Effect of Cooling Methods on Microstructure and Mechanical Properties of Hot-extruded Cu-15Ni-8Sn Alloy

The effect of cooling method on microstructures and mechanical properties of the hot-extruded Cu-15Ni-8Sn bars were investigated by optical microscope(OM), scanning electronic microscope(SEM), differential scanning calorimetry(DSC) and tensile testing. The results indicated that the content of solid solution atom Sn in the water-cooled alloy after hot extrusion is greater than that in the air-cooled alloy. The precipitation onset temperature and dissolution termination temperature of γ phase in the water-cooled alloy are 24 °C and 32 °C lower than that in the air-cooled alloy, respectively. Both water-cooled and air-cooled alloys are composed of γ phase and supersaturated solid solution α(Cu), and show the similar grain size about 23 μm. In the air-cooled alloy, a large number of dispersed rod-like and granular γ-phases are precipitated in the grains, while only granular γ phase is observed in water-cooled alloy with. The tensile strength, elongation and microhardness of the water-cooled and air-cooled alloys are 588±12 MPa and 855±41 MPa, 41.6±2.0% and 15.2±1.0%, 166±7 HV and 292±5 HV, respectively, while the former shows a typical ductile fracture mechanism and the latter has somewhat brittle fracture characteristics.