Tensile creep behaviors of Mg-10Gd-0.4Zr alloy in different heat treatment states

Mg-Gd alloys show good mechanical properties due to the strengthening β′ phases. However, the β′ phases are unstable and easily transform into other phases, which leads to the severe performance deterioration at elevated temperatures. In this paper, the tensile creep behaviors of Mg-10Gd-0.4Zr alloy in different heat treatment states were investigated at temperatures from 225 °C to 300 °C under stresses from 50 MPa to 80 MPa. The results showed that the creep behaviors of the solution treated (T4) and peak-aged (T6) alloys were dependent on the critical transition temperature. Microstructure evolution analysis revealed that the creep resistance of the T6 alloy was better than that of the T4 alloy below the critical transition temperature due to the existence of more β′ phases. With further increasing the creep temperature, the formation of β₁ phase and the widening of precipitate-free zones (PFZs) in the T6 alloy significantly deteriorated the creep resistance, while the precipitated β′ phases strengthened the T4 alloy effectively. It resulted in that the T4 alloy exhibited better creep resistance above the critical transition temperature. The stress exponents (3.7–6.2) and creep activation energy (127 kJ mol⁻¹) of T4 alloy indicated that the dominated creep mechanism was dislocation climb assisted by lattice self-diffusion of Mg atoms. The stress exponents (3.6–6.4) of T6 alloy were close to that of T4 alloy, while the creep activation energy changed from 94 kJ mol⁻¹ to 192 kJ mol⁻¹ with increasing creep temperature. Observation of dislocation configuration revealed that the creep mechanism of T6 alloy transformed into dislocation cross slip above the critical transition temperature.