Observation of Twin Transmission Process in Mg Alloys by In Situ EBSD

Abstract

Twin pairs are frequently observed at the common grain boundaries of Mg alloys. The formation of twin pairs is associated with a twin transmission (TT) process. Investigating the TT process is important for understanding the deformation mechanism of Mg alloys. Here, the authors report an in situ electron backscatter diffraction (EBSD) examination on the TT process in AZ31 Mg alloys subjected to tensile deformation. TT process is clearly observed by band contrast images resulting in the formation of twin chains as strain increases. Interestingly, transmitted twins are developed in advance in form of non‐indexing dislocations or twins in neighboring grain. The TT direction is roughly along the shear plane normal or shear direction of the paired twins. In addition, multiple twin variants are activated by TT mechanism and nucleation of new twins at pre‐existing twin boundaries.

Interactions between Dislocations and Boundaries during Deformation

“…Since hcp metals are favorable for the formation of deformation twins, they have been taken as one of the model metals to study the deformation twin-grain boundary interaction, especially pure Mg and Mg alloy. Similar results have been obtained in these studies [ 14 , 32 , 36 , 149 , 150 , 151 ] and several factors are engaged in the behavior of twin transmission across a grain boundary in coarse-grained metals, which has many characteristics in common with dislocation–grain boundary interaction.…”

“…On the other hand, compared with the dislocation–boundary interactions, there is few studies on the influencing factors of twin–boundary interactions. For example, the studies of twin–grain boundary interaction mainly focus on the grain boundary misorientation in hcp metals [ 14 , 32 , 36 , 149 , 150 ]. Whereas most researches of twin–twin interaction investigate what occurs during the interaction [ 13 , 38 , 39 , 148 ], but few of them involve investigations of influencing factors.…”

“…Twin transmission always occurs in the case of crossing a low angle grain boundary. The speed of twin transmission across low angle grain boundary is rapid, which is observed by in-situ SEM and in-situ optical microscopy (OM) mechanical testing [ 149 , 150 ].…”

“…The in-situ SEM technique can produce a high-quality image with a spatial resolution smaller than 1 nm and provide abundant information on microstructure, crystal orientations, phases, Schmid factors, and strains. It has been considered as one of the best methods for the investigation of the twinning behavior during deformation induced by the external stress [ 149 , 151 , 155 ]. Compared with the in-situ TEM technique, the in-situ SEM technique is capable for investigating the crystal orientation, microstructure and dynamic behavior at both large stains and large scales, which approaches the situation of macro-mechanical properties.…”