Influence of 0.1% Addition of Mg and/or Fe on Strength and Stress Relaxation Property of a Cu-0.52%Ni-0.19%P Alloy

The effects of addition of 0.1 mass%Mg and 0.1 mass%Fe on the strength and stress relaxation property of a precipitation-hardenable Cu-0.52 mass%Ni-0.19 mass%P alloy aged at 450°C have been investigated. Precipitation of Ni2P, Ni12P5, both Ni2P and Ni5P4, and both Ni12P5 and Ni5P4 phases was observed to occur on aging the Cu-Ni-P, Cu-Ni-P-Fe, Cu-Ni-P-Mg, and Cu-Ni-P-Fe-Mg alloys, respectively. The needle-like Ni5P4 phase was first found to be precipitated in the Cu matrix. In the peak-aging stage, the yield strength of the four alloys is controlled by the Orowan mechanism. The addition of Mg to the Cu-Ni-P alloy enhanced the strength of the alloy, but the Fe addition did not significantly affect the strength. The strength of the Cu-Ni-P-Mg alloy was nearly identical to that of the Cu-Ni-P-Fe-Mg alloy. The increase in strength by the addition of Mg is ascribed to the decrease in inter-precipitate spacing of precipitates. The Cu-Ni-P-Fe alloy exhibited better stress relaxation property than the Cu-Ni-P alloy, probably because atoms caused by pairs of Fe and P chemical bonding are dragged by moving dislocations. Adding Mg to the Cu-Ni-P alloys did not essentially change the stress relaxation property. This may be as a result of the combination of the increase in stress relaxation resistance caused by the Mg-atom-drag effect on dislocation motion and of the decrease in that due to the decrease in the amount of P atoms in the Cu matrix by adding Mg.