Microstructure evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250 °C

The microstructural evolution in a Mg–15Gd–0.5Zr (wt.%) alloy during isothermal aging at 250 °C, has been investigated using transmission electron microscopy. The decomposition of α-Mg supersaturated solid solution (S.S.S.S., cph) in the alloy with increasing aging time is as follows: β″ (D019) → β′(cbco) → β1(fcc) → β(fcc), which is similar to that of Mg–Gd–Y, Mg–Gd–Nd and Mg–Y–Nd alloys, but different from previously reported three stage sequence: S.S.S.S. → β″ (D019) → β′(cbco) → β(fcc). It is found that the metastable β″ and β′ phases coexist in the matrix at the very early stage of aging. Peak age-hardening is attributed to the precipitation of prismatic β′ plates in a triangular arrangement. At the over-aged stage, β1 phase appears to take place via an in situ transformation from a decomposed β′ phase but grows in a direction different from the previous one of β′ phase. Continued aging makes the β1 phase transform in situ to the equilibrium β phase and the orientation relationship between the precipitate and matrix phases is retained through the in situ transformation of the β1 phase.