Precipitation in a Mg–10Gd–3Y–0.4Zr (wt.%) alloy during isothermal ageing at 250 °C

The precipitation sequence in a Mg–10Gd–3Y–0.4Zr (wt.%) alloy during isothermal ageing at 250 °C has been investigated using transmission electron microscopy. It is found that the precipitation sequence involves super-saturated solid solution (S.S.S.S.) → β″(D019) → β′(cbco) → β1(fcc) → β(fcc), which is similar to that of WE54 alloy but different from previously reported three-stage sequence: S.S.S.S. → β″(D019) → β′(cbco) → β (bcc). The metastable β′ phase, which plays an important role for the age hardening of the alloy, has a shape of convex lens and is very thermally stable at 250 °C. Formation of β1 phase appears to take place via an in situ transformation from a decomposed β′ phase.