Metals & Alloys Flashcards
what is the symbol for an edge dislocation?
⊥
what is the symbol for a screw dislocation?
curved arrow
how do two like dislocations interact?
can repel each other
how do two unlike dislocations interact?
attract and annihilate each other
what does the ability of metal to plastically deform depend on?
- depends on the ability of dislocations to move
- to strength a material we try to restrict or hinder dislocation movement
what are the 4 main mechanisms for restricting dislocation movement to strengthen a material?
- solid-solution strengthening
- strain hardening or cold working
- reducing grain size
- precipitation strengthening
how does grain size reduction work?
reducing the gain sizes increases the number of grain boundaries - dislocations need more energy to pass through a grain boundary so it impedes dislocation movement, rolling with a polycrystalline metal can also induce this
are fine or large grained materials harder?
fine grained materials are stronger and harder because the yield strength is increased because there is a larger no. of grain boundaries
what is the Hall-Petch equation? what does it relate to?
σyield = σo + ky x d^-0.5
where…
σyield = yield stress
σo = starting stress for dislocation movement (constant)
Ky = strengthening coefficient
d = average grain diameter
(it relates to the effect of grain boundaries on the yield stress)
what is solid-solution strengthening?
- when you deliberately alloy metals with impurity atoms
- impurity atoms distort the lattice and generate stress
- stress can produce a barrier to dislocation motion
- can be done with substiituonal solid solution or interstitial solid solution strengthening
- more energy is required as a dislocation wants to move it has to to tear itself from the impurity atoms
what is an example of solid-solution strengthening?
Cu-Ni alloys, alloying increases the yield stress and tensile strength
in terms of solid-solution strengthening, what does the degree of strengthening depend on?
the relative atomic size…
for example Cu-Ni had a small difference whereas Cu-Sn has a large difference in atomic size meaning there is a larger lattice strain and will have greater strengthening by concentration
what is precipitation strengthening?
- dislocations can’t get through precipitates easily
- hard precipitates are difficult to shear, which takes up a lot of energy
- for example aluminium is strengthened with precipitates formed by alloying making it ideal for use in aircrafts because it has greater strength
what can we change in terms of precipitation strengthening?
decreasing the spacing, S, and putting the precipitates close together means the dislocations bend round more which increases the material strength
what is cold work hardening (or strain hardening)?
- hitting a metal with a hammer etc.
- metal becomes harder and stronger as it is deformed due to strain or ‘work’ hardening - improves the mechanical properties
- dislocations become entangled
- the stress required to cause further plastic deformation increases
how does cold work hardening work?
- dislocations entangle one another during cold work
- dislocation motion becomes more difficult
what is the equation that relates to cold work hardening ?
%CW = (Ao - Ad / Ao) x 100
where A is the area, use πro^2 and πrd^2 for a cylindrical rod
what is annealing?
- heating up the metal and cooling it slowly
- removes strengthening, as diffusion allows dislocations to rearrange and annihilate
- annealing is a form of recovery: it allows for recrystalisation and grain growth, removes stresses
- reduces dislocation density
- if you increase the temp. you increase the grain size
where are dislocations primarily seen?
primarily in metals and alloys
what is cast iron? where is it used?
- 2-4% carbon
- lower melting point due to high carbon content
- has pearlite and flakes of graphite
- graphite improves wear resistance by providing lubrication and a large reduction in toughness and ductility as source of cracks
what is metal fabrication and what are the main types?
metal fabrication is the processing of metals into finished objects, these methods include:
- casting - pouring liquid metals (low cost)
- forming/forging (if metal is ductile)
- machining such as cutting and grinding
- joining
how are most alloys initially formed?
by casting - conditions during solidification are important in determining the quality of the alloy
what is sand casting?
- low cost way of manufacturing simple shapes
- make the required ‘pattern’ which is the needed shape
- place the wooden pattern into moulding sand, then remove the positive mould former
- liquid alloy is poured into the negative void and allowed to solidify
- sand mould is broken up and casting is removed
what is investment casting?
- process of using lost wax
- used for high temp alloys (e.g turbine blades)
- provides high dimensional accuracy for materials that are difficult to machine at room temp.
- used for low volume but complex shapes
- master mould is produced in alloy which is machinable e.g brass
- the master is then used to create wax patterns
- wax pattern is coated with ceramic
- the coated wax is melted away leaving a high temp ceramic mould
- high Tm alloy is poured into the ceramic mould and removed on solidification
what is die casting?
- process of injecting liquid metal into a reusable mould under pressure
- produces complex shapes
- reduced porosity
- little metal waste
- restricted to lower Tm alloys (Cu, Zn, Mg, Al etc.)
what is forging?
shaping by hammer blows into an anvil ‘mould’ - can form complex, strong parts such as wrenches and crankshafts
what is rolling?
material passed through cylindrical roller to produce sheet type products and things like rails and I-beams
what is extrusion?
forcing metal through a shaped die to form parts with a constant cross section like tubing and rods
what is drawing?
pulling metal through a die usually to make metal wires, rods and tubing
