Geometric compatibility measure m? for twin transmission: A predictor or descriptor?

In this work, the geometric compatibility factor m ' is critically analyzed to assess whether it can be used to interpret/predict twin transmission (TT) across grain boundaries (GBs). This geometric measure is widely used to relate the likelihood of TT to the misalignment of both the shear and plane-normal directions within a twin set (i. e., incoming and outgoing twin). Using a large set of electron back scattering diffraction (EBSD) data, a detailed statistical analysis of twin-GB interactions is performed for {1012} tensile twins in hexagonal close-packed (HCP) metals Mg, Zr, and Ti at different strain levels. In addition, a full-field crystal plasticity model is employed to quantify the role of local stresses and the applicability of m ' as a criterion for the TT process. This combined study addresses the following three main questions: (i) What is the fidelity of m ' in describing experimentally observed TTs? (ii) Can m ' be used as a metric to predict/anticipate TT? (iii) Does m ' naturally capture local stress effects? As a descriptor, m ' cannot rationalize-25% of TT events observed in Mg or more than 50% of TT events in Zr and Ti. As a predictor, the m '-measure does not predict TT events in over-50% of twin-GB interactions analyzed. Further, the applicability of m ' to describe and predict TT events decreases with an increase in elastic anisotropy, plastic anisotropy, and macroscopic strain levels. Finally, the twinning simulations reveal that m ' does not capture the key effects of local stresses on variant selection upon twin transmission. The local stress induced by the twinning shear transformation plays a dominant role in driving the TT process compared to the geometric alignment of the constituting twins, i.e., m '.

Automated summary

This study critically analyzes the geometric compatibility factor m' for twin transmission across grain boundaries (GBs), and finds that m' cannot accurately describe and predict twin transmission events. The applicability of m' is influenced by the elastic anisotropy, plastic anisotropy, and strain levels. Local stresses play a dominant role in twin transmission compared to geometric alignment.