An extensive investigation of the effects of Mg(0.1, 0.2wt%) and P(0.03wt%) additions on precipitation characteristics and mechanical properties in Cu-2.5Ni-0.5Si-X(base alloy) has been carried out by means of microhardness test, tensile test, electrical conductivity measurement, and TEM.
On aging these alloys at 450℃, uniform fine precipitates of $δ-Ni_2Si$ with Pnma space group were formed for the main strengthening phase.
Aging of these alloys at 450℃ showed that Mg addition had little influence on the increase of maximum yield strength. However, the aging time required for the 0.2Mg alloy to reach maximum yield strength was reduced to 1/3 in comparison with the base alloy(Cu-2.5Ni-0.5Si). P was also effective in accelerating the precipitation of the 2nd phase. In the case of 0.03P alloy, maximum yield strength was greater than that of base alloy due to smaller grain size.
The presences of small amounts of Mg and P were found to increase the precipitation rate by lowering activation energy required for the precipitation of 2nd phase. The apparent activation energy for the precipitation decreased from 74.7 kcal/mole to 65.6 kcal/mole with the addition of 0.2Mg, while to 68.2 kcal/mole with the addition of 0.03P. It is suggested that the acceleration of the precipitation of $Ni_2Si$ by Mg and P addition is due to the dislocation substructure resulted from the thermal stress originated in the water quenching after solution treatment. This was confirmed by investigating the dislocation substructure of as-quenched alloys through transmission electron microscope.
At the initial stage of the aging, the more Mg content gave rise to the higher conductivity. As precipitation progresses the tendency was reversed because the role of Mg to stimulate the depletion of Ni and Si atoms from matrix decreased and the ability of the resolved Mg atoms to disturb free electron motion became more important at the later stage of the aging. In case of 0.03P alloy, the conductivity was higher than that of the base alloy through overall aging times because of very small amount of P compared with Mg in the 0.2Mg alloy.
High strength and good electrical conductivity are the most required properties for the electrical connector alloys. Based on the combination of high strength and good electrical conductivity, it was found that the optimum magnesium content is near 0.1wt% in Cu-2.5Ni-0.5Si alloys.
