The effects of magnesium(0.05wt.\%) addition to Cu-1.5Ni-0.3Si0.03P(PMC-102) on precipitation behavior and tensile properties have been investigated by means of tensile tests, electrical resistivity measurements, and transmission electron microscopy(TEM). The crystal structure of the main strengthening precipitate($Ni_2Si$) and its orientation relationship with the matrix have been also studied. In the solution-treated condition, the Mg-added alloy (PMC-102M) exhibited higher strength and electrical resistivity compared to the Mg-free alloy(PMC-102). This was due to the solid-solution effect of the magnesium atoms. In the peak aged condition, the yield stress of the Mg-containing alloy was higher than that of the Mg-free alloy, while the reverse was true in electrical resistivity. TEM observations revealed that the Mg-containing alloy had more uniform and finer distribution and higher volume fraction of the precipitate compared to the Mg-free alloy, resulting in the higher strength. A lower equilibrium solute content of the matrix was responsible for the increased electrical conductivity. The solvus of the precipitates determined from the electrical resistivity measurements increased with the addition of magnesium. Due to the increased solvus temperature, the supersaturation of Mg-containing alloy was larger than that of the Mg-free alloy. The higher supersaturation will enhance nucleation rate of precipitates, resulting in finer distribution and higher volume fraction of precipitate. An anisotropy in tensile properties was observed in cold-rolled sheets of both PMC-102 and PMC-102M alloys. The deformation textures observed in both the two alloys were found to be $\{110\}<112>$. The application of tension annealing or tension levelling on cold-rolled sheets, which were intended to reduce residual stress, resulted in a great increase in tensile elongation coupled with a slight decrease in tensile strength. The thermal softening tests performed on aged specimens showed that the Mg-addition also enhanced the resistance to thermal softening. On aging at $450\,^\circ\!C$, the main precipitating phase in both PMC-102 and PMC-102M alloys was identified as $\delta-Ni_2Si$ with C23 crystal structure by means of X-ray diffraction and energy dispersive specrometry. No evidence of precipitating phases other than $\delta-Ni_2Si$ was observed. TEM micrographs showed that the $\delta-Ni_2Si$ precipitates were in the shape of discs in $\{110\}$ planes of the matrix. Computer aidedanalyses of electron diffraction patterns have shown that the orientation relationship between the precipitates and the matrix is $\{110\}_{cu}//(001)_{\delta}$, $<100>_{cu}//\{100\}_{\delta}$. The comparison of lattice parameters determined from X-ray diffractions performed on extracted products with those determined from TEM diffraction patterns revealed that the coherent precipitates were under compression stress along a$\delta$ axis, while under tension stress along c$\delta$ axis.
