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
in what conditions is plastic deformation done?
at high temp. to reduce stress required and provide ductility in the finished part
what are the main difference between cold working and hot working?
COLD WORKING:
- more energy to deform
- oxidation: good finish
- higher strength
- fracture resistant
HOT WORKING
- recrystalisation
- less energy to deform
- oxidation: poor finish
- lower strength
what is an alloy?
an alloy is a metallic substance made up of more than one element
name some examples of alloys
- bronze (Cu-Sn)
- brass (Cu-Zn)
- solder (Pb-Sn)
(bronze alloys are used for propellors in boats because they are corrosion resistant, also used for musical instruments)
why do we alloy metals?
- to improve the mechanical properties
- they can have higher strengths
- easier casting (controlled solidification) because lower melting point
- electrical/magnetic properties
- can tailor properties for a given application
what are the properties of metals and alloys controlled by?
- bonding
- crystal structure
- microstructure / defects
what are components in terms of alloys?
the elements or compounds which are mixed initially (e.g Al and Cu)
what are the phases in terms of alloys?
the physically and chemically distinct material regions that result (e.g α and β) - they have different structures
what is the phase boundary/solubility limit on a phase diagram?
the boundary between a liquid and saturated solution (i.e a liquid + solid) - the point at which you can’t fit any more solid in
what are ferrous alloys?
metals that contain iron
what are some benefits of iron? and one con?
- abundant
- extraction is economical
- versatile and tailored properties
- high stiffness (210GPa), good strength and ductility
- CON: susceptible to corrosion
if we add carbon to iron what do we get?
steel
what type of steel is used to reinforce concrete?
low carbon steel
what phases are included in the iron-carbon phase diagram?
ferrite and cementite
during heat treatment it will pass through the austenite phase producing distinctive microstructures
what is ferrite?
pure iron, α-iron - BCC
what is cementite?
iron carbide Fe3C (a ceramic)
what does α + Fe3C mean?
pure iron (ferrite) and iron carbide (cementite) or PEARLITE
what is austenite?
γ-iron (higher temp phase) - FCC
what is the main difference between ferrite and austenite?
ferrite is body centred cubic
austenite is face centred cubic
at what temp does ferrite form?
room temp.
at what temp does austenite form?
912 - 1394 degrees C
at what temp does liquid iron form?
1538 degrees C
what happens if you increase the temp. from ferrite?
ferrite is a a soft, ductile, magnetic phase - if you increase the temp. you get austenite which is non magnetic
what do we have above 6.7wt% carbon?
pure cementite, Fe3C iron carbide - cementite is hard and brittle and can be used to enhance the strength of some steels
what is the eutectoid?
going from a solid to solid phase
what is the eutectic?
going from a liquid to solid phase
what is pearlite?
- pearlite is the laminated structure of ferrite and cementite formed during cooling by the diffusion of carbon
- has alternating lamellar of ferrite and cementite
- occurs at the eutectoid as the temp. decreases
- it has soft, ductile ferrite as well as hard, brittle cementite making it brittle
what is martensite?
- formed when the steel is cooled very quickly, pearlite doesn’t form because the carbon hasn’t got enough time to diffuse
- non equilibrium phase (don’t see it on phase diagram)
- martensite is very hard and brittle
- BCT unit cell
what happens if you cool austenite very slowly?
you get pearlite (BCC) which is soft and ductile
what happens if you cool austenite very quickly?
you get martensite (BCT) which is hard and brittle
what unit cell is the structure of martensite?
BCT - body centred tetragonal
how can we use martensite formation to improve metal properties?
we can use quenching (heat treatment) to improve the strength and hardness of steels
why is welding an issue with medium to high carbon steels?
when the weld cools down martensite can form and because martensite is brittle it will result in a weaker weld - therefore you can’t weld high carbon steel due to martensite formation
some info on low carbon steels?
- less than 0.25wt% carbon
- produced in greatest quantities
- unresponsive to heat treatments (difficult to form martensite)
- strengthening achieved by cold working
- relatively soft and weak, but excellent ductility
- machinable, weldable (no martensite)
- cheap, low cost to produce and manufacture with
what are some applications for low carbon steels?
- car body components
- structural shapes (I-beams)
- reinforced concrete
- sheets used in pipelines and buildings
some info on medium carbon steels?
- 0.25-0.6wt% carbon
- heat treatable to improve mechanical properties, but thin sections only
- stronger but less ductile than low C steels
what are some applications for medium carbon steels?
- railway wheels and tracks
- gear and crank shafts
- high strength and wear resistance, moderate toughness
some info on high carbon steels?
- 0.6-1.4wt%
- hardest, strongest and least ductile
- heat treatable in thick sections but avoid welding
- additions of Cr, V, W, form hard carbides with excellent wear resistance and hardness
what are some applications for high carbon steels?
- cuttings tools and blades
- drill bits
what is hardenability?
ease of martensite formation
why is martensite not observable in the iron-carbon equilibrium phase diagram?
martensite is a non-equilibrium phase formed during rapid cooling when diffusion is too slow to allow pearlite formation
why might you add chromium, vanadium or tungsten to a high carbon steel?
forms hard carbides which give excellent wear resistance and hardness
what crystal unit cell represents the structure of magnesium?
HCP
what method is suitable for the manufacture of a steel beam?
rolling of a billet - softening point of steel is too high so extrusion cannot be used and it’s rolled
what does increasing carbon content do in terms of martensite production?
facilitates martensite formation
