Microstructural simulation of phase decomposition in Cu-Ni based alloys

The microstructure simulation of spinodal decomposition was carried out in the aged Cu-70at.%Ni and Cu-46at.%Ni-4at.%Fe alloys, based on a solution of the Cahn-Hilliard partial differential equation by the finite difference method. The calculated results were compared to those determined by atom-probe field ion microscope analyses of the solution treated and aged alloys. Both the numerically simulated and experimental results showed a good agreement for the concentration profiles and microstructure in the aged Cu-Ni alloys. A very slow growth kinetics of phase decomposition was observed to occur in this type of alloy. The morphology of decomposed phases consists of an irregular shape with no preferential alignment in any crystallographic direction. The wavelength of composition modulation was determined to be about 1 nm and it remained constant after aging at 523 K for times as long as 200 h. No phase decomposition was observed to occur for the numerical simulation of aging at temperatures lower than 480 K for a time as long as 20 years. In contrast, the simulated and experimental results of phase decomposition for the aged Cu-Ni-Fe alloys showed an equiaxial shape of the decomposed Ni-rich phase aligned in the elastically-softest crystallographic direction <100> of Cu-rich matrix. The growth kinetics rate of phase decomposition in Cu-Ni-Fe alloys is appreciably faster than that in Cu-Ni alloys. The wavelength of composition modulation was determined to be about 3 nm and it remained constant after aging at 723 K for times as long as 100 h.