Dislocation transmutation by tension twinning in magnesium alloy AZ31

The interaction between dislocations and {10 (1) over bar2}<(1) over bar 011 > twins appears to play an important role in the strain hardening behavior of Mg. Detailed transmission electron microscopy study was performed to investigate the concept of dislocation "transmutation" across twin boundaries. A previously proposed dislocation transmutation reaction is confirmed. For (10 (1) over bar2) twins, the transmutation reactions involve [a(1)] or [a(3)] matrix dislocations resulting in (c a2) dislocations, which populate the vicinity of the twin boundary. No other < c +/- a(2)> slip systems are observed within the twins, despite the fact that the < c +/- a(2)> dislocations have similar or lower resolved shear stress, as compared to other slip systems. This suggests the (c + a) slip mode is source limited, since the observed slip systems are the only ones that can result from transmutation. The formation of a unit (c + a) dislocation in the twin is proposed to involve two consecutive reactions, necessitating dislocation pile-up in the matrix, and the associated dislocation configurations are evaluated in terms of elastic strain energy considerations. Dislocation reactions are proposed which could explain the presence of basal stacking faults of either I-1 or I-2 type inside twin. At the low stress levels typical of {10 (1) over bar2}<(1) over bar 011 > twinning dominated flow of textured polycrystals, the observed < c +/- a(2)> dislocations are most likely sessile. However, this could serve as a source mechanism for later deformation, and as a forest hardening mechanism against other slip systems. The interfacial reaction products could result in a dragging effect on twin boundary advancement, and establish a basis for subsequent rapid hardening. (C) 2016 Elsevier Ltd. All rights reserved.