Rate-dependent transition of dislocation mechanisms in a magnesium alloy

The limited slip systems of magnesium (Mg) and its alloys hamper their widespread applications in key areas. Rational design of such lightweight alloys requires fundamental knowledge of their microscopic plasticity mechanisms which, however, remain partially unresolved. Here, to obtain a better understanding of the plastic deformation mechanisms of Mg alloys, we performed tensile straining over a wide range of strain rates from 10-5 s- 1 up to 2000 s-1, revealing for the first time the occurrence of a rate-dependent transition of the dislocation mechanisms. Such a shift of plasticity mechanisms is identified by two distinct activation volumes. Systematic experimental characterizations, such as transmission electron microscopy under two-beam conditions and synchrotron X-ray diffraction analysis, were employed to analyze both qualitatively and quantitatively the characteristics of dislocations at changing strain rates, revealing that the rate-dependent dislocation mechanisms are accompanied by the change of dislocation activities from easy-glide (a) dislocations to glissile (c + a) dislocations with increasing strain rates. Specifically, temporarily glissile (c + a) dislocations, enabled by a thermally activated transformation of dislocation cores from the dissociated configuration to the compact one, govern the plasticity at high strain rates. This is in stark contrast to the dominance of the easy-glide (a) dislocations at low strain rates. Meanwhile, it is found that abundant (c + a) dislocations do not necessarily lead to enhanced ductility, contrary to the common belief. We expect that these results will contribute to a further understanding of the plasticity mechanisms of Mg alloys.