in an alloy caused by cooling the alloy too fast
what does rapid cooling of a molten alloy result in?
• prevents atoms diffusing through lattice - causes coring
what can coring in an alloy lead to?
reduced corrosion resistance of solid alloy
how is coring in alloys resolved?
homogenising annealing!
what is homogenising annealing (alloys)?
• once solid cored alloy is formed REHEAT to allow atoms to diffuse and so cause grain composition to become homogeneous
when annealing, what is essential about the temperature used? why is this?
must be BELOW recrystallisation temperature - grains altered if it goes above
why is an alloy more fracture resistant than a metal?
• alloys have 2 types of metals of different atomic size • defects don’t ‘roll’ over the lattice plane when a force is applied like in metals • more energy/force is needed for the defect to overcome the different sized atoms and move along the lattice to the grain boundary • hence, requiring greater stress to move dislocations in a solid solution
what are the properties of a Eutectic alloy?
• metals are soluble in liquid state • metals insoluble in solid state (so 2 phases) • each metal forms physically distinct grains