Steels: twinning-induced plasticity. Lecture 11 of 12
Channel: bhadeshia123
Date: 2020-10-03

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 it typically is 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. 


The addition of aluminium counters this because it raises the stacking
fault energy of the austenite. Silicon has the opposite effect of
reducing the stacking fault energy, but like aluminium, it leads to a
reduction in the density of the steel; the combination of Al and Si at
the concentrations used typically reduces the overall density from some
7.8\,g\,cm$^{-3}$ to about 7.3\,g\,cm$^{-3}$.




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