2P3 Materials Flashcards
What factors determine the increase in yield stress due to precipitation hardening?
- inversely proportional to the obstacle spacing
- the strength of the obstacle (large second phase particles are stronger than weaker solid solution paricles)
What is a common heat treatable aluminium alloy?
Al-Cu
In Cu-Al, what other compound is formed when cooled below the solubility limit?
CuAl2
What is the driving force for precipitation formation in Al-Cu alloy?
Gibbs free energy, diffusion enables the transformation.
Where are precipitates most likely to form in metals and why?
heterogeneously on grain boundaries, because there is the most space in the Al lattice and therefore faster diffusion.
What is the problem with a slow cooled Al-Cu microstructure?
Precipitates are large and spaced far apart. Lower gibbs free energy for larger precipitates. Weak hardening
What is the difference between a TTT ad a CCT diagram and what are they used for?
TTT: Time temperature Transformation. Indicates the fraction of CuAl2 that will be formed if the material is held at a certime temperature for a certain time.
CCT: Continuous Cooling Transformation.
This shows the amount of CuAl2 that will be formed if cooling at a fixed dT/dt, still plotted T against time. From this a critical cooling rate can be found where no precipitation phase transformation occurs.
What happens to a material cooled faster than the CCR?
It will form a single phase supersaturated solid solution (SSSS), which is metastable and not in equilibrium.
This is solid solution hardening, not optimum for material strength.
What is age hardening?
After an alloy is quenched, it is reheated which allows precipitates to form from the metastable SSSS. This can either be done at higher temps or at room temperature.
How does effectiveness of precipitation hardening change as a material is aged?
When they are small, the effect of hardening is dominated by the strenght of the precipitates, strength is proportional to radius ^.5
When they are large, the spacing gets larger, so the strenght is dominated by dislocations bypassing obstcales, dislocations are proportional to 1/spacing.
This leads a time at which the aluminum is peak aged, after which it will continue growing to an equilibrium microstructure (even weaker than as-quenched)
What are metastable precipitates?
When the lattice of the precipitates remains coherent with the Al lattice. This reduces interface energy but also strains the lattice. These are weak obstacles. Lots of fine metastable precipitates are found in a peak aged lattice.
What are non-heat-treatable Al-Alloys?
Alloys that do not form metastable precipitates, and therefore cannot be aged. Almost all cooling rates would quench these materials, and form a SSSS. Usually they rely on solid solution and work hardening.
Examples are Al-Mg (‘5000 series’)
What is a hypo-eutectoid steel?
A steel with <0.8 wt %. C
What is a normalised steel?
A slow cooled steel such as hot rolled.
What are mild steels?
Steels with <0.1-0.2 wt% C.
What is the microstructure of a slow cooled, hypo-eutectoid steel?
Two phase region of austenite an ferrite, then turns into ferrite and pearlite.
How is hardeness related to pearlite concentration and why?
higher pearlite fraction, the harder the material. Cementite is very hard (Fe3C)
What happens when steel is held above the A1 Temperature?
Ferrite forms first, creating a ferrite and austenite microstructure.
What happens when steel is held at or below the A1 temperature but above the nose of the C curve?
Ferrite forms first, then at the carbide line it starts to form pearlite, creating a ferrite + pearlite microstructure
What happens if steel is held below the nose of the C curves?
Diffusion is more inhibited, austenite transforms directly into ferrite + iron carbide, in a fine scale dispersion. This is known as bainite.
What happens when steel is quenched from an austenite phase?
It forms martensite, a metastable phase. SSSS of C in Fe. Has to transform from FCC to BCC without diffusion, creating straining of the lattice. This is achieved by shear, creating needles of martensite.
Why does martensite form?
Gibbs free energy is lower than that of austenite and temperatures well below A1. So there is a thermodynamic driving force.
There also needs to be undercooling.
A certain amount of transformation can reduce the free energy enough to stop further transformation.
What happens to the C curves at higher wt%C?
They shift to the right as more C means longer difussion times?
What happens to martensite temperatures at higher wt%C?
Start and finish temperatures decrease with higher wt% C. More carbon means more lattice strain, therefore greater undercooling.
What are the yield stresses for slow cooled steels?
200-450Mpa (0.1%-0.8% C)
What is the yield stress for a Bainite microstructure?
