Effects of Gd solutes on hardness and yield strength of Mg alloys

Relative contribution of individual strengthening mechanisms to the yield strength of Mg–0–15 wt% Gd alloys were investigated. Alloys with different grain size were prepared by adding Zr and hot extrusion. Hardness and tensile/compression yield strength were tested on the alloys after solid solution treatment and extrusion. Hall-Petch constants were calculated with hardness and tensile/compressive data. The results showed that the hardness of Mg–Gd alloys with similar Gd content and different grain size were almost the same, which indicates that grain size had little effect on hardness. The hardness linearly increased with rising Gd content (dHv/dc ≈ 25 kg mm−2/at%Gd). The tensile and compressive yield strengths enhanced with the increase of Gd content for all alloys in different conditions. In addition, the tensile/compressive (t/c) yield asymmetry of extruded alloys decreased with increasing Gd content. Large t/c yield asymmetry ratio (1.77) was observed for pure Mg, and with increasing Gd content this value decreased to 1. With the increasing of tensile strength, the stress intensity factor, ky, decreased from 0.27 MPa m1/2 for Mg–2 wt% Gd alloy to 0.19 MPa m1/2 for Mg–5 wt% Gd alloy, then increased to 0.29 MPa m1/2 for Mg–15 wt% Gd alloy. However, ky increased linearly form 0.16–0.31 MPa for compression test. The influence of grain size strengthening was eliminated, and the yield strength of tension and compression both linearly increased with cn, where c is the atom concentration of Gd, and n = 1/2 or 2/3.