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

The evolution of beta' precipitate in a Mg-3.3Y-0.1Zr (at.%) alloy before and after creep at 300 degrees C under 50/70 MPa was comprehensively studied using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Before creep, beta' precipitates randomly distribute in aged Mg-3.3Y-0.1Zr (250 degrees C x 32 h). After creep, linear beta' precipitate chains form upon the thermal-mechanical coupled role. The linear beta' precipitate chains deviate from (2 (1) over bar(1) over bar0)(alpha) by similar to 15 degrees and are roughly perpendicular to the applied stress. In a single precipitate chain, the beta' precipitates inevitably belong to the same orientation variant. A variant selection mechanism of beta' phase is proposed herein, accounting for the preferential growth of the beta' orientation variant with (010)(beta') (//{10 (1) over bar0}(alpha)) being similar to 45 degrees with respect to the applied stress. In addition to the linear beta' precipitate chains, dynamic precipitation of nano-scale beta' precipitate is found during creep. While maintaining the same orientation relationship with the alpha-Mg matrix as reported in the literature, the nano-scale beta' precipitates extend parallel to {10 (1) over bar0}(alpha) (habit plane), showing a plate-like shape, which is different from the traditional belief that beta' phase shows a nearly equiaxed shape in Mg-Y alloy. Such crystallographic feature of the nano-scale beta' precipitate is rationalised as the interfacial energy is more dominant over the elastic energy during the early formation stages of beta' phase.