Enhancing strength–ductility synergy in a Mg–Gd–Y–Zr alloy at sub-zero temperatures via high dislocation density and shearable precipitates

The strength–ductility trade-off dilemma is hard to be evaded in high-strength Mg alloys at sub-zero temperatures, especially in the Mg alloys containing a high volume fraction of precipitates. In this paper, we report an enhanced strength–ductility synergy at sub-zero temperatures in an aged Mg–7.37Gd–3.1Y–0.27Zr alloy. The tensile stress–strain curves at room temperature (RT), −70 °C and −196 °C show that the strength increases monotonically with decreasing temperature, but the elongation increases first from RT to −70 °C then declines from −70 °C to −196 °C. After systematic investigation of the microstructure evolutions at different deformation temperatures via synchrotron X-ray diffraction, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), it is found that a high dislocation density with sufficient dislocations promotes good tensile ductility at −70 °C, which is attributed to the minimized critical resolved shear stress (CRSS) ratio of non-basal to basal dislocations. In addition, more shearable precipitates can further improve the ductility via lengthening the mean free path of dislocation glide. The present work demonstrates that an excellent strength–ductility synergy at sub-zero temperatures can be achieved by introducing a high dislocation density and shearable precipitates in high-strength Mg alloys.