Formation of grain boundary segregation layers in Cu-based alloys during high pressure torsion

High pressure torsion (HPT) of Cu-based alloys leads to the (1) grain refinement and (2) dynamic equilibrium between precipitation and dissolution of precipitates with a certain concentration in solid solution c_ss. If the concentration in initial sample c_i is below c_ss, it increases to c_ss during HPT. If c_i > c_ss, it decreases to c_ss during HPT. It has been shown earlier that c_ss ≈ 3.0 wt. % Ag for Cu–Ag alloys and c_ss ≈ 2.5 wt. % Co for Cu–Co alloys. We measured lattice parameter a in two Cu–Ag and two Cu–Co alloys before and after HPT using X-rays diffraction. They contained 2.0 and 8.0 wt. % Ag and 2.0 and 5.0 wt. % Co. It means, that the total concentration was either below (2.0 wt. % Ag and 2.0 wt. % Co) or above (8.0 wt. % Ag and 5.0 wt. % Co) of respective c_ss. The alloys with 2.0 wt. % Ag and 2.0 wt. % Co were homogenized and contained only Cu-based solid solution. The alloys with 8.0 wt. % Ag and 5.0 wt. % Co were annealed in such a way that they contained the Cu-based solid solution with c_i ≈ 2.0 wt. % Ag or c_i ≈ 2.0 wt. % Co as well as Ag or Co precipitates, respectively. During HPT, Cu or Co precipitates partly dissolved and concentration in solid solutions increased from c_i to c_ss. However, if Cu or Co precipitates are absent, c_i decreased. It is because Ag and Co strongly segregates in Cu/Cu grain boundaries (GBs). The grain refinement during HPT leads to the formation of new GB segregation layers which soak (consume) the Ag or Co from solid solution. The thickness of GB segregation layers was estimated.