The formation of nanograin structures and accelerated room-temperature theta precipitation in a severely deformed Al–4 wt.% Cu alloy

The grain size achievable and long-term stability of a severely deformed aluminium copper alloy have been investigated when copper is used in solution to inhibit recovery. It is shown that copper is more effective than magnesium in inhibiting dynamic recovery. A grain width of only ～70 nm was obtained in an Al–4 wt.% Cu alloy, after processing by equal-channel angular extrusion to a strain of ε_eff = 10, resulting in a lamellar nanograin structure. However, post-processing, the severely deformed solid solution was found to be unstable at room temperature and copious precipitation of θ occurred at grain boundaries within the deformed state, leading to recovery of the deformation structure and a loss of strength. The solute level fell to equilibrium within ～9 months. The precipitation kinetics were shown to occur at many orders of magnitude higher than can be predicted by classical nucleation and growth theory. The reasons for this discrepancy are discussed.