Synergistic strengthening and toughening in Mg-doped Cu-15Ni-8Sn as-cast alloy: inhibition of discontinuous precipitation

Abstract

Cu-15Ni-8Sn-xMg (x = 0, 0.1, 0.2, 0.4 wt%) alloys were fabricated via conventional casting, and the microstructural evolution, solidification process, and mechanical properties were investigated through microstructural characterization, thermodynamic calculations, and tensile testing. The results demonstrate that Cu matrix (α₁) continuously nucleates and grows from melt during the solidification process, and the remaining liquid phase undergoes a divorced eutectic reaction to form (Cu, Ni)₃Sn (γ) and Cu (α₂) phase, and then the discontinuous precipitates (DPs) of α₁+γ are formed from α₂ with cooling proceeding. Mg addition significantly suppresses the formation of DPs in the as-cast microstructure, and with increasing Mg content, Cu₂Mg and Cu₄MgSn precipitates progressively form. The non-equilibrium solidification thermodynamic calculations indicate that Mg lowers the solidus temperature of the Cu-15Ni-8Sn alloy from approximately 761 °C to 490 °C. The reaction time of divorced eutectic is prolonged, resulting in facilitating the formation of γ phases, with a decrease in the volume fraction of α₂ phases, and thus the discontinuous precipitation is inhibited. The inhibition of discontinuous precipitation enhances the mechanical properties of Cu-15Ni-8Sn-xMg, with the 0.1 wt% Mg alloy exhibiting the optimal combination of a tensile strength of 608 MPa and an elongation of 10.1%. The present study establishes a foundational basis for suppressing discontinuous precipitation, with implications for designing alloys with enhanced mechanical properties.