Abstract: The strength-ductility trade-offdilemma 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 <c+a> dislocations promotes good tensile ductility at-70 °C, which is attributed to the minimized critical resolved shear stress (CRSS) ratio of non-basal <c+a> to basal <a> dislocations. In ad-dition, 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.

Key words: Magnesium alloys, Strength-ductility synergy, Dislocation density, Critical resolved shear stress, Shearable precipitate