On the evolution of β' precipitate during creep in a Mg–3.3Y–0.1Zr(at.%) alloy

Abstract

The evolution of β' precipitate in a Mg–3.3Y–0.1Zr (at.%) alloy before and after creep at 300 °C under 50/70 MPa was comprehensively studied using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Before creep, β' precipitates randomly distribute in aged Mg–3.3Y–0.1Zr (250 °C × 32 h). After creep, linear β' precipitate chains form upon the thermal-mechanical coupled role. The linear β' precipitate chains deviate from 21¯1¯0α by ~15° and are roughly perpendicular to the applied stress. In a single precipitate chain, the β' precipitates inevitably belong to the same orientation variant. A variant selection mechanism of β' phase is proposed herein, accounting for the preferential growth of the β' orientation variant with (010)β′ (// 101¯0α) being ~45° with respect to the applied stress. In addition to the linear β' precipitate chains, dynamic precipitation of nano-scale β' precipitate is found during creep. While maintaining the same orientation relationship with the α-Mg matrix as reported in the literature, the nano-scale β' precipitates extend parallel to 101¯0α (habit plane), showing a plate-like shape, which is different from the traditional belief that β' phase shows a nearly equiaxed shape in Mg–Y alloy. Such crystallographic feature of the nano-scale β' precipitate is rationalised as the interfacial energy is more dominant over the elastic energy during the early formation stages of β' phase.