Local hardening and asymmetric twin growth by twin-twin interactions in a Mg alloy

In this study, the role of twin-twin interactions on the distributions of local defects (e.g., dislocations) and stress fields in a magnesium alloy is investigated. A co-zone (101¯2)-(1¯012) tensile twin junction in a deformed Mg-3wt.%Y alloy is analyzed using transmission electron microscopy (TEM). The results show that the morphology of the impinging (1¯012) twin is asymmetric, and the non-interacting boundary of the recipient (101¯2) twin is irregular. Detailed analysis of TEM images reveals that type-II pyramidal [1¯21¯3](12¯12) dislocations concentrate in the vicinity of the twin-twin junction site. The same 〈c + a〉 dislocations are also observed inside the interacting twin domains along with a few 〈a〉 dislocations. The 〈c + a〉 dislocations emanating from the impinging (1¯012) twin boundary have edge character and are extended with faults parallel to the basal plane. In contrast, the 〈c + a〉 dislocations connected to the recipient (101¯2) twin are predominantly screw orientation and compact. Elasto-viscoplastic fast Fourier transform based crystal plasticity calculations are performed to rationalize the observed twin morphology and local dislocation distribution. The model calculations suggest that the local stress fields generated at the junction site where the two twins meet are responsible for the experimentally observed concentration of 〈c + a〉 dislocations. The calculated stress fields are asymmetric with respect to the junction site, explaining the observed asymmetric morphology of the impinging twin. Overall, these findings show strong effects of twin-twin interactions on the distribution of dislocations as well as the evolution of the twinned microstructure and as such, can help advance understanding of twinning in Mg alloys and their effect on mechanical behavior.