Influence of Substitution of Co for Ni in Cu–Ni–Si Alloy on Work and Precipitation Hardening

Abstract

The influence of substitution of Co for Ni in Cu–Ni–Si alloy on work and precipitation hardening was investigated using Cu–1.5Ni–0.36Si, Cu–0.75Ni–0.75Co–0.36Si and Cu–1.5Co–0.36Si alloys (in mass％). The alloys were peak–aged at 550°C and then cold–rolled to various rolling reductions. The larger the Co content was, the higher the precipitation hardening attained at peak aging. On the other hand, the work hardenability of the alloys decreased in the order of Co substitution. Based on the hypothesis that the yield stress after the thermomechanical treatments was mainly contributed by the precipitation strengthening and work (dislocation) strengthening, the effects of the substitution of Co for Ni on the strength of the alloys were discussed by the estimation of each contribution. The amount of precipitation strengthening was estimated from the volume fractions of precipitates obtained by electrical resistivity measurements. The estimates exhibited the same tendency as the actual age–hardening. Also, the amount of dislocation strengthening was estimated from the dislocation density derived from X–ray diffraction measurement. The amount of work hardening observed was in qualitatively accordance with the estimated dislocation strengthening. However, the sum of estimated precipitation and dislocation strengthenings showed a different tendency from the measured yield stress. Therefore, solid solution strengthening and grain refinement strengthening should be considered.