The effect of cold rolling on age hardening of Cu-3Ti-3Ni-0.5Si alloy

This study investigates the age hardening behavior of the cold rolled Cu-3Ti-3Ni-0.5Si alloy upon different deformations. The microstructure and the phase composition after precipitation were characterized by X-ray diffraction, optical microscopy, scanning electron microscopy, and transmission electron microscopy, and the electrical conductivity and mechanical properties of the alloy were tested as well. The as-cast Cu-3Ti-3Ni-0.5Si alloy consists of Ni2Ti phase, NiTi phase, and Cu matrix, and Ni2Si, Ni3Si, and Ni3Ti phases precipitated from the Cu matrix after aging treatment. TEM analysis shows that Ni2Si phase is coherent with the Cu matrix after 50% deformation and aging at 500 °C for 120 min, with an orientation relationship of [2¯01]Ni2Si//[111¯]Cu. However, Ni2Si phase loses its coherence with the Cu matrix if the size of the Ni2Si phase is above 4.68 nm. Additionally, large deformation promotes most striped precipitates to transform into fine and spherical precipitates, so the appropriate deformation and aging temperature are beneficial for the improvement on the electrical conductivity and hardness. After 90% deformation and aging for 30 min at 500 °C, Cu-3Ti-3Ni-0.5Si alloy has a good combination of tensile strength, elongation, hardness and electrical conductivity, which are 615 MPa, 25.1%, 268 HV and 33.79 %IACS, respectively. The fracture morphology exhibits numerous fine equiaxed dimples with an average size of about 1 μm, and small amounts of large dimples with an average size of about 10 μm, and the assigned failure mechanism of the Cu-3Ti-3Ni-0.5Si alloy is a ductile fracture.