Effect of Magnesium on Microstructure Refinements and Properties Enhancements in High-Strength CuNiSi Alloys

Microalloying is an effective method to improve the comprehensive properties of copper alloys. The effects of magnesium on the microstructure, mechanical properties and anti-stress relaxation properties of CuNiSi alloys have been investigated. Results demonstrated that magnesium plays significant roles in refining the dendritic microstructure of the as-cast ingot, accelerating the precipitation decomposition, improving the mechanical properties and increasing the anti-stress relaxation properties. The incremental strength increase is due to the Orowan strengthening from the nanoscale Ni2Si and Ni3Al precipitates. As compared with the Cu-6.0Ni-1.0Si-0.5Al (wt%) alloy, the ultimate tensile strength of the designed Cu-6.0Ni-1.0Si-0.5Al-0.15Mg (wt%) alloy increases from 983.9 to 1095.7 MPa, and the electrical conductivity decreases from 27.1 to 26.6% IACS, respectively. The stress relaxation rates of the designed Cu-6.0Ni-1.0Si-0.5Al-0.15Mg alloy are 4.05% at 25 degrees C, 6.62% at 100 degrees C and 9.74% at 200 degrees C after having been loaded for 100 h, respectively. Magnesium significantly promotes nucleation during precipitation and maintains small precipitate size.