Enhanced strength of Cu-Ni-Si alloy via heterogeneous nucleation at grain boundaries during homogenization

Copper alloys are utilized either as cast or wrought alloys. Typically, cast alloys contain a higher quantity of elements to enhance strength. The increased content of additive elements inevitably escalates the precipitation driving force. However, this can result in heterogeneous nucleation, leading to the formation of large particles at grain boundaries, consequently impeding the strengthening process. Managing the shape and structure becomes pivotal to address this challenge. The Cu alloy, enriched with 6 wt% Ni, 1.3 wt% Si, and 2.4 wt% Mn, possesses a high precipitation driving force due to the abundance of solutes in the matrix at high temperature. Upon cooling in air following heat treatment at 980 °C for 8 h (homogenization), both homogeneous and heterogeneous nucleation and growth processes occur within the grain interior and at the grain boundaries. The phase heterogeneously formed at the grain boundary is identified as Ni2Si compound in the Cu-Ni-Si alloy, while the phase at the grain boundary is recognized as Mn6Ni16Si7, known as the G-phase. The presence of G-phase at the grain boundary results in higher strength and ductility compared to the alloy with Ni2Si. The substantial coverage of the grain boundary by the film-shaped G-phase renders the grain boundary more stable than the irregularly shaped Ni2Si, consequently contributing to the elevated strength and ductility of the Cu-Ni-Si alloy. •A hard intermetallic compound at the grain boundary, with low interfacial energy, can potentially increase ductility.•A notable increase in both strength and ductility in the Cu-Ni-Si alloy with Mn occurred after homogenization.•The mechanisms for increased ductility despite large particles at grain boundaries in the alloy were elucidated.