500-700 MPa, very good precipitation hardening
What is the yield strength for Martensite and why?
1000-3000MPa
BCC is heavily supersaturated with carbon, giving solid solution hardening.
Distortion of the lattice enhances this.
Higher C means more distortion and more hardness.
What is the flaw with martensite?
Very small fracture toughness, extremely brittle.
What temperature to temper martensite?
450-600oC
What is the effect of tempering martensite on the steels mechanical properties?
Yield strength reduces (450-800), fracture toughness increases
What microstructural changes occur during tempering?
Carbon in SSSS diffuses to form Cementite precipitates, and the lattice changes back to undistorted BCC. As tempering happens, the cementite coarsens.
How is hardenability defined and determined?
Hardenability - how easily a steel forms martensite on cooling.
On TTT diagram, higher hardenability is indicated by longer transformation times.
On CCT diagram, higher hardenability is indicated by a slower CCR.
Largest bar diameter that will give 100% martensite at its centre after quenching is also an indicator of hardenability.
What sort of hardenability do you want for welding?
Low hardenability, thermal cycling during welding could form martensite which would make the weld brittle.
What factors affect the hardenability of steel?
Carbon concentration: higher C gives higher hardenability, by delaying diffusion phase transformation. Eutectoid steels require bar diameter of <30mm (water quenched) to form 100% martensite.
Further alloying, can also delay diffusional phase transformation, up to 5 wt% of Ni, Cr, Mo, V, W can be added. Known as low alloy steels.
Why are high alloy steels used?
High hardenability is great for cutting tools which need high hardness at high temperatures. Alloy carbides are stable at high temperatures retaining precipitation hardening in service. High temperature solid solution hardening.
What is carburising?
Immsersing steel in molten cyanide, which has high C content, making it easy to form martensite which can be tempered. Higher C will make martensite harder.
What is transformation hardening?
Imposing a heat source to the surface, so a thin layer is austentiseed and then cooled quickly to form a layer of martensite. Could be done with a flam, laser, electron gun, or high frequency induction coils to induce eddy currents. Air cooling or water quenching may be used.
Define Phase
All parts of a material with the same atomic structure are a single phase
Difference between weight% (wt%) and atom% (at%)
wt% = weight of component A/total weight
at% = number of atoms (or mols) of A/total number of atoms (or mols)
What defines the consititution of an alloy?
The phases present;
The weight fraction of each phase;
the composition of each phase
What is an equilibrium constitution?
The state of lowest Gibbs free energy, for a given composition, temperature and pressure, thermodynamically stable.
What is the liquidus and solidus line?
Liquidus defines the bottom of the single-phase liquid field, solidus defines the upper limit of the single phase solid line.
Define the eutectic poin
The lower limit of the single phase liquid field formed by the intersection of the two liquidus lines.
How do you determine the composition of the phases in a two phase substance?
By drawing a tie line horizontally at the constitution point and using the intersection, and the lever rule. Take note of wt% or at%
How do intermediate compounds usually appear on a phase diagram?
As a thin vertical line, which can tolerate a little of the other atom.
What phases are formed during the heating of pure iron?
Ferrite (low temp, BCC), austenite (FCC, 910oC), δ (BCC, 1391oC)
How is the solubility different in Ferrite compared to Austenite?
Austenite is FCC so has larger interstitial holes, therefore is much more soluble for carbon and therefore is much happier as a single phase with carbon.
What wt% and at% is cementite formed?
6.7 wt% C, 25 at% C
What is a peritectoid point?
The upper limit on the formation of a single solid phase, an inverted V meeting a horizontal.
Difference between eutectic, eutectoid, peritectic, peritectoid?
eutec - normal V intersecting horizontal line
peritec - inverted V meeting horizontal line
-tic - liquid is involved
-toid - all phases are solid
What are some techniques for observing phases and microstructures?
Dilatometry - high resolution measurement of dimensional changes
Electrical Resistivity
Calorimetry (differential thermal analysis) - measurement of release or take-up of latent heat
Optical Microscopy - differential reflection of light
X-ray diffraction - diffraction of X-ray
SEM - differential back-scattering of electrons
TEM - Diffraction of an eletron beam.
What is the condition for homogeneous nucleation?
Solid colonies will grow stably as long as they can reach a critical radius, where the free energy released is greater than the surface energy barrier. Heterogenous nucleation reduces the surface energy barrier.
