Precipitation behavior and mechanical properties of Cu-Ni-Si, Cu-Ni-Si-Mg and Cu-Ni-Si-Mg-Mn alloys were investigated by hardness test, tensile test, Differential Thermal Analysis and Transmission Electron Microscope.
Aging of Cu-Ni-Si at 450℃ produced two peaks with aging time. The apparent activation energy of the formation of the first peak was 13Kcal/mol, and that of the second peak was 65Kcal/mol. The first peak was due to the formation of $Ni_2Si$ precipitate. TEM analysis indicated that the second product was $L1_2$ structure of off-stoichiometric nickel silicide($Ni_{3+x}Si_{1-x}$), whose lattice parameter was 3.62 Å. They had a disc shape parallel with {100} plane of the matrix. Their thickness and diameter were about 100 Å and 1000 Å, respectively.
In Cu-Ni-Si-Mg alloy, double peaks on the curve of hardness vs aging time were also appeared at 550℃. The first peak was produced by the precipitation of off-stoichiometric nickel silicide, while the second peak due to the formation of magnesium compounds. The magnesium compounds had $L1_2$ structure, and lattice parameter was 4.24 Å. They were very fine and randomly distributed in copper matrix.
The distribution and shapes of precipitates in Cu-Ni-Si-Mg-Mn alloy were same as those of Cu-Ni-Si-Mg alloy and no Mn-compounds were formed with the addition of Mn. However, Manganese in Cu-Ni-Si-Mg-Mn system retarded the formation of nickel silicide transition phase. The height of the first peak of DTA curve in Cu-Ni-Si-Mg-Mn system was lower than that of Cu-Ni-Si-Mg system. Manganese addition had detrimental effects on the mechanical properties of Cu-Ni-Si-Mg alloy. Best tensile properties were obtained in Cu-Ni-Si-Mg, where Y.S.(ksi), U.T.S.(ksi) and E.L.(%) were 69.53, 90.68, 12.85, respectively.
