Effect of heat treatments on the microstructure and mechanical properties of Zn-15 wt% Al alloy

The effect of heat treatment on the micro-structural and mechanical properties of Zn-15wt%Al alloys was investigated systematically in this work. After being annealed below 275 °C, granular α particles were distributed homogeneously in the η phase matrix, while a lamellar microstructure along with isolated η grains was observed when the alloys were annealed above 275 °C, followed by an air cooling (AC) treatment. A subsequent tensile test showed that the strength of alloys always increased with the annealing temperature. However, when the alloys were annealed above 275 °C, followed by a water-quenched (WQ) treatment, their strength firstly decreased with the aging time and then became unchanged. It was revealed by an in situ investigation that the γ′ phase retained from quenching decomposed gradually into an equiaxed microstructure during aging, leading to a reduction in the strength. Transmission electron microscopic (TEM) studies showed that dislocations were rarely inspected after a severely plastic deformation in the WQ-treated alloys, indicating that grain boundary sliding and rotation would be a dominant deformation mechanism. However, for the AC-treated alloys, dislocation tangles were easily found in lamellas due to the severe plastic deformations, demonstrating that in addition to grain boundary sliding and rotation, dislocation slip was another important deformation mechanism. A strength model, incorporating phase transformation dynamics with the law of mixtures, was established for the WQ-treated alloys annealed above 275 °C. Meanwhile, the number of α particles in the γ′ phase was assumed to be a key parameter for understanding the evolution of mechanical properties of Zn-15wt%Al alloys during the heat treatments. The more α particles in a unit volume of the γ′ phase, the easier plastic deformation the alloys is prone to, a lower strength and a larger elongation of the alloys will be. (paper)