What is a phase reaction?
When a phase composition changes with temperature.
What is a eutectic reaction?
When a single phase liquid transforms into two solid phases at constant temperature
What is a eutectic microstructure?
Formation of plates/needles.
Why does eutectic reaction form eutectic microstructure?
minimises diffusion distances, also lower surface energy in some orientations aligned with the crystals. However, there is a price to pay in terms of surface energy
What is the eutectoid transformation in steel?
Austenite to pearlite (made up of ferrite and cementite), forms lamellar structure
What makes pearlite distintict from most eutectic structures?
Much finer plate like structure, as formed by solid state diffusion rather than liquid.
What is the eutectoid point for steel?
0.8 wt% (pearlitic steel)
What are properties of ferrite and pearlite?
ferrite has high intrinsic strength, with high toughness. Pearlite is effective at obstructing dislocation motion.
What does entropy measure in terms of micro-states?
The uncertainty in the number of micro-states for a given macro-state
Equation for entropy in terms of microstates, Ω
S = kb * ln(Ω)
What is the entropy of mixing components a and b with na and nb atoms respectively?
ΔS = kb ln[n!/(na!nb!)]
when n is large
ΔS = -kb (naln(xa) + nbln(xb) = -n kb(xalnxa + xb lnxb)
This makes it extensive.
Because ln(n!) = nln(n) -n for large n
How to derive ideal gas equation from microstates eq?
TdS = pdV
Ω = Ωv(T) * Ωc(V) [microstates of velocities, microstates of positions)
dS/dV = kb 1/Ωc * dΩc/dV
Ωc = KV^n (position microstates is volume to the power of the number of particles)
dS/dV = nkb/V
How does latex become rubber?
Vulcanisation -> heating at 170oC
Difference in behaviour of polymers above and below Tg?
Below, too weak for chains to slide past therefore high modulus in GPa.
Above, they behave viscoelastically, become entangled and can store deformation. transient elastic response. Become liquid at steady load.
What are elastomers?
Crosslinked networks of polymer chains, such as rubber.
Remains solid above Tg
The equation for force on a polymer chain and its derivation?
f = -T dS/dr
where r is the extension
TdS = dU - fdr
Change in U is small as temperature and bond stretching is uncahnged.
What is the elastic constant of a polymer chain?
kb T/na^2
where n is the length of polymer (number of monomers in the chain) and a is the legnth of each indicvidual monomer.
Can derive by considering a 1D random walk with a certain number of chains facing each way, and substituing this into the microstates and then expanding with stirlings approximations and then a taylor expansion.
What is the mean end to end distance of a polymer chain?
The radius of gyration rg, rg = sqrt(mean r^2) = sqrt(n)*a
What is rc for an elastomer?
Mean distance between crosslinks.
What is the spring constant between cross-links?
k = kb T/ rc^2
What is the useful work available?
dU = dQ - dQ = TdS - TdSirr - pdV - dW’
=>
dW’ = -dU + TdSirr - pdV
How does useful work relate to Gibbs free energy?
-dG > dW’
G = U + pV -TS
dG = dU + pdW - TdS if dp=dT=0
What does the condition dW’ = 0 imply?
It implies that dG < 0, at equilibrium G is at minimum
How does Gibb’s Free energy compare for solid’s liquids and gases?
Gs = U + pV -TS
Gl = U + pV -TS
Gg = U + pV - TS
entropy:
gas>liqiuid>solid
volume:
gas>liquid=solid
Internal energy:
gas>Liquid>Solid
gas has higher intersept but steeper gradient due to higher entropy.
What is the change in Gibbs free energy due to mixing?
Gm = Gu + ΔUm + pΔV - TΔSm
Um -> change in internal energy due to interactions
pΔV -> changes in density
TΔSm -> entropy of mixing
Forms a convex curve for miscible materials, can be non-convex for non-miscilbe materials.
What is the Gibb’s free energy of a homogeneously nucleating solid?
