Effect of deep cryogenic treatment on the microstructure and tensile property of Mg-9Gd-4Y–2Zn-0.5Zr alloy

The effects of deep cryogenic treatment (DCT) on the tensile property and microstructure of Mg-9Gd-4Y–2Zn-0.5Zr(wt.%) alloy were investigated by comparing the alloy treated by solution treatment, deep cryogenic treatment and aging treatment (ST + DCT + AT) and that treated by solution treatment and aging treatment (ST + AT). The lamellar 14H-type long-period stacking ordered (14H-LPSO) in grain was precipitated in the alloy treated by ST + AT. However, the particle and needle-like Mg5RE (RE = Gd, Y) phases instead of the lamellar 14H-LPSO were precipitated, and a high-density dislocation existed in the alloy treated by ST + DCT. The greater ultimate tensile strength (UTS) and yield strength (YS) of the alloy treated by ST + DCT + AT were obtained because of the precipitation of the Mg5RE phases. The trend, the UTS and YS firstly increased then decreased with the increasing of soaking time, is corresponding to the volume fraction increasing and size-coarsening of Mg5RE precipitates, respectively. The greater elongation (EL) was obtained after the short-time DCT for 0–2 h (DCT0h-DCT2h) and the smaller EL was obtained for the long soaking time (>DCT2h). The fracture mode of the samples is all quasi-cleavage fracture.