Microstructure of hot-deformed Cu–3.45 wt.% Ti alloy

Hot compression tests at temperatures ranging from 673 to 1173 K were carried out on solution-treated Cu–3.45 wt.% Ti at a constant true strain rate of 1.4×10−4 s−1. According to literature data, an effective hardening of Cu–Ti alloys results from continuous precipitation of small metastable β′ particles of Cu4Ti-phase which follows the spinodal decomposition of the solid solution during static aging of a solution treated alloy. This process was also responsible for an effective hardening of the material during hot compression of the solution treated alloy in the early stages of the test. At low deformation temperatures (673–773 K), shear banding and growth of intergranular cavities were found to be responsible for a flow stress decrease and for sample fracture at large strains. During hot deformation at intermediate temperatures (873–973 K), softening was also found to follow the flow stress maximum. According to the literature, the discontinuous precipitation of stable β particles of Cu3Ti-phase results in a decrease of strength during static aging, i.e. an overaging effect. However, during hot deformation of a solution treated alloy in this temperature range dynamic precipitation coarsening within shear bands is also observed to proceed in competition with cellular growth of β particles. Phase transformation of fine-scale precipitates into colonies of coarse Cu3Ti particles produces a ‘soft’ structural component within the hardened matrix. This as well as intensive shear banding was responsible for the effective flow softening of the material. Moreover, discontinuous precipitation at grain boundaries is found to reduce the growth of cavities and to delay intergranular fracture. Thus, samples deformed at temperatures from 973 to 1023 K did not fracture in the deformation range used in the experiments. It is suggested that intergranular crack growth is retarded by dynamic recovery within shear bands as well as by dynamic recrystallization above the solvus temperature.