Coarsening mechanism of over-aged 6-Ni2Si nanoscale precipitates in Cu-Ni-Si-Cr-Mg alloy

6-Ni2Si precipitate is the crucial strengthening phase in Cu-Ni-Si alloy, and its coarsening behavior and mechanism impose dramatic effects on mechanical properties. In this paper, Cu-3Ni-0.7Si-0.1Cr-0.05 Mg (wt%) alloy was prepared to systematically investigate the coarsening behavior and elucidate the coarsening mechanism of 6-Ni2Si precipitate during over -aging treatment. Featured core -shell structure was observed via careful characterization during coarsening of 6-Ni2Si phase. Specifically, the core structure was stable 6-Ni2Si, while the shell structure was a metastable interface composed of Cu, Ni and Si elements. At later over -aging stage, the core -shell structure completely transformed into a stable and coarsened 6-Ni2Si phase being coherent with the Cu matrix. It is confirmed for the first time that the coarsening process of 6-Ni2Si precipitate can be interpreted by the transinterface -diffusion -controlled (TIDC) coarsening theory. The core -shell structure was caused by the disparity between the diffusion activation energy and diffusion rate of Ni, Si, and Cu from the perspective of diffusion kinetics. Moreover, the presence of trace amounts of Cr and Mg components exhibited little effect on the coarsening process of 6-Ni2Si. During over -aging, tensile strength dramatically decreased owing to diminishing precipitation strengthening effect from coarsening of 6-Ni2Si precipitates.