Twin migration in Fe-based bcc crystals: theory and experiments

We establish an overall energy expression to determine the twin migration stress in bcc metals. Twin migration succeeds twin nucleation often after a load drop, and a model to establish twin migration stress is of paramount importance. We compute the planar fault energy barriers and determine the elastic energies of twinning dislocations including the role of residual dislocations (b(r)) and twin intersection types such as < 1 1 0 >, < 1 1 3 > and < 2 1 0 >. The energy expression derived provides the twin migration stress in relation to the twin nucleation stress with a ratio of 0.5-0.8 depending on the resultant residual burgers vector and the intersection types. Utilizing digital image correlation, it was possible to differentiate the twin nucleation and twin advancement events experimentally, and transmission electron microscopy observations provided further support to the modelling efforts. Overall, the methodology developed provides an enhanced understanding of twin progression in bcc metals, and most importantly the proposed model does not rely on empirical constants. We utilize Fe-50at.%Cr in our experiments, and subsequently predict the twin migration stress for pure Fe, and Fe-3at.%V from the literature showing excellent agreement with experiments.