The effect of grain size and grain orientation upon deformation twinning in a Fe–22Mn–0.6wt%C twinning-induced plasticity steel was investigated by using microstructure observations by electron channelling contrast imaging and electron back-scatter diffraction. Samples with average grain sizes of 3 and 50μm were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which led to the proposal of a Hall–Petch-type relationship for the twinning stress using the same Hall–Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress was more complicated. At low strain, a strong influence of grain orientation on deformation twinning was observed which fully complies with Schmid's law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to <111>//tensile axis directions where the twinning stress was larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid's law was observed. Deformation twins were now also observed in grains unfavourably oriented for twinning according to Schmid's law. This deviation was explained in terms of local grain-scale stress variations. The local stress state controlling deformation twinning was modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighbouring grains.

The Effect of Grain Size and Grain Orientation on Deformation Twinning in a Fe–22wt%Mn–0.6wt%C TWIP Steel. I.Gutierrez-Urrutia, S.Zaefferer, D.Raabe: Materials Science and Engineering A, 2010, 527[15], 3552-60