Softening by severe plastic deformation and hardening by annealing of aluminum-zinc alloy: Significance of elemental and spinodal decompositions

An Al-30 mol% Zn supersaturated solid solution alloy was severely deformed using high-pressure torsion (HPT) at 300 K and subsequently annealed at 373-673 K. The hardness and tensile strength significantly decreased and the tensile ductility increased with straining by HPT and reached a steady-state level at large imposed strains. Despite this softening behavior, the lattice strain was increased, Zn-rich particles were precipitated and the initial coarse grains were refined significantly to a size of ~190 nm while being accompanied by decomposition to Al- and Zn-rich phases because of rapid atomic diffusion. The subsequent annealing led to a hardening, but microstructural observations showed that decrease in the lattice strain, increase in the grain size and reduction in the fraction of precipitates occurred by annealing. It was shown that the unusual softening/hardening behavior of the Al-Zn alloy was mainly due to the contribution of spinodal decomposition. The formation of nano-sized lamellae by spinodal decomposition resulted in increase in hardness after solution treatment and after post-HPT annealing, while this lamellar structure was destroyed by HPT, which resulted in softening. The softening was less significant when the hardness was evaluated at low homologous temperatures.