Metals- Twinning

Question 1

Where is twinning routinely found?

Answer 1

Hexagonal metals at room temperatures.

BCC metals at cryogenic temperatures

Question 2

What is twinning?

Answer 2

The simultaneous or cooperative shearing of successive atomic planes

Question 3

Where are twins?

Answer 3

Extend across whole grain but stop at grain boundaries can can’t transition into grains of different orientation. Retain smooth interphase with the grain boundary

Question 4

Why is it called twinning?

Answer 4

Because the crystal either side of the twin boundary are mirror images of each other

Question 5

Twinning plane

Answer 5

The twin’s structure is a mirror image of the parent structure about the twinning plane which is the plane at the interface between deformed and undeformed structure.

Question 6

Twinning direction

Answer 6

The direction in which atoms move

Question 7

Shear strain from twinning

Answer 7

All cubic structures have same shear strain 0.707. For hcp it depends on the c/a so varies between materials

Question 8

What is hear strain equal to from twinning?

Answer 8

Magnitude of displacement (from twinning direction)/interplanar spacing between twin planes

Question 9

Features of dislocation slip that can be compared to deformation twinning

Answer 9

No change in crystal orientation in slipped regions
Deformation as a result of slip appears step like
Movement on discrete planes
Stress required to initiate slip lower but strongly dependent on T
Time dependency, especially when diffusion required for climb

Question 10

Features of deformation twinning that can be compared to dislocation slip

Answer 10

Deformed/twinned section of grain exhibits mirror orientation compared to undeformed part of grain.
Deformation appears wedge like.
Relatively small movements on every plane.
Stress to initiate twinning higher but independent of T.
Stress to initiate much larger than stress to propagate.
Progresses very fast (speed of sound)

Question 11

When is twinning prominent?

Answer 11

In cases where dislocation slip is inhibited

Question 12

Conditions where twinning favoured over dislocation slip

Answer 12

Low crystallographic symmetry
Low temperatures
High strain rate applied
Low stacking fault energy

Question 13

Low crystallographic symmetry

Answer 13

For hcp crystals number of potential active slip systems is reduced. Hence slip inherently inhibited. Twinning very prominent in hcp for which the resolved shear stress on the basal plane (easiest slip plane) is low

Question 14

Low temperatures

Answer 14

Whilst stress to initiate twinning quite large it is independent of T. Hence at low temperatures (where stress required for dislocations to slip is very high) deformation preferentially occurs by twinning

Question 15

High strain rate applied

Answer 15

This is the rate of deformation. The strain rate of deformation used also has pronounced effect on stress required to initiate slip and very little effect on stress to initiate twinning. Hence at high strain rates twinning is preferred and can be initiated at higher temperatures

Question 16

Low stacking fault energy

Answer 16

Twinning stress strongly dependent upon the stacking fault energy of the material by:
σtwin proportional to γ^1/2 where γ is stacking fault energy.
Result of the rate of work hardening in the material. Low stacking fault energy materials show high rates of work hardening as cross slip more difficult so dislocation motion inhibited.

Question 17

Twinning and grain size

Answer 17

Grain size does not determine twinning prevalence in isolation. Has a different effect for different materials

Question 18

What can twinning lead to?

Answer 18

A reduced ductility.

Produces coherent grain boundaries and so provides a mechanism for work hardening

Question 19

When can twinning occur?

Answer 19

Either during deformation or during heat treatment/annealing following recrystallisation

Question 20

Shape of deformation twins

Answer 20

Extend across entire grain and lenticular in shape, tapering off as it reaches grain boundaries or other twin boundaries (need to maintain strain compatibility at the grain boundaries)

Question 21

Shape of annealing or recrystallisation twins

Answer 21

Appear as bands through the grain. Don’t taper off at grain boundaries and may stop abruptly in a grain