Precipitation behavior of Cu-3.0Ni-0.72Si alloy

Abstract

CuNiSi alloys have been widely applied in electronic and electrical industries. The precipitation behavior of some of the CuNiSi alloys is still not well understood. In this study, the precipitation behavior of the Cu-3.0Ni-0.72Si alloy aged at 600 °C for different times was investigated by transmission electron microscopy, atom probe tomography and phenomenological theory of precipitation crystallography. A new orientation relationship (OR) between the precipitates and the Cu matrix was found in the over-aged condition and a coarsening mechanism of the metastable precipitates was put forward. The two- and three-dimension invariant line theories were successfully applied in interpreting the evolution of the ORs and the morphologies in the Cu/δ-Ni2Si (Cu/δ) system. At the early stage of aging, the fine metastable δ’-(Cu, Ni)2Si precipitates are coherent with the Cu matrix, with a quasi-Bain OR of (110)Cu||(100)δ′ and [001]Cu||[001]δ′, and four pairs of parallel conjugate planes: (1¯1¯1)Cu||(3¯01)δ′, (111)Cu||(301)δ′, (1¯11)Cu||(021)δ′, and (11¯1)Cu||(02¯1)δ’. The precipitates have a δ-Ni2Si structure, with some Ni atoms substituted by Cu atoms. During growth, the core region of the metastable δ’-(Cu,Ni)2Si precipitate transforms into stable δ-Ni2Si, with a quasi-NW OR of (1¯11)Cu||(021)δ and [110]Cu||[100]δ, while a layer of metastable δ’- (Cu, Ni)2Si still exists around the core. With prolonging aging time, the δ-Ni2Si precipitates with the OR of (1¯11)Cu||(021)δ and [110]Cu||[100]δ grow two-dimensionally to form a plate-like shape, while those with the OR of (111)Cu||(301)δ and [1¯10]Cu||[010]δ grow one-dimensionally to form a fiber-like shape.