Metal & Alloys
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give examples of uses of metal in dentistry
cocr - partial denture stainless steel - crown or denture base titanium - orthodontic wire
what are the mechanical properties of metal
high elastic limit - very rigid, high UTS - resist tension, high fracture stress - a lot for it to fracture
what is ductility/malleability
the difference between elastic limit and fracture stress, the stress it can undergo causing permanent deformation but not causing fracture. ductility - tensile stress, malleability - compressive stress
what affects the mechanical properties and what is the knock on affect from this
grain size, impurities and lattice structure. these are affected by the processing of the metal, therefore, processing affects the mechanical properties
what is the difference between a metal and an alloy
metal - atoms arranged in lattice structure alloy - atoms of 2 or more metals arranged in lattice structure
describe the over all process of producing metal
molten metal allowed to cool, reaches a temperature where solid formation begins, stays at this temperature until all metal is transformed to solid, then allowed to cool further
describe crystal growth
nuclei of crystallisation - atom from which other atoms crystallise. crystals grow from here to form dendrites - 3d lattice structure, growing in an orientation. as grains impinge on other crystals - grain boundary - change of orientation
what are the 2 types of grain growth
radial growth - small crystals, many of them, good mechanical properties fibrous growth - large crystals, few of them, poor mechanical properties
what affects grain growth
the way in which the metal is cooled. fast cooling - small grains formed, slow cooling - large grains formed
how can a metal be cooled quickly
quenching - putting it into cold water
what is the problem with defects in lattice structure
misalign the structure. if nothing done about it, the defect will move along the lattice - slip - propagation of dislocation. in doing so, weakens the material
how can defects be reduced
cold work - applying a force in cold temperature eg hammering. moves the defect along the lattice structure until it gets to a grain boundary where it is stopped. this prevents the defect from moving and improves the UTS, EL and FS
what is the disadvantage of cold work
results in an increase in internal residual stress
how can the residual stress be dealt with
stress relief annealing - heating up the metal after cold work to realign the lattice. doesnt affect grain size or shape or mechanical properties but reduces the stress
what can be done if a poor result has come from cold work and stress relief annealing
recrystalisation - heating the metal back up to molten state, then allow for cooling. un-does all cold work
what is a phase
distinct homogenous structure
if two metals in an alloy form two separate lattice structures, how many phases are there
2
if two metals in an alloy form one lattice structure with 2 types of grains, how many phases are there
1
if a molten alloy is decreased down to crystallisation temperature, what are the possible results
two metals are insoluble in one another - 2 separate phases are formed intermetallic two metals are soluble in one another - 1 phase formed with 2 types of grains
what is a solid solution
solid metal lattice with 2 grain types - 1 phase
how can 2 grains in 1 phase be arranged
substitution - one atom replaces another, either random or ordered interstitial - smaller atoms fill in the gaps between larger atoms
how does cooling of an alloy for crystallisation differ to that of a metal
metal - happens at one temperature alloy - happens at a range of temperatures, continues to cool while crystallisation is happening
how does the composition of metal in an alloy effect the crystallisation temperature
different compositions results in a different range of temperatures for crystallisation
what is shown in a phase diagram
temperature of cyrstallisation against composition of alloy. liquidus - above this line, the alloy is molten, below, it begins crystallisation. solidus - below this line, the alloy is fully solid, crystallisation has finished