Deformation in the γ-Mg17Al12 phase at 25–278 °C

Abstract

The complex metallic phase γ-Mg17Al12 is found in most Mg-Al alloys, however, its mechanical properties are largely unknown. Here, its deformation at 25–278 °C is studied using several instrumented indentation methods (quasi-static, rate jump and stress relaxation) in conjunction with atomic force and electron microscopy. Two distinct deformation regimes with a transition around 150 °C are identified by all techniques. Serrated flow, slip steps and weak thermal activation characterise the low temperature regime, while homogeneous flow in terms of both load-displacement data and distribution of dislocations is seen above 150 °C. The results are discussed in terms of the underlying deformation mechanisms and the role of thermal activation. The transition from lattice or solute diffusion controlled, serrated flow to homogeneous flow is consistent with the onset of dislocation climb. Preliminary observations by TEM are in agreement with expectations based on gamma surface calculations in the literature. The comprehensive use of different instrumented indentation and microscopy techniques facilitates a complementary and consistent characterisation and has allowed us to extend the insights from previous studies at high temperatures across the transition to the low temperature deformation regime. This is essential to unravel the role of Mg17Al12 as a load-bearing phase in technical alloys and the effect of its complex crystal structure on plasticity.