Steels: twinning-induced plasticity steels
Channel: bhadeshia123
Date: 2022-03-11

There are three essential modes by which steels can be permanently
deformed at ambient temperature, without recourse to diffusion.
Individual dislocations whose Burgers vectors correspond to lattice
vectors can glide, leading to a change in shape without altering the
crystal structure or volume. In contrast, a displacive transformation
(e.g. martensite or bainite) results not only in a plastic strain, but
also a change of crystal structure and density; this is the phenomenon
exploited in the TRIP steels. 

The third mode of deformation is mechanical twinning, in which the
crystal structure of the steel is preserved but the twinned region is
{reoriented} in the process. Mechanical twinning results in a much larger
shear strain (0.7071...), compared with displacive transformations
where the shear is typically 0.26. There is a particular class of
extraordinarily ductile alloys of iron, known as the TWIP steels, which
exploit mechanical twinning to achieve their properties.

TWIP stands for twinning-induced plasticity. The alloys are austenitic
and remain so during mechanical deformation, but the material is able to
accommodate strain via both the glide of individual dislocations and
through mechanical twinning. The alloys typically contain a large amount of manganese, some aluminium and silicon (e.g. Fe-25{Mn}-3Si-3Al wt%) with carbon and nitrogen in some versions present
essentially as impurities. Larger concentrations of carbon may be added to enhance strength. At high manganese concentrations, there is a
tendency for the austenite to transform into epsilon-{martensite}
(hexagonal close packed) during deformation.

The alloys have a rather low yield strength at  200-300 MPa but the  ultimate tensile strength can be much higher, in excess
of 1100 MPa. This is because the strain-hardening
coefficient is large, resulting in a great deal of uniform elongation,
and a total elongation of some 60-95\%. The effect of mechanical
twinning is two-fold. The twins add to plasticity, but they also have a
powerful effect in increasing the work-hardening rate by subdividing the
{untwinned} austenite into finer regions.

Associated teaching materials can be found on:
http://www.phase-trans.msm.cam.ac.uk/teaching.html
H. K. D. H. Bhadeshia