the effect of carbon content of the mechanical properties - McMaster ...

M.A.Sc. Thesis – E. E. Yang McMaster – Materials Science & Engineering
mirror image about the line PQ and the crystal structure before and after twinning seen in
Figure 2.6b with the atoms being sheared to the left (Kelly 1970). If Shockley partial
dislocations moved on every {111} plane (similar to inserting a crystal plane), an
extrinsic stacking fault is observed, leading to a twin stacking sequence (Olson 1976a).
As the twin formation changes the crystallography and not the chemical composition of
the structure, twining is not a diffusion-driven phase transformation. In the case of high-
Mn TWIP steels, the creation of twins is often regarded as a type of phase transformation
with the twins and parent matrix both being austenite.
a)
Figure 2.6 Twin Nucleation a) Stacking Sequence b) Crystal Structure Before &
After Twinning (Kelly 1970)
One model for twin nucleation focuses on the movement of partial dislocations.
As a twin is defined within a matrix with a twin-matrix interface, a shape change places
an elastic strain field on this interface (Figure 2.7a), with dislocations representing the
strain field (Figure 2.7b). When the twin is sufficiently thin, the stress experienced can be
represented by a dislocation pile-up as seen in Figure 2.7c (Kelly 1970).
b)
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