IB Materials ALL

What is composition

Percentages at the ends of the tie line

Why do compounds form

That single phase lattice has lower free energy

What is Austenite

Gamma FCC high temp

What is ferrite

Alpha BCC lower temp

What is cementite

Fe3C compound

Eutectoid vs eutectic

Oid solid, tic liquid, both points (on either side of line)

Eutectic v peritectic

Eu is v shape peri is hat shape

How do pure metals form solids

Nucleation, atoms transfer at solid liquid interface, solid region impingement (glob together), solid grains

Describe dilute solidification

Same as pure but once grains fully solid, solute precipitates in them

Describe eutectic solidification

Liquid transforms directly to two solid phases, diffusion distance small so plates or needles form

Describe off eutectic solidification

One nucleates, regions grow, liquid region has eutectic solidification, precipitates form in initial grains

What is pearlite

Ferrite and cementite layers formed at eutectic point

How does entropy relate to number of states

Entropy is Kb x ln(#states)

For n total atoms, x of one type, how many configurations

nCx

For a polymer what is the radius of gyration

How far away the end could be

Define immiscible

Can’t mix, like oil and water

Why are foams bendy

Cell walls thin and easy to deform

A polymer above Tg is

Viscoelastic

What causes elasticity of polymer chains

Entropy- larger end to end distance means fewer configurations, more entropy

How to increase elastic modulus of elastomer

Increase cross linking

Under isobaric and isothermal conditions, what happens to Gibbs

Variation must be negative, at eqm it is a minimum

What determines what state a material is in

The lowest Gibbs free energy at those conditions

What makes a Gibbs graph have a boob shape (G against composition)

It’s a smiley face because mixing lowers available energy, but is pushed up in the middle because entropy is highest at equal proportions

Does surface energy help solidification

No it is a barrier to it

What determines the stable nucleus size when a liquid nucleates

The difference between surface energy (positive G) and bulk energy (negative G)

When is pi not 3.1415…

When it’s in an equation representing osmotic pressure

What is the equation for osmotic pressure and when is it valid

R (proper R 8.314) x temp x conc. Valid for low conc, less than 1 mol per litre

What is the rough time for diffusion

Distance squared over diffusion coefficient

Name four diffusion mechanisms and give their speed

Bulk interstitial diffusion (fast), bulk vacancy diffusion (slow) short circuit diffusion along grain boundary (fast), short circuit diffusion along dislocation cores (fast)

What is the average energy per atom

3 x Kb x T

What is the probability of an atom having energy greater than q

Exp(-q/KbT)

What forces drive diffusion

Change of G due to phase change, surface energy, stress field, electric field

What is the Arrhenius law

Rate of process is proportional to exp(-Q/RT) where Q is the activation energy

Explain a trade off for temperature of phase transformation (freezing eg)

It needs to be large enough to promote atomic movements, but low enough that there is enough thermodynamic driving force for nucleation

What are the two stages of silicon doping

Pre-deposition (dopant left on surface at high temperature until the correct quantity has entered) and drive-in (surface supply removed, temp increased, diffusion into material)

How is electrode potential measured

By having one electrode to be tested and one arbitrary reference electrode doing electrolysis

What does a positive standard potential mean

Less electronegative, does not want to lose electrons

What does it mean if a metal has a more negative standard potential

It is more electronegative, it is more prone to lose electrons

In electrolysis where do electrons go

From the anode to the cathode

What is a half cell reaction

Metal —> ion and electron (anode) or ion and electron —> metal (cathode)

How do you avoid galvanic corrosion

Do not join metals with very different electro potentials

How does a sacrificial anode work

Blocks of zinc joined to metal, zinc is preferentially corroded so protects boat hull. Corrosion is concentrated on the more reactive metal

If an oxide protects the metal it forms on then rate of oxide formation is

Proportional to the square root of time as it is limited by diffusive transport

What is precipitation hardening

Strong second phase particles pin dislocations

What is the issue with slow cooling a metal

The precipitates have time to grow and reach a size where they can be sheared and are no longer obstacles to dislocations

What does small undercooling mean

Small change in G so low thermodynamic driving force

What does a large undercooling mean

Low thermal energy so rearrangement of atoms by diffusion is slow

What is a TTT diagram

Time Temperature Transformation- Plot of temperature against time showing phase changes due to precipitation. They apply to isothermal transformations only

What is a CCT diagram

Continuous cooling transformation. C curves show the fraction transformed for a fixed cooling rate.

What is the CCR

The critical cooling rate, the slowest the metal can cool without precipitation

What is a SSSS

Supersaturated solid solution- when a metal is quenched it is not at equilibrium but is metastable, one element is trapped in the lattice of the other

What are the three steps of age hardening

Solution heat treatment, to dissolve alloying elements and achieve single phase solid solution (get very hot)

Quenching, cool faster than CCR to form a SSSS

Ageing, reheat to allow time for precipitation to occur

What does ageing do to a metal

Precipitates nucleate and grow

What is martensite

Austenite (FCC), when quenched, transforms to BCC without diffusion by straining to make martensite

What is Bainite

Iron with fine dispersion of cementite precipitates

What are the properties of martensite

Very high yield strength, negligibly low fracture toughness (very brittle)

What is hardenability of a steel

How easily it forms martensite

What is carburising

Immersing steel at high temperature into a carbon rich environment so that carbon diffuses into the surface

What are the three zones inside a cast

Chill zone- at mould wall, cools faster than CCR, small grains, lots of heterogeneous nucleation

Columnar zone- grains grow away from cast wall, not enough undercooling for new nuceation

Equiaxed zone- enough cooling for nucleation, larger grains than chill zone, mostly heterogeneous nucleations on impurities

Why don’t you want flakes or needles of graphite in your steel casting

Graphite is brittle so they act as sharp cracks. A small alloying addition of Mg causes the graphite to form spheres instead and makes it stronger.

What is segregation

When a metal cools through a two phase region, the first solid to form is purer. As it cools, the concentration of the solute around the solid increases so diffuses in more readily, so across grains there is a concentration gradient.

What does cold working a metal do

Changes the shape of grains (squashes them) and does work hardening (increases dislocation density)

What does annealing (heating to half of Tm and cooling) do

Dislocations rearrange to reduce lattice strain (recovery) and new large crystals grow.

Why does a metal need to be annealed periodically when rolled

The yield stress gets higher when cold rolled and needs to be lowered to be worked

What is forging

Compressive plastic deformation between two dies

How does a thermoset differ from an elastomer

Thermosets have more cross linking

How do elastomers differ from thermoplastics

Thermoplastics have no cross linking

Name two things that increase the crystallinity of a polymer

Slow cooling, molecules with no branching

What are the three stages of creep

Primary- dislocation motion

Steady state- dislocations hit obstacles

Tertiary- voids form and grow

How does grain size affect creep

Large grain = slow creep

What are some strategies to reduce creep

Alloying to reduce dislocation creep

Controlling grain size

Continuous cooling of part