Influence of Co on Strength and Microstructure of Cu-Ni-Co-Si Alloy

Cu-2.0wt%Ni-0.5wt%Si, Cu-1.4wt%Ni-0.6wt%Co-0.5wt%Si (0.6%Co) and Cu-1.0wt%Ni-1.0wt%Co-0.5wt%Si (1.0%Co) alloys produced by combining cold rolling to a 25% and a 90% reduction with aging treatment are employed to investigate the effects of Co on the strength and microstructure of Cu-Ni-Co-Si alloys. Aging the 0.6%Co and 1.0%Co alloys at 525, 425 and 325℃ produces orthorhombic (Ni, Co)2Si precipitates that have the same crystal system as Ni2Si precipitates formed in the 0%Co alloy. The larger the amount of Co in the three alloys is, the higher the dislocation density in the alloys peak-aged and rolled to a 25% and a 90% reduction is. The amounts of deformation twins observed in the 0.6%Co and 1.0%Co alloys peak-aged at 525℃ and rolled to a 90% reduction are much larger than that observed in the 0%Co alloy peak-aged at 525℃ and rolled to a 90% reduction. The strength and electrical conductivity of the three alloys initially aged at 525℃, rolled to a 25% reduction and re-aged at 425℃ (A25RA), or aged at 525℃, rolled to a 90% reduction and re-aged at 325℃ (A90RA) becomes higher as the Co content increases. The increase in strength with increasing the Co content is attributed to decrease in the inter-precipitate spacing and increase in the dislocation density for the A25RA alloys, and increase in the amount of deformation twins in addition to decrease in the inter-precipitate spacing and increase in the dislocation density for the A90RA alloys.