Physical background of significant increase in mechanical properties and fatigue strength of groove-rolled Cu-Ni-Si alloy with discontinuous precipitates

Abstract

Prolonged aging of Cu-Ni-Si alloy induced the formation of discontinuous precipitates (DPs) of fiber-shaped Ni2Si intermetallic compounds, rendering good conductivity and relatively inferior strength. To improve the strength of Cu-Ni-Si alloy with DPs, groove-rolling technique was utilized to plastically deform the aged Cu-6Ni-1.5Si specimen. As the rate of area reduction increased from 20% to 50% and 80%, the diameter and interspacing of Ni2Si fibers decreased. Both tensile strength and fatigue limit stress, which are essential for the design of electrical and electronic mechanical components, of Cu-6Ni-1.5Si specimen increased significantly with groove-rolling. It was suggested that an extra strengthening was achieved by Ni2Si fiber reinforcement in addition to work hardening. The increase in yield strength by fiber reinforcement was estimated by Orowan strengthening principle, assuming ideal fiber arrangement. The estimated yield strength based on Orowan mechanism was found to be much greater than the experimentally measured value. This was because actual morphology and arrangement of fibers were largely different from the ideal state, necessitating an error factor, k, which varied with the true strain, εt, due to groove-rolling. The k showed initial steep increase and subsequent mild increase to εt, followed by decreasing trend after εt ≒ 0.7. The physical background of enhanced fatigue strength of rolled specimen was discussed based on the detailed microstructural observation on crack initiation and propagation behavior.