G(r) = 4/3 π* r^3 * ΔGv + 4πr^2*γ
Where Gv is gibbs free energy of solidification per unit volume (negative).
and γ is surface energy per unit area
What is the critical radius for a homogenesouly nucleating solid?
r* is the radius above which the nucelaus is stable. Defined by dG/dr = 0
r* = 2*γ/|ΔGv|
What is critical radius in terms of enthalpy change per unit volume?
r* = 2*γ/|ΔHv| * Tm/(Tm-T)
∆G=∆U+P∆V −T∆S=∆H−T∆S
∆G(Tm) = ∆H −Tm∆S =0 =⇒ ∆S =∆H/Tm
∆G(T) = ∆H−T∆S =∆H−T∆H/Tm
How can you model heterogeneous nucleation?
By considering a spherical cap, radius r contact angle θ
What is the critical radius for heterogeneous nucleation?
Same as homogeneous,
r* = 2*γ/|ΔHv| * Tm/(Tm-T)
However, less atoms are needed for same radius
What is osmotic pressure?
π= RTc, where c is solute concentration in moles/m^3
Where does osmotic pressure come from?
Minimising Gibbs free energy of mixing two substances.
What is a colloid?
A liquid with small solid particles suspended, two phases.
What is a depletion force?
When two plates are separated by a distance smaller than the size of colloids, there is an attractive force between the plates, F=RTcA
How is depletion used to purify water?
Adding large size contaminants to water causes the formation of aggregates that are large enough to sink
What are hydrogels?
Polymer networks in food products and biomaterials, such as agarosee and gelatin, very low polymer desnity. Swell with water via osmosis.
What parameters are needed to characterise random motion of molecules?
Length scale λ and a time scale τ, time between colissions
What is the standard deviation of the distance from origin of a random walk?
t/τ λ^2
What is Fick’s First law?
Net flux of atoms is proportional to the concentration gradient
J =−DdC/dx
What is Fick’s second law?
∂C/ ∂t =− ∂ /∂x( −D∂C/∂x)
Rate of change of concentration at a point
What is the response to a delta function input to the diffusion equation?
A Guassian distribution
What are the mechanisms of diffusion in a solid?
Bulk interstitial diffusion - fast (moving betwween interstitial sites)
Bulk vacancy diffusion -slow- solute atoms exchange places with vacancy
Short circuit diffusion along a grain boundary - fast (open crystal structure)
Short-circuit diffusion along dislocation cores - fast (large atomic spacing around dislocation)
What is the Arrhenius law?
Thermally activated processes:
Rate of process ∝ exp [− Q /RT]
Comes from the probability distribution of atom exceeding energy barrier
What is D0
The diffusion constant which depends on the type of diffusing atoma nd what material?
How does growth rate of solid phase depend ?
q - ∆G/2 for liquid to solid
q+ ∆G/2 for solid to liquid
Therefore growth rate of solid = pls-psl, so its proportional to Tm-T/T exp(-q/kbT)
Forms a bell curve, for rate against T/Tm
What does nucleation require?
Small scale flucations to get atoms to move around,
fluctuations of energy of the order of ∆G∗ bringing together a nucleus of size r∗
How does rate of nucleation vary with temperature?
rate of nucleation ∝ exp(−∆G∗/ kBT )· exp(−q/kBT)
What are the two stages of Silicon doping?
Pre-disposition -> high concentration of dopant at a high temperature
Drive in -> high temperature to encourage diffusion of dopant to get near-even distribution
What is the solution to pre-disposition?
erf
What happens when a metal is placed in water?
Metal ions detach and go in solution, leaving electrons at the interface, giving the metal a potential
How do you measure the potential of a metal electrode?
By comparing it to a reference electrode.
What happens in a cell?
One compound is oxidized, meaning it loses electrons at the anode.
One compound is reduced, meaning it gains electrons, at the cathode.
What happens to iron in an acidic environment?
Anode: Fe -> Fe2+ + 2e-
Cathode: 2H+ + 2e- -> H2
What happens to iron in a neutral environment with oxygen?
Fe -> Fe2+ + 2e-
O2 + 2H2O + 4e- -> 4OH-
What is rust?
Fe(OH)2
What will make a metal more likely to corrode?
The lower the standard potential?
What can cause localised corrosion?
Defects in protective coating (paint, natural oxide layer) ->pitting
Electrolyte present in crevices can encourage anodic reaction.
Cracks -> corrosion promotes crack growth.
What are the conditions for aqueous corrosion?
Anodic reaction
Cathodic reaction
Conductive path
Solution containing ions to transport chargers (electrolyte)
Solutions to avoid corrosion:
Displacing the anodic reaction (galvanic protection)
Controlling enviornemnet so cathodic reaction is avoided.
