Metals Flashcards
what is a metal?
an aggregate of atoms in a crystalline structure
what is an alloy?
contribution of two or more types of metal atoms in a crystalline structure
what does it mean if the stress-strain curve has a large gradient?
it is rigid
what does large stress give?
small strain
what is ductility?
amount of plastic deformation prior to fracture
what does the crystalline structure depend on?
method of production and shaping
what factors affect mechanical properties?
crystalline structure, grain size and grain imperfections
what are the different lattice structures?
cubic, face-centred, body centred
describe the cooling curve of pure metal
molten form flows readily, as temperature drops some metal atoms change in state to solid, temperature then remains constant with more atoms crystallising. When all atoms have crystallised the temperature drops to room temperature
what are nuclei of crystallisation?
crystals which grow as dendrites and continue to grow until they come into contact with each other
what does it mean if grains are equi-axed?
crystal growth is of equal dimension in each direction
apart from equi-axed grains, what other grain structures are available
radial, fibrous
if a material is fast cooled (quenched) what are the grains like?
more nuclei, small fine grains
if a material is slow cooled what are the grains like?
few nuclei, large coarse grains
what are nucleating agents?
impurities or additives which ac as foci for crystal growth
where do grain boundaries occur?
where there is a change in orientation of the crystal planes
what are the advantages of small fine grains?
high elastic limit and increased FS, UTS, hardness
what is the disadvantage of small fine grains?
decreased ductility
what are factors needed for rapid cooling
small bulk, heat metal just above Tm, mould - high thermal conduction, quench
what happens if there is a dislocation within the lattice?
it will fracture unless the dislocation is pushed along to the end of the lattice
what are dislocations?
imperfections/defects in the crystal lattice
what is SLIP due to?
propagation of dislocations and involves rupture of only a few bonds at a time
how can you impede the movement of dislocations in metals and alloys?
have grain boundaries to prevent dislocation moving from one grain to another, have smaller finer grains
what is the advantage of impeding dislocations?
metal/alloy has increased Fs, El and Sh
what is cold working?
work done on a metal/alloy at a low temperature (below recrystallisation temperature) causing SLIP and making the material stronger and harder
what does cold working increase?
El, Fs and Sh
what does cold working decrease?
ductility, impact strength and corrosion resistance
what is residual stress
stresses developed in the metal itself where atoms become oriented out of their equilibrial position in the grain/lattice
what does residual stress cause?
instability in metal lattice, distortion of metal overtime
what is residual stress relieved by?
subjecting metal or alloy to annealing
what is annealing?
heating metal so that greater thermal vibrations allows migration of atoms
what does annealing do?
eliminates stresses by allowing atoms to re-arrange within grains
when does recrystallisation happen?
when metal/alloy is heated
what does recrystallisation do?
allow new smaller equiaxed grains, lower El, UTS, hardness, increased ductility
what is the relationship between recrystallisation and cold working?
recrystallisation spoils the benefits of cold work but allows further cold work
what is the recrystallisation temperature dependent on?
amount of cold work - the more cold work the lower the recrystallisation temperature
what does excessive temperature rise cause?
large grains to replace smaller coarse grains which yields poorer mechanical properties
what are the metal/alloy manipulation processes
quenching, cold working, stress relief annealing, recrystallisation
what is grain structure affected by?
dislocations, processing methods
