Microstructural Subsequence and Phase Equilibria in an Age-Hardenable Cu-Ni-Si Alloy

We investigated the microstructural subsequence and phase equilibrium of Cu-4.3 at% Ni-2.2 at% Si alloy specimens during isothermal aging over a temperature range of 698 K to 873 K. During aging in this temperature range, the microstructure of the specimens evolved in the following sequence; continuous precipitation of fine disk-shaped delta-Ni2Si in the matrix, discontinuous precipitation of cellular components containing coarse fiber-shaped delta-Ni2Si at grain boundaries, and then occupation of the specimen with cellular components. By analyzing the variations in the hardness and electrical conductivity with aging time as well as temperature, we described the kinetics of the highest number density of fine delta-Ni2Si continuous precipitates. Based on the chemical analyses of the specimen in a phase equilibrium state using an extraction procedure, we proposed a revised solvus of solutes Ni and Si in the Cu solid solution phase, as well as Cu in the d-Ni2Si intermetallic phase in a Cu-delta-Ni2Si pseudo-binary system.