Competition between the Formation and Decomposition of a Solid Solution in Al–Cu Alloys under High-Pressure Torsion

The competition between the formation and decomposition of a solid solution under high-pressure torsion in an Al–6 wt % Cu alloy pre-annealed at 300 and 530°C has been identified for the first time. After annealing at 300°C, the solid solution in the matrix is almost Cu-free, while annealing at 530°C yields a solid solution containing about 5 wt % Cu, which is close to the maximum possible value. In the first sample, high-pressure torsion leads to the enrichment of the solid solution with Cu due to the partial decomposition of Al2Cu phase particles. In the second sample, on the contrary, the solid solution decomposes with the formation of Al2Cu particles. As a result of the competition between these processes, a dynamic equilibrium is established, and the Cu concentration in the matrix approaches a steady-state value of css = 2 wt % from both sides. The value css corresponds to the solubility of Cu in the solid solution at Teff = (400 ± 20)°C. This phenomenon can be explained by the increased concentration of defects (in particular, vacancies) in the steady state under high-pressure torsion, which is equivalent to a temperature increase.