High pressure torsion of Cu–Ag and Cu–Sn alloys: Limits for solubility and dissolution

The high-pressure torsion (HPT) of binary copper alloys with 3, 5, 8, 10 wt. % Ag and 14 wt. % Sn has been studied at room temperature T _HPT. Before HPT, the Cu–Ag alloys have been annealed at 12 different temperatures between 320 and 800 °C and Cu–14 wt. % Sn has been annealed at 9 different temperatures between 310 and 500 °C. Thus, before HPT the Cu–Ag alloys consisted of Ag-particles in the Cu-based matrix with silver content c _init from almost zero to 8 wt.%. The Cu–14 wt. % Sn samples had Cu-based matrix with tin concentration c _init from almost zero to 14 wt.% Sn and precipitates of ε or δ Hume-Rothery intermetallic phases. After about 1.5 plunger rotations a certain steady-state concentration c _ss of the alloying element is reached in the matrix. The measured c _ss values were 5.5±0.1 wt. % Ag and 13.1±0.1 wt. % Sn. If the initial concentration c _init in Cu matrix was below c _ss ( c _init < c _ss), it increased towards c _ss during HPT. If c _init > c _ss it decreased towards c _ss. We observed that c _ss did not depend on c _init in broad interval of c _init and was, therefore, equifinal. The equifinal c _ss values corresponded to the certain equilibrium solubilities of silver and tin in Cu matrix and allowed to estimate the (elevated) effective temperature as T _eff (Ag) = 700±10 °C and T _eff (Sn) = 400±10 °C, respectively. The observed phenomena are discussed using the ideas of non-equilibrium thermodynamics of open systems. During HPT the decomposition of a solid solution competed with dissolution of precipitates. As a result, a dynamic equilibrium established between precipitation and dissolution at steady-state deformation stage. In this dynamic equilibrium a certain steady-state concentration c _ss of the alloying element is reached in the matrix. In Cu-based alloys, the obtained T _eff is always higher than T _HPT and correlates with activation enthalpy of dopant diffusion in Cu. Other HPT-driven phenomena such as accelerated mass transfer, intermetallic phase formation, grain boundary faceting and grain boundary segregation are taken into account to evaluate the effective temperature T _eff.