metals and alloys Flashcards
metal vs alloys
metal - single species of atoms in a crystalline structure
alloys - combination of metal atoms in a crystalline structure
metals are the building blocks of alloys
when would a material need to display ductility
when subject to tensile strength (pulling)
ductility - ability to deform without fracture
when would a material need to display malleability
when subject to compressive strength
malleability = ability to be pressed/hammered etc
are mechanical properties identical in every metal
no
aside from choice of metal what else can influence a metals mechanical properties
crystalline structure
this structure depends on method of production and how it was shaped e.g cold working, swaging, annealing
describe the temperature changes/metal composition as a metal cools from a molten state to a solid
in molten form atoms are arranged randomly and flow readily
as it drops to melting point atoms start to crystallise as they go from liquid to solid
temperature is constant at this stage
only when crystallisation has complete will the temperature begin to cool again
what term is used to describe what crystals grow as to form a 3D lattice network
dendrites
what are crystals also known as
grains
grain boundaries
form where a series of individual grains make contact with one another - stopping each other from further growth
equi axed grains
form when atom crystals grow equally in each direction
how is radial grain structure achieved
by cooling molten metal quickly in a cylindrical mould
quenching
fast cooling
what effect does quenching have on crystalline structure
causes more nuclei (of crystallisation) to form therefore forms more grains , these are small in size
what effect does slow cooling have on crystalline structure
less nuclei (of crystallisation) form therefore less grains, these grains are large in size
what may be added to molten metals to act as additional nuclei of crystalisation and therefore produce more small grains
nucleating agents such as impurities or addidives
are small or large grains advantageous and why
small grains - they have a higher EL, FS, UTS and hardness however decreased ductility
grain
single crystal with atoms in given directions
dislocation
‘imperfection’ in crystal lattice structure causing a discontinuity , this area is most likely to fracture
slip
propagation of dislocations , towards a grain boundary
dislocations move one lattice bond at a time until they reach a grain boundary
what makes small grains have better mechanical properties
many small grains = many grain boundaries , this impedes the movement of dislocations making it stronger
cold working
work such as bending, rolling or swaging done at a low temp - one below the metals recrystalisation temperature
benefits of cold working
the applied force/ pressure from the work causes slip and produces a harder and stronger material
disadvantages of cold working
lots of cold working can increase residual stress (internal stresses)
this causes instability of the lattice and may result in distortion of the metal over time
how are residual stresses relieved
annealing - heating of the metal/alloy which causes increased thermal vibrations allowing the metal atoms to migrate and re arrange so as to eliminate instability
stress relieving annealing
when annealing is done to cold worked materials
doesn’t alter grain structure or mechanical properties and allows further cold working to be carried out
recrystallisation
can be carried out if cold working doesnt work out so that the metal can be recycled
however this changes grain structure therefore changes mechanical properties - decreased EL, FS, hardness but increased ductility
metalloid
element with properties in between a metal and a non metal e.g silicon , chloride
what advantages do alloys have in comparison to metals
increased mechanical properties, increased corrosion resistance
solution vs phase
solution - homogenous mixture at an atomic scale
phase - physically distinct homogenous structure - can have more than one component
solid solution
on crystallisation 2 metals come together to form a common lattice i.e one phase
substitutional solid solution
atoms of one metal replace the other in the crystal lattice
2 types
- random - randomly replaced, atoms of similar size
- ordered - orderly replaced in lattice arrangement, atoms of similar size
interstitial solid solution
atoms markedly different in size, smaller atoms don’t have a specific location and are located within the larger atoms lattice structure
metal vs alloy cooling curve
metal - crystallisation of metals occurs at a constant temp before cooling resumes
alloy - crystallisation of alloy occurs over a temperature range as the different metals will have different melting points
liquidus
line representing temp at which crystallisation begins - above this ally is in a liquid state
solidus
line representing temp at which crystallisation ends - below this alloy is in a solid state
slow cooling of alloys
ensures grain composition is homogenous however forms large grains which have decreased mechanical properties
fast cooling of alloys
produces small grains and many grain boundaries which can impede dislocations therefore better mechanical properties
however grains formed are not homogenous they experience coring
coring of alloy grains
happens when alloys are quenched
occurs as the metals within the alloy have different melting points
happens as atoms are prevented from freely moving through the lattice so grains have a varying composition
extend of coring dependent on separation between liquidus and solidus
downfall of coring
may decrease corrosion resistance
how can you overcome the downfall of decreased corrosion resistance caused by coring
homogenising annealing
reheat the solid core to allow atoms to diffuse and grain composition to become homogenous
why do alloys have improved mechanical properties in comparison to metals
alloys resist movements of dislocations more
differing atom sizes means more force is needed to move the dislocations
eutectic alloys
metals are soluble when molten but insoluble when solid so form 2 phases
brittle and poor corrosion resistance
partially soluble alloys
a range of compositions are not possible , do not cool to form 50:50 grains
instead forms alpha and beta grains , with them being made up majority of the respective metal
can’t form grains between the solubility limit lines
when annealed what do partially soluble alloys undergo
precipitation hardening which makes them stronger and harder
