Discontinuous Precipitating Behavior for Cu-Ni-Si Alloy with Mn Addition

We investigated the age-induced precipitation behavior of a Cu-4.0 wt.% Ni-1.1 wt.% Si alloy with 0.7 wt.% Mn addition, in comparison to a Cu-4.7 wt.% Ni-1.1 wt.% Si alloy without Mn addition. In the Cu-Ni-Si alloy without Mn addition, fine orthorhombic δ-Ni2Si precipitates were dispersed within the matrix grains, while lamellar structures composed of δ-Ni2Si and Cu laminates were coarsely developed at the grain boundaries. On the other hand, in the Cu-Ni-Si alloy with Mn addition, a small amount of Mn6Ni16Si7 particles (commonly referred to as the G phase, having a cubic structure) with a size of less than 100 nm was formed at the grain boundaries, although the lamellar structures containing coarse δ-Ni2Si laminates had significantly disappeared. As a result, the decrease in hardness after peak-aging (over-hardening aging) was suppressed. DFT analysis revealed that adding Mn to the Cu-Ni-Si alloy reduces the interfacial energy between the G phase and the Cu matrix. This supports the experimental fact that Mn addition to Cu-Ni-Si alloys promotes the formation of G phase at grain boundaries.