CHE217 - 2.3 Strengthening Mechanisms

Solid Solution Strengthening

• Impurity atoms distort the lattice and generate stress

• Stress can produce a barrier to dislocation motion

Substitutional impurities

Smaller and larger __ __ tend to diffuse into strained regions around dislocations.

• Amount of solute

• Size of solute atoms

• Shear modulus of solute atoms

Strength of hardening effect is proportional to:

Strain Hardening

• Also called as COLD WORKING

• Plastic deformation of ductile materials to increase strength

• Temperature is lower than recrystallization temperature (~ 1/3 to ½ of melting temperature)

• Increase of dislocation density with plastic deformation.

Parameters affected due to strain hardening

• Increasing hardness

• Increasing yield strength

• Decreasing ductility (materials are more brittle)

Annealing

• Removes effects of coldworking

• Softens metal and reverts to strain-free condition

Recovery

• Low temperature annealing

• Eliminate residual stresses introduced during deformation

• Does not reduce strength

Recrystallization

• Medium temperature annealing

• Eliminates all effects of the strain hardening produced

Grain growth

• Movement of grain boundaries by diffusion to reduce the amount of grain boundary areas

Grain size strengthening

• Materials with fine grain size are stronger than materials with coarse grains.

• Barrier strength increases with misorientaation

• Grain boundary barrier to dislocation motion → slip plane changes orientation/discontinues

Hall-Peth Equation

• σ_y = σ_o + k_yd^(-1/2)

• Strength varies with grain size

• d can be controlled by rate of solidication

Obtaining smaller grain sizes

1. Grain refiners

2. Cooling materials faster during casting

3. Cooling materials faster during heat treatment

Precipitation Hardening

• Age hardening - strength develops through heat treatment with time

• Strength is enhanced by precipitation of very fine and uniformly dispersed hard phase

• Fine precipitates distort the parent lattice and act as obstacles to dislocation movement

Requirement for age hardening alloys

1. Phase diagram must show decrease in solubility

2. Matrix phase must be ductile and strengthening precipitate is hard

3. Precipitate phase should be coherent with the matrix but distort sufficiently to block motion

4. Alloy should be quenchable

Noncoherent precipitate

• No relationship with the crystal structure

Coherent precipitate

• There is a definite relationship between the precipitate’s and matrix’s crystal structure.

Solution heat treatment

• Form a homogeneous single phase solid solution

Quenchihng

• Produce a supersaturated solid solution of alloying elements in the matrix.

Ageing

• Form fine dispersion of precipitates

• Natural ageing: Room temperature

• Artificial ageing: Above room temperature

Summary of Mechanisms