<p>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 <i>c</i>_ss = 2 wt % from both sides. The value <i>c</i>_ss corresponds to the solubility of Cu in the solid solution at <i>T</i>_eff = (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.</p>