quiz 2 - strengthening mechanisms

Avoiding the movement of dislocations

Metals can be strengthened by?

Solid solution strengthening

Mechanism where impurity atoms distort the lattice and generate stress which produces a barrier to dislocation motion

• amount of solute

• size of solute atoms

• shear modulus of solute atoms

Strength of hardening effect is proportional to:

Alloys

These are usually stronger than pure metals

Ductile metals

These increase in strength when deformed plastically at cold working conditions

Strain Hardening

Referred to as cold working

Strain Hardening

Mechanism where there is an increase of dislocation density with plastic deformation

• hardness increases

• yield strength increses

• ductility infreases

Strain hardening changes the following parameters of the mayerials

Annealing

Process of removing the effects of strain hardening

Annealing

This softens the metal and reverts to strain free condition at high trmperature

• recovery

• recrystallization

• grain growth

Three stages of annealing

Recovery

stage of annealing at low temperature designed to eliminate residual stresses introduced during deformation without reducing the strength of the cold worked material

Recrystallizatin

Stage of annealing at medium temperature designed to eliminate all the effects of the strain hardening produced by cold working

Grain growth

Stage of annealing where the movement of grain boundaries by diffusion in order i reduce the amount of grain boundary areas

Fine grain size

Which grain size is stronger?

Grain size strengthening

mechanism where the grain boundary barrier to dislocation motion causes slip plane discontinues ot change orientation

Barrier strength

What increases with misorientation

Small angle grain boundaries

These are not effrctive in nlocking dislocation

High angle grain boundaries

grain boundaries that are block slip and increase strength of mayerials

• strength varies with grain size

• grain size can be controlled by rate of solidification

Hall-Petch Equation states that

• Treating with impurities called “grain refiners” —> increase number of nucleayion sites → higher nucleation rates

• cooling the materials faster during casting

• cooling the material faster during heat treatment

How to obtain smaller grain sizes

Precipitation hardening

Mechanism where strength is enhanced by precipitation of very fine uniformly dispersed hard phase within a softer matrix

Fine precipitates

these distort the parent lattice and act as obstacles to dislocation movement

Non coherent precipitates

has no relationship with the crystal structure of the surrounding matrix

Coherent precipitate

forms so that there is a definite relationshop between the precipitate’s and matrix’s crystal structure

• solution heat treatment

• quenching

• ageing

stages of heat treatment in precipitation hardening

solution heat treatment

process to form homogeneous single phase solid solution

quenching

process that produces supersaturated solid solution of alloying elements in the matrix

ageing

process that forms fine dispersion of precipitates

natural ageing

ageing at room temperature

artificial ageing

ageing above room temperature