High-manganese austenitic TWIP (Twinning-induced Plasticity)
steels present a great interest due to their exceptional mechanical
properties and because of their peculiar work hardening mechanisms
that are still not fully understood. Stacking fault energy (SFE)
is definitely a parameter that greatly influences the work hardening
rate of the austenitic steels since it dictates the deformation
mechanism that preferentially occurs. It depends mainly on the
chemical composition and temperature [1–5]. If the SFE is large,
only dislocation glide takes place. On the contrary, if the SFE is
small, dissociation of perfect dislocations into partials is favoured
and mechanical twinning takes place as in the case of TWIP steels.
Furthermore, the Fe–Mn–C TWIP steels strained in tension also
present a so-called Portevin–Le Châtelier effect, with the appearance
of deformation bands resulting from dynamic strain ageing
[6–12]. Due to these different mechanisms, the exact nature of their
exceptional properties remains a matter of debate. It is often stated
that the formation of twins increases the work hardening rate by
bringing about obstacles to dislocation glide in a so-called dynamic
Hall & Petch effect [13–17]. An extra-hardening would also arise
from the difference in dislocation mobility in the twinned regions
and in the parent grain [18,19]. Karaman et al. [18] measured different
levels of hardness for the twins and the matrix in a single
crystal Hadfield steel. Recent publications [20,21] also showed that
mechanical twins in Fe–Mn–C TWIP steels contain a large density
of sessile dislocations. That would explain the hardness difference
of the twins compared to the softer austenitic matrix (at least at
the beginning of deformation). The mechanical twins could then
be considered as a second phase. A composite effect, as previously
stated by Gil Sevillano [22], could then take place and help to
explain the extraordinary properties of these TWIP steels.