MAE 381 EXAM 3 (strengthening mechanisms)

If we want to make a material harder and stronger → _________ dislocation motion.

restrict

1) Grain Size Reduction

decreasing size of crystalline grains - Grain boundaries act as a barrier to dislocation motion

Two Reasons that Grain Boundaries act as a Barrier:

1) Dislocation must change slip direction as it crosses boundary (More difficult for grain dislocations to cross high angle grain boundaries → pile up instead of crossing)
2) Atomic disorder results in a discontinuity of slip planes

Rate of Solidification

slower cooling → larger grains

Heat treatments

grains grow at elevated temps

2) Solid Solution Strengthening

Adding impurities atoms to metals.

impurity atoms increase lattice strain, different sizes distort the crystal lattice, making dislocation movement more difficult

Why do impurity atoms cause lattice strain?

Impurity atoms are different sizes from the host atoms, so they distort the surrounding crystal lattice, then they store lattice energy.

Where do smaller impurity atoms prefer to locate near an edge dislocation?

In the compressive region where atoms are squeezed together → reduces overall lattice strain

Where do larger impurity atoms prefer to locate near an edge dislocation?

In the tensile region where atoms are further apart →reduces overall lattice strain

3) Strain hardening

Plastic deformation increases hardness and strength by increasing dislocation density -. dislocations are harder to move when they are closer together

Why does plastic deformation increase dislocation density?

Dislocations multiply during plastic deformation.

How do two positive edge dislocations on the same slip plane interact?

They repel each other because their stress fields interact. This repulsion can obstruct their movement. (more likely)

What happens when two edge dislocations of opposite signs meet on the same slip plane?

They attract each other. If they meet, their extra half-planes can join and the dislocations can annihilate. (not as common)

Why does strain hardening depend on dislocation density?

Dislocations are harder to move when they are closer together

As yield strength and tensile strength increase with additional cold work we sacrifice_____.

ductility

Recovery-________

relief of some internal strain energy, usually by heat treatment.
why heat? → enhance atomic diffusion

Why does a cold-worked metal have high internal strain energy?

Plastic deformation increases dislocation density. These dislocations distort the crystal lattice and store strain energy

Recrystallization

The formation of a new set of strain free grains within a previously cold-worked material → usually requires annealing heat treatment

Grain boundaries have ______ energy.

High

Increasing grain size _________ overall energy.

decreases

Where does grain growth occur?

by the migration of grain boundaries