High temperature creep behavior and creep microstructure evolution of T6 state Mg–15Gd alloy

Abstract The creep properties of Mg alloys are critical for their high temperature applications. But for Mg–Gd alloys, there is a lack of systematic research on the creep behavior and microstructure evolution above 250 °C. In this study, the high temperature tensile creep behavior and microstructure evolution of the T6 state Mg-15Gd (wt.%) metal mold casting alloy were investigated at temperatures from 235 °C to 300 °C and stresses from 50 MPa to 90 MPa. The experimental alloy was composed of α-Mg matrix, elliptic shape β′ precipitates, cuboid-shaped GdH2 phase and a small amount of M5Gd phase. It was found that the creep strain and creep rate of the experimental alloy increased but the creep life decreased with increasing the creep temperature and the applied stress. At a fixed creep temperature of 260 °C, the calculated stress exponent n value was about 2.8 in the applied stress range of 50–90 MPa, suggesting that the creep deformation may be controlled by dislocation slip. Meanwhile, under a defined creep stress of 50 MPa, the calculated activation energy Q value of the alloy was about 108 kJ mol−1 in the temperature range of 235–300 °C, indicating that the pipe diffusion may be a dominant affecting factor. During the creep process, the α(Mg) grain size coarsened and the quantity, size and type of the precipitated phases changed with three-stage: β′ (cbco) →β1 (fcc) →β (fcc). Observation of creep microstructure evolution under 50 MPa and 260 °C confirmed that the creep mechanism was dislocation glide and the creep fracture mechanism was brittle transgranular cleavage fracture.