Altering metal alloy properties

SMAS

Have the ability to remember the shape of the material sample before deformation and return to it when heated/

Define annealing

Cooling down a material slowly In order to relieve internal stresses thus softening and increasing toughness of the material

Define Normalising

Cooling a material sample fast via air - between annealing which uses a furnace to slowly cool and quenching which uses liquid to cool super fast.

Define Quenching

Cooling down a material so quickly that it strengthens the material as the atoms cannot return to their natural lattice structure in time.

Define Tempering

Heat treatment process that reheats a hardened material like martensite to reduce brittleness/increase toughness whilst retaining most of the strength.

How does grain size influence strength of the material?

Note: this is where the hall-perch equation comes in - d = average grain size.

Larger grains means softer/less strong and vv

Grain boundaries act as barriers for the transfer of dislocations during plastic deformation (difficult to continue dislocation from one grain to the other)

therefore more grains = more boundaries = more resistance to plastic deformation = stronger.

4 ways of strengthening metals

Solid solution hardening

Strain hardening

Precipitation hardening

Grain size decreasing

NOTE: draw back of strengthening tends to be a decrease in toughness/increase in toughness this can be somewhat countered by the likes of tempering.

Define dislocation

A linear defect in a lattice material including an abrupt change in the atoms’ arrangement

Solid solution hardening key pts

Note: hardening/strengthening = same thing basically

Uses impurities/solute atoms to disrupt the lattice structure, whether smaller or bigger than solvent atoms, they cause lattice strains and are considered to be dislocations.

These dislocations/impurities prevent layers from sliding over eachother and hence strengthen the material

How do you WEAKEN via grain size?

Must make the grains larger therefore heat to HIGH TEMP and keep there for LONG TIME.

Strain Hardening/ cold working key pts

%CW equation comes in

Deform plastically in order to strengthen!

Does this by increasing the dislocation density therefore layers can’t slide over eachother…

Precipitation hardening key pts

Increases strength of material via formation of second phase across all of material sample.

These second phase areas make it hard for dislocations to propagate through the whole sample and hence strengthen mat.

to make the second phase though you need:Solution heat treatment + precipitation heat treatment.

Solution heat treatment (part of precip)

Heat up to single phase system (inc in temp inc in solubility of solute in solvent)

Quench rapidly into what is usually the 2phase region - cooled so quickly that alpha retains said additional solute in solid solution and hence is super saturated - i.e. usually contains less of solute at that temp.

Precipitation heat treatment/AGING (part of precipitation hardening)

Once solution heat treatment is complete, the super saturated solution is reheated to a high temperature STILL WITHIN THE 2 PHASE REGION.

HOLD FOR SET TIME.

Then allowed to cool down resulting in the formation of beta phase areas across the whole material sample evenly distributed everywhere, each acting as tiny barriers that dislocations find very hard to transfer through, thus strengthening the material.

OverAGING

If you heat up the material during aging/precipitation heat treatment for too long at temp T2, the millions of small 2nd phase areas across the material will grown into bigger areas and STRENGTH IS LOST.

Fine and course pearlite - how does this happen?

When cooling through/near the eutectoid point but cooling either fast or slower.

Cooling fastER i.e. through normalising = hardER = fine pearlite

Cooling slowER i.e. through annealing = softER = coarse pearlite (bigger pieces)

Martensite

Made of super saturated SINGLE phase - its a form of steel produced when cooled via quenching i.e so fast that the solution doesnt have time to seperate into two different phases.

Bainite

Bainite - formed at cooling rates between that of pearlite and martensite i.e. not quenched but not slow annealing or normalising either, is also a 2 phase form like pearlite made of the same two phases, ferrite and cementite except its even finer than the finest pearlite.

What do the lines represent on a time-temperature transformation plot

Gradient of straight lines indicate rate of cooling

Curved lines represent the temperatures and times at which the formation of each type of microstructures forms.