Discontinuous precipitates in age-hardening CuNiSi alloys

•The discontinuous precipitates in the alloy are identified to be δ-Ni2Si.•δ-Ni2Si precipitates exhibit fiber-shape after initial aging and then grow quickly.•δ-Ni2Si precipitates preferentially extend across {111}Cu matrix plane.•Coarse δ-Ni2Si fibers in cellular components are detrimental to alloy hardening.

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 ⟨100⟩δ//⟨110⟩Cu and (013)δ//(11̅1)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.