The grain-scale tension-compression (T-C) asymmetric slip behavior and geometrically necessary dislocation (GND) density in an aged and twin-free Mg-10Y sheet were statistically studied using slip trace analysis and electron backscatter diffraction (EBSD) analysis. A significantly asymmetric slip activity, i.e., higher tensile slip activity and proportion of non-basal slip, was manifested. Prismatic 〈a〉 (37.1%) and basal 〈a〉 (27.6%) slips dominated the tensile deformation, followed by pyramidal Ⅱ 〈c + a〉 slip (20.0%). While during compression, basal 〈a〉 slip (61.9%) was the most active slip mode, and only 6.9% pyramidal Ⅱ 〈c + a〉 slip was observed. The critical resolved shear stress (CRSS) ratio was estimated based on ~800 sets of the identified slip traces, which suggested that the CRSS_{pyr Ⅱ}/CRSS_bas for compression was ~3 times than that of tension. The pyramidal Ⅱ 〈c + a〉 slip was more active when the slip plane was under tension than under compression, which was consistent with the calculated asymmetric CRSS_{pyr Ⅱ}/CRSS_bas. The activity of multiple slip, cross slip and slip transfer, as well as the GND density were also T-C asymmetric. This work thoughtfully demonstrated the T-C asymmetric slip behavior and plastic heterogeneity in Mg alloys which was believed to be responsible for the macroscopic T-C asymmetry when twinning was absent. The present statistical results are valuable for validating and/or facilitating crystal plasticity simulations.

Rare earth (RE) can produce excellent precipitation hardening in Mg alloys. However, when forming a solid solution, it also deteriorates formability, a problem that can usually be overcome by raising deformation temperature. Here we report an unexpected observation of high temperature brittleness in a Mg-Gd-Y-Ag alloy. As the temperature reached 500 ℃, the formability decreased drastically, leading to severe intergranular fracture under only 0.5% strain. This was caused by failure of grain boundaries, which are weakened by segregated interfacial compounds.