Coupling strengthening mechanism of precipitate phases and sub-grain boundaries in the aging heat treatment of deformed Cu-Ni-Co-Si alloy

This study investigates the mechanism by which the microstructure of cold-rolled Cu-Ni-Co-Si alloys influences the precipitation strengthening and recrystallization processes during aging treatment. The cold-rolling reductions applied were 30 %, 50 %, 70 %, and 80 %, with aging temperatures of 450°C, 500°C, and 550°C, and holding times ranging from 60 to 480 min. The results indicate that with the increase in cold rolling reduction, a significant accumulation of dislocations occurs within the matrix, leading to an increase in texture density. Additionally, under stress dominance, the alloy structure formed after cold rolling exhibits a large number of shear bands and solute segregation gradients, which also rise with the increase in cold rolling reduction. Microstructural observations after aging reveal the precipitation of nanoscale δ-(Ni,Co)₂Si strengthening phases during aging, and ion spectrum confirm the stability of δ-(Ni,Co)₂Si during the aging process. The δ-(Ni,Co)₂Si phase interacts with dislocations during aging and couples with sub-grain boundaries, improving the mechanical properties of the alloy. The grains undergo varying degrees of recrystallization, with both continuous static recrystallization (CSRX) and discontinuous static recrystallization (DSRX) occurring during the process. For the sample with 80 % cold-rolling reduction, the hardness after peak aging at 450°C, 500°C, and 550°C were 255.3 HV, 284.8 HV, and 271.7 HV, respectively. After aging at 500°C for 60 min, the dislocation cells in the matrix transform into subgrains. The distribution of δ-(Ni,Co)₂Si phases along the sub-grain boundaries and high-density dislocations inhibits the recrystallization process, resulting in the alloy achieving a conductivity of 38.88 % IACS and a tensile strength of 798.38 MPa.