Effect of strain rate and sample thickness on mechanical properties of spray deposited and extruded Al–Zn–Mg–Cu alloy

Abstract

The microstructure of spray deposited and extruded Al–Zn–Mg–Cu alloy after regression re-aging (RRA) treatment was investigated by transmission electron microscope and electron back-scatter diffraction. The influence of strain rate and sample thickness on the mechanical properties were evaluated. The results showed that the main precipitate phases in the RRA Al–Zn–Mg–Cu alloy included short rod-shaped η′, coarse rod-shaped η and G.P. zones with nanoscale dimensions. The yield and ultimate tensile strength of RRA alloys were determined by sample thickness and strain rate to regulate the mass fraction of precipitate phases and the restraint of internal grains by sample surfaces, the number of activated slip systems and dislocation density. The RRA Al–Zn–Mg–Cu alloy exhibited a mixed-mode fracture due to the nanoscale precipitate phases restraining grain boundary sliding, the precipitate-free zones and high angle grain boundary inducing crack nucleation and propagation, and the post-necking elongation producing cleavage planes. The strain hardening is conducted by deformation texture. The strain hardening exponent n and the strain rate sensitivity coefficient m were determined by the sample thickness. The deformation texture intensity of RRA alloy was related to the strain rate controlled by texture components. The high strain rate improved the content of Cu texture {112}<-1-11> to elevate the mechanical properties of RRA alloy.