TEM study on the microstructural evolution in an Mg–Y–Gd–Zn alloy during ageing

The multi-phase microstructural evolution in an Mg–Y–Gd–Zn alloy during ageing at 225 °C was studied in TEM and the age-hardening mechanism is elucidated in this paper. Two independent precipitation reactions were observed in this alloy, i.e. α-Mg(SSS) + β(Mg24(Y,Gd,Zn)5) → α + β + 18R and α-Mg(SSS) → βʺ(D019) → β′(cbco) → β1(fcc) → β. The age-hardening contribution from precipitates is discussed based on their size, shape, orientation and distribution at corresponding ageing stages. The major hardening precipitate is found to be βʹ phase with a c-axis base-centered orthogonal (cbco) structure, rather than the long period stacking ordered (LPSO) phase that prevailed in some earlier studies.