Chapter 8 Strain hardening and

annealing

Reading

All of Ch. 8 except subsections in Sec. 8-1 on strain-hardening

exponent, strain-rate sensitivity and Bauschinger Effect.

Homework No. 10

Problems 8-19, 8-22, 8-28, 8-54, 8-64

Strengthening mechanisms in metals

A correlation exists between dislocation motion and mechanical behavior of metals.

Macroscopic plastic deformation motion of large #s of dislocations.

The ability of a metal to plastically deform depends on the ability of dislocations to move.

Limiting the dislocation motion hardness and strength increase greater mechanical forces required to initiate

plastic deformation.

Strengthening mechanisms in metals

Strengthening principle: restricting or hindering dislocation motion renders a material harder and stronger.

Mechanisms for strengthening single phase metals:

grain size reduction

solid solution alloying

strain hardening

Strengthening by grain size reduction

A grain boundary poses a barrier to dislocation motion for two reasons:

A dislocation moving in grain A to pass into grain B of different orientation will have to change its direction of motion. This is rather difficult.

Slip planes are discontinuous from one grain to the other.

Strengthening by grain size reduction

Dislocation pile-up

Strengthening by grain size reduction

A fine grain material is harder and stronger than one that is coarse grained.

Toughness also improves with finer grain.

Small-angle grain boundaries are not as effective as large-angle grain boundaries in interfering with dislocation motion.

Solid-solution strengthening Alloys are almost always stronger than their pure metals,

because the solute atoms strain the solvent lattice.

These strain fields interact with those of the dislocations restricting the dislocation movement.

Solid-solution strengthening Solute atom and its segregation

towards dislocations causes reduction of the strain fields.

As solute atoms are attached to the dislocations, the resistance to slip is greater since dislocations have to be torn away from them to move.

Solid-solution strengthening

Hardness and strength increase with increase of alloy concentration.

Ductility usually decreases.

Solid-solution strengthening

Solid-solution strengthening

Solid-solution strengthening

Strain hardening (Work hardening)

Cold Work: Mechanical deformation of a metal at relatively low

temperatures (below about 1/3 of the melting temperature in K).

% C.W. is defined relative to the reduction in cross sectional area of the material.

1000

A

AACW% do

Strain hardening (Work hardening)

The fibrous grain structure of a low carbon steel produced by cold working: (a) 10% cold work, (b) 30% cold work, (c) 60% cold work, and (d) 90% cold work (250). (Source: From ASM Handbook Vol. 9, Metallography and Microstructure, (1985) ASM International, Materials Park, OH 44073. Used with permission.)

Common metal working methods

Rolling

Open die forging

Closed die forging

Direct extrusion

Indirect extrusion

Wire drawing

Stamping

Strain hardening Process whereby a metal is plastically deformed, making it

harder and stronger. Stress-strain diagram & strain hardening.

A material is stressed beyond the yield strength before the stress is removed.

Now the material has a higher yield strength and tensile strength but lower ductility.

By repeating the procedure, the strength continues to strength and the ductility continues to decrease until the material becomes very brittle.

Strain hardening

Dislocation multiplication and strain field interactions dislocation motion is hindered by the presence of other dislocations.

As the dislocation density increases, dislocation motion resistance by other dislocations becomes more pronounced.

Annealing

To make a material more ductile after cold working

Stages of annealing

Thermal recovery - Stress relief

- Dislocation rearrangement

Recrystallization - Birth of new strain-free grains

Grain growth

Effect of annealing time at a fixed annealing temperature

Brass

Cold-worked brass

After 3 s at 580°C, new grains appear.

After 4 s at 580°C, many more grains appear.

8 s at 580°C, complete recrystallization has occurred.

1 h at 580°C, substantial grain growth has occurred.

Effect of annealing temperature

Brass

Annealed at 400°C Twin boundaries

Annealed at 650°C

Annealed at 800°C

Recrystallization

Effect of prior cold work on recrystallization temperature

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During hot working, the elongated anisotropic grains immediately recrystallize. If the hot-working temperature is properly controlled, the final hot-worked grain size can be very fine.

Hot working