Understanding the Anomalous Discontinuous Precipitation in Cu-Ni-X (X = Co,Fe) Alloys Based on CALPHAD Method and Phase-Field Simulation

Abstract

The anomalous discontinuous precipitation which is called “Cellular Decomposition (CD)” has been observed in Cu-Ni-X (X = Co,Fe) ternary systems, i.e., the precipitate morphology in cell region is a nanoscale fine fiber, the direction of the fibers does not take specific crystal orientation, and the morphology is wavy. In this study, we elucidated the reason why such an unusual cellular decomposition occurs by means of the Calphad method and phase-field simulations. In particular, we focused on the spinodal decomposition inside the grain before CD. The results showed that the Ni component preferentially segregates at the interface region between precipitate and matrix phase in the two-phase microstructure by spinodal decomposition, and the Ostwald ripening is suppressed due to the low solute solubility of the matrix phase in the Cu-Ni-X (X = Co,Fe) system. Hence, the spinodal microstructure formed inside the grain before CD is trapped into high-energy level, which induces the discontinuous precipitation with high speed grain boundary migration, that provides “Cellular Decomposition”.

Fig. 2　Two-dimensional simulation result of spinodal decomposition in Cu-25.6 at％Ni-16.4 at％Co alloy at 773 K for t = 10 [(a), (c)] and t = 140 [(b), (d)]. The local composition is indicated by RGB color combination in (a) and (c). The composition of Ni component is represented by gray scale contrast in (b) and (c).