Evolution process of precipitate-free zones in a Mg-Gd alloy during creep

Abstract

Magnesium alloys, the lightest metallic materials for structural applications, have met the bottleneck in the applications at 300°C due to limited creep resistance. The degradation of creep resistance closely depends on the microstructural deterioration, especially the formation of precipitate-free zones (PFZs), but the detailed evolution process remains unclear in this regard. The present study adopted a quasi-in-situ methodology to track the evolution process of the PFZs in Mg-2.5Gd-0.1Zr (at.%) alloy during creep at 300°C under 60 MPa. In the early creep stage, the widening of PFZs and phase transformation of intragranular precipitates are repressed by the applied stress. In the steady and accelerated creep stages, propagation of dislocations generates misorientation between PFZs and their parent grains, leading to the formation of Type-A PFZs. Meanwhile, vacancy diffusion leads to inverse migration of grain boundaries, and produces PFZs with serrated grain boundaries between split rows of grain boundary particles, causing the formation of Type-B PFZs. Secondary intergranular cracks tend to develop in Type-B PFZs in the accelerated creep stage, but the strain accumulation in Type-A PFZs is the key contributor to premature creep failure.