Understanding common grain boundary twins in Mg alloys by a composite Schmid factor

It is well known that twinning likely nucleates and forms pairs at grain boundaries (GBs) during the deformation of Mg alloys. Therefore, the crystallography of GBs plays an important role in selecting twin variants. In this regard, the Schmid factor (SF) cannot predict from which GBs twinning prefers to nucleate. To solve this problem, a composite Schmid factor (CSF) is proposed in this paper that incorporates both the SF and a geometric compatibility factor (m). First, the link of the CSF with the SF and m' is defined and discussed based on theoretical derivations, which is further verified in terms of crystal plasticity finite element simulations. Then, experiments were carried out to investigate the effects of this parameter in predicting {10-12} extension twins in AZ31 Mg alloys. A CSF threshold for the activation of common-boundary twins was noticed and it decreases with the applied strain. By contrast, the SF threshold and its dependence with strain is not obvious. Moreover, the CSF is more effective in predicting the variants of common-boundary twins especially for small strain.