Insulating the path between anode and cathode (oxide/coating)
Avoiding water accumulation
What is impressed current protection?
A DC generator connected to an anode (stable or consumable)
What is the chemical reaction involved in forming an oxide layer?
M -> M 2+ + 2e-
O2 + 4e- -> 2O2-
O2 + 2M -> 2MO
How do you know whether an oxide reaction will take place?
if G(P,T) > 0, reaction will not spontaneously happen, metal surface is stable
if G(P,T)<0, reaction will spontaneously happen, oxide layer is stable
What are the three main types evolution of the oxygen layer?
Linear loss -> oxide does not stay at surface (volatlie)
Parabolic gain -> oxide adheres to metal, barrier to diffusion
Linear gain -> oxide stays on the metal surface but cracks/peels off allowing more oxygen to reach
How does diffusion work in Silicon Oxide?
Oxide is placed on the surface which diffuses to the edge of silicon oxide where more reaction occurs. Jdiff = Jreact
What is the yield stress of pure aluminum?
40MPa
What is the difference between how strong obstacles pin dislocations compared with weak obstacles?
θ -> 0 for strong obstacles
higher θ for weak obstacles
What is the solvus line?
The line which marks a transition between single phase solid to a two phase solid, where precipitates are formed.
What is the driving force for the formation of CuAl2 in a Cu-Al alloy?
Reduction in Gibb’s free energy, diffusion enables this to occur.
Why does slow-cooled aluminium icrostructure provide ineffective precipitation hardening?
Large precipitates that are spaced far apart. This occurs because the larger the precipitates, the lower the Gibb’s free energy.
Where is the temperature which marks the solvus boundary on a TTT diagram?
Dashed line above all the curves.
What are TTT diagrams for?
quench and hold processes, holding a sample at a certain temperature below the solvus boundary.
What is the CCR?
Critical cooling rate, slowest cooling rate that avoids the C-curves, which therefore forms no precipitation phase.
What are the axes on both TTT and CCT.
TTT -> Tempertaure / log(time)
CCT -> Temperature / log(time)
What is the difference in terms of yield strength between a quenched and slow cooled aluminum alloy?
Quenched, effective solid solution hardening, therefore higher yield stress.
Slow cooled, ineffective large precipitate formation.
What are the three processes in age hardening?
Dissolving alloying elements,
Quenching, forming SSSS,
Ageing, re-heat allow for precipitation to occur via diffusion.
What happens over time during artificial aging?
Yield stress rises, as precipitates grow up to a peak value. However, the yield stress decreases as precipitates get bigger, until eventually its the same as the equilibrium microstructure (worse than SSSS)
What happens over time during natural aging?
yield stress gradually increases up to a plateau, precipitates grow but their growth is limited by diffusion at low temps.
What are the two methods by which dislocations pass through precipitates?
When precipitates are small they are weak obstacles, so can be sheared through directly.
strength ∝ radius^1/2
When they are large, the spacing is large. Therefore strength is now dominated by dislocations “bypassing” (breaking and reforming of dislocation)
Strength ∝ 1 / spacing ∝ 1 / radius
What are the difference between metastable and equilibrium precipitates?
metastable -> precipitate is coherent, lattice matches the Al, reducing interface energy but allowing shearing through.
Equilibrium-> when reaches CuAl2 its no longer coherent, lower gibbs free energy outweighs interface energy benefits.
What is the martensite phase transformation?
FCC lattice is trained to form BCC, this is achieved by shear, creating a needle of martensite within the austenite grain.
Why is there no time dependence for the formation of martensite?
Because it doesn’t rely on diffusion
What determines the Martensite Start Temperature?
A certain temperature below A1, the gibbs free energy of martensite is favourable. However, there needs to be a certain degree of undercooling to enable the transformation.
What happens as the carbon content of hypo-eutectoid steel increases?
Propotion of ferrite to pearlite decreases (more pearlite)
C curves move to longer times, larger amount of C to diffuse
Martensite temperatures decreases, larger amount of undercooling is needed due to more carbon in SSSS
What is the martensite finish temperature for eutectoid steel?
-50oC
Compare the yield stresses of slow cooled, bainite, and martensite?
slow cooled 200-450MPa
Bainite 500-700MPa
martensite 1000-3000MPa
What gives martensite a very high yield stress?
