Discontinuous precipitates in age-hardening Cu-Ni-Si alloys

The microstructural evolution and characterization of discontinuous precipitates in a Cu-4.3 Ni-2.2 Si (in at.%) alloy were studied, and compared with those of continuous precipitates in the same alloy. During prolonged aging, coarse cellular components containing fiber-shaped δ-Ni₂Si and copper solid-solution phases nucleate and grow quickly but discontinuously at the grain boundaries, accompanied by the consumption of fine δ-Ni₂Si particles formed by continuous precipitation. In terms of the crystal structure, all the precipitates are of the same type of orthorhombic δ-Ni₂Si. However, in terms of the crystallographic features, the δ-Ni₂Si discontinuous precipitates have micro-scale fibers that are aligned with the orientation relationship of _100δ/110_Cu and (013)_δ/(111)_Cu, and with a preferential extending direction on the (111)_Cu plane, which differs from the fine δ-Ni₂Si continuous precipitates at the early stages of aging. The evolution of the discontinuous precipitates can be explained by the existing classical theories of phase transformation, as discussed by Hu et al. regarding the subsequence of the continuous precipitates. In this study, we also confirmed that the development of coarse δ-Ni₂Si discontinuous precipitates of the cellular components leads to a serious drop in the strength in the later stages of aging.