The effect of strain rates on the microstructure and the mechanical properties of an over-aged Al-Zn-Mg-Cu alloy

Experiments were carried out to investigate the compressive mechanical properties of an Al-Zn-Mg-Cu alloy after spray forming and extrusion followed by T74 heat treatment (393 K for 8 h + 433 K for 24 h). The results show that the yield and ultimate strengths are enhanced after the T74 heat treatment under strain rates ranging from 1.0 × 10⁻³ s⁻¹ to 5.0 × 10³ s⁻¹ due to the large number of precipitates. Elongated grains are fragmented forming grains with low angle grain boundaries (LAGB's) under 3.0 × 10³ s⁻¹ and 5.0 × 10³ s⁻¹ before and after the heat treatment influenced by the dislocation accumulation. Adiabatic shear bands (ASBs) are formed after high strain rate compression assisted by local shear and temperature rise in a narrow region forming fine recrystallized grains accompanied by the initiation of micro-cracks. There is a slight variation in the volume fraction of precipitates before and after compression. Precipitates are also fragmented under high strain rate compression to form spherical and irregular morphologies from the original platelet morphology before compression.