Super saturated with carbon, giving solid solution hardening.
Distortion of lattice, as C atoms are too large to fit in interstitial holes
What occurs during the process of tempering?
Martensite is reheated to around 450-600 oC
What is the effect of tempering martensite?
Yield strength reduces 450-800MPa
Fracture toughness significantly increases
What microstructural changes occur during tempering?
Carbon in SSSS diffuses to form Fe3C precipitates
Fe lattice changes returns to undistorted BCC.
How does temperature affect the tempering process?
High temperature, means faster coarsening.
Higher temperature also changes the volume fraction of precipitates (phase diagram)
What is hardenability?
How easily a steel forms martensite on cooling.
Higher hardenability => longer transformation times on TTT diagram, slow CCR, larger diaemeter that will give 100% martensite.
What are the advtanges of cooling a sample of steel slower whilst still creating martensite?
Lower thermal stress, less thermal distortion, lower risk of quench cracking.
High hardenability?
When do you want a steel with low hardenability?
When you don’t want martensite on cooling, such as when welding.
high hardenability => poor weldability.
What effect does Carbon concentration have on the hardenability of steels?
Increase the C content increases the hardenability (delays the diffusional phase transformations)
What effect does further alloying have on the hardenability of steel?
Increases hardenability by delaying diffusional phase transformations.
Alloying elements also need to be rearranged by diffusioned as solubility is different in austenite versus ferrite.
Why are high alloy steels useful?
Increases hardenability, (5-20% wt% of Mo W, and V)
quench and temper leads to the formation of alloy carbides, retaining precipitation hardening at high temp.
contribute to high temperature solid solution hardening.
What are two methods to case harden steel?
Carburising, placing steel in high temp carbon-rich atmosphere (such as molten cyanide)
Transformation hardening, applying a heat source to surface so it forms austenite which is quickly cooled to form martensite. Laser or induction coils.
How are grains formed on solidification after casting?
Polycrystal, grains originate from randomly oriented crystal nuclei.
What are the three zones of a typical casting grain structure?
Chill zone, near edge of mould the liquid cools rapidly, nucleation starts with sites for heterogeneous nucleation and high undercooling, small grain sizes due to high nucleation rate.
Columnar zone, preferentially orientated grains within chill zone grow towards the centre, undercooling isn’t great enough for new nuclei.
Equiaxed zone, nucleation occurs homogenously or heterogeneously within the remaining liquid, random directions. Lower undercooling so fewer nuclei.
Why are fine grain castings desirable for castings?
To enable grain boundary hardend.
𝜎 =𝐴+ 𝐵 /sqrt(𝑑) where d is grain boundary size.
How to achieve fine grain castings?
By using inoculants, solid particles added to the melt.
TiB2 added to Al castings. (Titanium diboride)
What is the typical composition of casting alloys?
Near eutectic: to reduce melting temperature, low ‘freezing range’ increasing fluidity,
What is the propertites of Al-Si casting alloys?
Two phase eutectic microstructure, higher strenght than pure Al, but needles of Si are detrimental to toughness
What is the typical range for cast irons?
1.8 wt% C -> 4.3 wt% C to achieve near eutectic.
What is the microstructure of a cast iron?
Pearlite + graphite (pure C)
What are the properties of a cast iron?
high strength and wear resistance, but poor toughness due to the brittle nature of graphite
How can you improve the properties of a cast iron?
Adding Mg improves toughness, graphite forms spheres instead of flakes, ‘poisoning’
What is segregation in cast alloys?
Concentration gradients accross grain boundaries. The first solid to form is purer, and the concentration changes as it is cast.
What is the difference between microsegregation and macrosegregation?
Accross a grain, gradient of concentration -> microsegregation
Accross a whole casting, from mould to casting -> macrosegregation
Where do impurities in casting alloys end up?
At the grain boundaries, as they are the last to solidify, and at the centre of the casting.
What are the consequences of segregation?
non-uniform distribution of main alloying elements, varying yield stress.
Impurities on grain boundaries, form solid phases or bubbles of gas, damaging toughness.
Solutions to segregation?
Reduce grain size with inoculants.
Add alloying additions that react with impurities, dispersing them.
Homogenise the casting, by heating it up to allow impurities to diffuse.
Advantages to deformation processing instead of casting?
