Experimental investigation of microstructure and ageing behaviour of bulk Zn–(1–18) wt% Al–(0–0.06) wt% Mg alloys

Fine scale microstructure and hardness properties of bulk Zn–Al–Mg alloys have been investigated after solution-annealing, during ageing at room temperature and after ageing at 95 °C. Small-angle X-ray scattering measurements evidence nucleation, growth and coarsening of tiny elongated precipitates (probably of the (Al) phase) in Zn–Al–Mg alloys during ageing, with a characteristic size of 10 nm, and a characteristic time scale of 1 h at 95 °C to complete the precipitation reaction. Small additions of magnesium strongly enhance this precipitation. A strong effect on both hardness and two-stage precipitation enthalpy was evidenced. The nodules of both α-(Al) and η-(Zn) phases formed by decomposition of the former α′ phase showed a bimodal size distribution. An ageing treatment at 15 and 95 °C mainly affected the bulk of former α′ particles i.e. the finest areas of this microstructure.