Effects of Natural Aging on Age-Hardening Behavior of Cu-Be-Co and Cu-Ti Alloys Severely Deformed by High-Pressure Torsion

Solution-treated Cu-1.8 mass% Be-0.2 mass% Co and Cu-3 mass% Ti alloys were subjected to severe plastic deformation using high-pressure torsion (HPT) process to examine the change in densities of grain boundary, dislocation and vacancy during natural aging of the two alloys at 293 K after HPT-straining, and the influence of the natural aging on the age-hardening behavior of the alloys on subsequent artificial aging. Application of HPT processing under an applied pressure of 5 GPa for 10 revolutions at 1 rpm to each alloy produced an ultrafine-grained structure. Aging the HPT-processed alloys at 293 K for the longest period of 2.59 Ms (1 month) did not essentially change the dislocation densities and grain sizes of the alloys; however, the vacancy concentrations of the alloys gradually decreased with increasing natural aging time. The attained peak hardness of the Cu-Be-Co alloy on subsequent artificial aging at 593 K decreased as natural aging time increased, while the natural aging at 293 K for 2.59 Ms had no influence on the age-hardening behavior of the Cu-Ti alloy during artificial aging at 623 K. This result is ascribed to the difference in formation mechanisms of strengthening precipitates between two alloys; G.P. zones are directly formed in Cu-Be-Co alloy while beta'-Cu4Ti phase is formed via spinodal decomposition without nucleation events in Cu-Ti alloy.