‘wrought’ metals, have improved mechanical properties, lower temperatures, better surface finish and dimensional accuracy.
What microstructural changes occur during cold working?
Grains change shape (“pancaked” grains)
Dislocation movement creates work hardening
How do mechanical properties change during cold working?
Higher strength, but ductility dereases (risk of fracture)
How are metals annealed?
Heat to 0.5Tm, reducing yield strength and increasing ductility.
Where does work go during plastic deformation?
95% is dissapated as heat, 5% remains in material in dislocations.
when fractured, elastic energy stored is released
What 2 microstructural mechanisms occur during annealing?
Recovery, dislocations rearrange to reduce strain in crystal lattice.
Tmin = 0.1Tm
small drop in strength
Recrystallisation, new grains nucleate.
Tmin = 0.3Tm
large drop in strength
What is hot working and what are its advantages?
Process at 0.7Tm
low tool forces (low yield strength)
large strains in each pass
no need for separate heat treatment
How do pressures compare in different types of forging?
Open die, 1-2Y
Impression die, 3-5Y
Closed die, 5-10Y
What are the differences between thermoplastics, thermosets, and elastomers?
Thermoplastics - have no cross links, easy to mould and recycle
Elastomers -> low degree of covalent cross linking
Thermosets -> high degree of covalent cross linking
How does crystallinity of thermoplastics affect both its density and Young’s modulus?
More crystallinity -> closer molecular packing -> denser than amorphous (leads to shrinking)
More crystallinity -> higher YM -> YM high through glass transition until temperature breaks down van der Waals in crystalline regions
What are the regions of the TTT diagram for thermoplastics?
Above Tm, amorphous (stable)
Between Tg and Tm there is a set of C curves
Slow cooling leads to fully crystalline polymer.
Diffusion allows chains to rearrange.
Below Tg van der waals bonds are too strong for further crystallisation.
How does crystallisation occur in thermoplastics?
Nucleation and growth of plate shaped ‘lamellar’ crystallites within amorphous regions. Radially arranged lamellar structures form ‘spherulites’
What is the condition for a transparent polymer?
The avoidance of crystallinity, and crystallites will scatter light making the image appear cloudy.
How can crystallisation be controlled?
Choosing correct polymer, complex molecules are slower to rearrange, and harder to crystallise.
Controlling cooling rates (use TTT diagram)
How can deformation processing strengthen thermoplastics?
By stressing the polymer in one direction, the molecular chains untangle lining up in the loading direction. This leads to orientation strengthening.
What polymer processing methods take advantage of orientation strengthening?
Fibre drawing, fibres drawn out of molten die.
Stretch blow moulding. Preform called a ‘parison’ is pre-heated adnd placed into a mould. This is stretched and then filled with compressed air to fill the mould.
This leads to strengthing in hoop and longditudinal dierctions.
What is film blowing?
An extruded polymer is inflated by internal pressure than wound onto a drum. This creates alignment within molecular structure. Creates packing bags.
What are the two consequecnes of creep?
Continued straining at a constant applied stress,
Stress relaxation at a constant applied strain.
What are the three stages of creep?
Stage I primary creep - short time period
Stage II: Steady-state creep -dominates creep life
Stage III: Tertiary creep - leads to rupture.
How can Stage II creep be modelled?
strain rate ∝𝐴exp (− 𝑄/𝑅𝑇) 𝜎^n
Arrhenius law, and a power law.
What are the two main creep mechanisms?
Power law (dislocation creep),
Dislocations can escape obstacles by climbing over them via difusion, some of the dislocation moves onto a different slip plane. n = 3->8
Diffusional creep
Grains elongate in the direction of applied stress through atomic rearrangement (diffusion along grain boundaries, and through the crystal).
n=1 (less sensitive to applied stress as doesn’t depend on dislocation motion)
How does grain size affect diffusional creep?
Smaller grains -> faster creep as more diffusion along grain boundaries.
How does failure occur during creep?
Creep rupture. Voids nucleate on grain boundaries, reducing load bearing cross section, accelerating creep rate. Tertiary creep and rupture
What are the advantages of nickel-based alloys?
Higher melting temperature (reducing T/Tm)
High alloy content gives solid solution and precipitation hardening.
How can grain size control manage creep?
Casting with columnar grains, increases diffusion distances.
Casting as a single crystal, no grains.
