In-situ EBSD study on twinning activity caused by deep cryogenic treatment (DCT) for an as-cast AZ31 Mg alloy

This work studied the activation of the twins in a commercial cast AZ31 Mg alloy by deep cryogenic treatment (DCT) using in-situ EBSD. The initial as-cast microstructure has a range of grain sizes. The DCT mainly activated twins in the areas of fine grain size, in which the twin area fraction increased with increasing DCT time. These twins were {10–12} extension twins, which were evoked and facilitated by the large tensile interior stress that was caused by the large temperature difference during the DCT. Furthermore, only {10–12} extension twins were activated. The activated {10–12} extension twins had the highest Schmid factor and conformed to Schmidt's law. One or two other kinds of tensile twinning variants of the six tensile twinning variants (V1–V6) were activated by the DCT. These activated variants had the highest Schmid factor and the lowest orientation difference (less than 10o), while those non-activated variants had an orientation difference of ∼60o. Theoretical evaluation indicated that these two kinds of tensile twinning variants were the para-position variant rather than the ortho-position variant or the meta-position variant.