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$^\circ$C has been identified for the first time. After annealing at 300$^\circ$C, the solid solution in the matrix is almost Cu-free, while annealing at 530$^\circ$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 Al$_2$Cu phase particles. In the second sample, on the contrary, the solid solution decomposes with the formation of Al$_2$Cu 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 $c_{ss} = 2\,$ wt % from both sides. The value css corresponds to the solubility of Cu in the solid solution at $T_{\text{eff}}=400\pm20\,^\circ$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.