altering material properties

Grain size reduction

size of grains in polycrystalline metal influences mechanical properties

• slip or dislocation motion during deformatio must occur across common boundary

yield strength variation by grain size

as grain size decreases, yield strength increases.

yield strength in terms of grain size

yield strength = constant + constant/sqrt average grain size

strain hardening

basically cold working, a metal becomes stronger as it is plastically deformed

• degree of plastic deformaton defined by amount of cold working

• but there is a reduction in ductility due to increased tenisle strenght

• strain hardening increases dislocation density meaning its harder to deform

rolling effect on grains

grains are elongated and sistorted, so more grainoundaries so dislocation density increased

precipitation hardening

hardness of some alloys enhanced by forming extremely small and uniformly dispersed particles of a second phase in original pjhase

solution heat treatment

1) heat to T0 (single alpha phase)

2) quench (rapidly cool) to T1 (alpha and beta phase)

3) so fast that diffusion of beta is prevented

4) this creates non-equilibrium structure of alpha phase with super saturated beta phase

precipitation heat treatment

• supersaturated alpha is heated to alpha+beta region where diffusion is faster

• beta starts to form

• it is cooled after a set time

overaging

if heated for too long, 2nd phase grows and strength increase is lost

fine and coarse pearlite

full annealing forms coarse pearlite (equilibrium) which is very slow cooling, normalsing which is faster cooling forms finer pearlite and less time for atoms to diffuse so they diffuse a shorter distance leadning to dinal structure

bainite

• transformation of austenite

• consists of two phases ferrite and cementite, like pearlite

• structure is very fine, resolved by microscope

• bainite forms as needles or plates and at lower temps than pearlite

martensite

• if quenched then there will be no time for atoms to diffuse through structure

• hard and brittle

• martensite can be tempered to restore ductility (heated)

non equilibrium phase structures of steel

full annealing (very slow cooling): leads to coarse pearlite

normalising (fast cooling): leads to fine pearlite

forced air cooling: leads to finer ferrite and cementite layers called bainite

quenching (fast cooling in opil:: gives no time for carbon to diffuse so austenite leads to hard martensite

solid solution hardening

high purity metals are always softer than their alloys

• increasing impurities increases tensile and yield strength

• impurities cause lattice strains which impede dislocations

• higher dislocation density means higher strength

tensile lattice strain

caused by smaler substitution

compressive lattice strain

caused by larger substitution

shape memory alloys

ablity to remember initial shape after deformation when heated

• nickel - titanium alloy