Precipitation evolution and properties of Cu-15Ni-8Sn alloys via Al microalloying during ageing

Abstract

Studies have shown that microalloying Cu–Ni–Sn alloys using Al inhibits discontinuous precipitation reactions; however, the mechanism has not been investigated yet. Thus, this study explored the properties and precipitation evolution of Cu-15Ni-8Sn alloys without and with 1 wt% Al(Cu-15Ni-8Sn-1Al) during ageing at 500 °C through X-ray diffraction (XRD), scanning electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. The results revealed that in Cu-15Ni-8Sn alloy, DO₂₂ ordering first occurred after spinodal decomposition, subsequently transitioning to an L1₂ ordering phase, during which the discontinuous precipitates gradually nucleated at the grain boundaries and formed ingrains. The alloy hardness increased sharply with ordering and then decreased rapidly after reaching its peak value, due to the dominance of the discontinuous precipitation reaction. However, the DO₂₂ ordered phase did not appear during the ageing of Cu-15Ni-8Sn-1Al alloy, and a Ni₃(Al_x, Sn_1−x) intermetallic compound with L1₂ structure was formed, consuming Ni and Sn atoms, thereby significantly suppressing the discontinuous precipitation reaction and improving the stability during prolonged ageing. The tensile strength (yield strength) of Cu-15Ni-8Sn and Cu-15Ni-8Sn-1Al alloys at peak hardness were 855 (755) and 936 MPa (892 MPa), respectively, and their main strengthening mechanisms were solid solution strengthening and precipitation strengthening. The addition of 1 wt% Al effectively improved the precipitation strengthening effect of Cu–Ni–Sn alloys but severely decreased the ductility.

Graphical Abstract