Morphological changes and hardness evolution in Cu–30Ni–5Cr and Cu–45Ni–15Cr spinodal alloys

Abstract

The development of increased strength in Cu–Ni–Cr alloys, compared with binary Cu–Ni alloys, is dependent upon heat treatment. These alloys have compositions which permit them to be solution treated at elevated temperature and then aged at a lower temperature, in a two phase field, to produce hardening. Decomposition into two phases may occur by nucleation and growth or by a spinodal reaction, depending on alloy composition and heat treatment temperature. As part of a more extensive study of ternary Cu–Ni–Cr alloys, the decomposition of Cu–30Ni–5Cr and Cu–45Ni–15Cr (wt-%) has been studied in the spinodal range. The evolution of microstructure has been determined together with the coarsening kinetics for the modulated spinodal decomposition products. Specimens rapid quenched from 1050°C, were aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the early stages of aging the modulation wavelength remained constant while the hardness increased continuously. After a certain period of aging, the hardness remained constant at its peak value, while the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.MST/1733