Developing a Mg alloy with ultrahigh room temperature ductility via grain boundary segregation and activation of non-basal slips

Poor room temperature (RT) ductility is an inherent disadvantage of magnesium (Mg) alloys, which limits their wide application. Grain refinement is an accepted method to improve ductility and strength simultaneously. However, obtaining ultrafine grains (about 1 μm and below) usually requires severe deformation processes, which is not conducive to be widely used. Here, by utilizing a distinctive design strategy, i.e., introducing beneficial solute to achieve grain boundary (GB) segregation and effectively reduce the critical resolved shear stress (CRSS) ratio between non-basal slips and basal slip, thus efficiently activating non-basal slips, we obtained a Mg-0.3Er (at%) alloy with moderate-fine grains (∼8 μm) having elongation near 50% at RT. Based on the results of in-situ electron back-scatter diffraction and slip trace analysis, and weak beam dark-field observation, we confirmed that minor Er element and appropriate fine grains would contribute to the activation of considerable non-basal dislocations and the suppression of twinning. In addition, through energy dispersive spectrometry element analyses for systematic investigation, we found that the GB segregation level of the alloy annealed at 300 °C is higher than that of the alloy annealed at 360 °C. The higher GB segregation level would impact the cohesion strength and be conducive to activate more non-basal dislocations to accommodate local stress, thus suppressing the GB cracking and finally improving the ductility. This study not only suggests an effective way for large-scale development of high ductility Mg alloys at RT, but also provides a new insight to elucidate the origins of ultrahigh ductility by revealing the fraction of non-basal slips and the level of GB segregation.