Microstructural behaviour and mechanical hardening in a Cu-Ni-Cr alloy

Abstract

In this investigation electron microscopy and diffraction have been employed to characterize the development of the modulated structures and associated sideband phenomena in a Cu-31.6 Ni-1.7Cr alloy and the microstructural behaviour has been correlated with the age-hardening response. The microstructural behaviour is consistent with the notion of spinodal decomposition of a rather asymmetric alloy within the ternary miscibility gap in the temperature range 650 to 750° C. The modulated structures which form during precipitation tend to undergo a morphological change during subsequent coarsening involving the sequence: cuboids → rods → platelets (or rafts); the driving force for this transformation is the minimization of the surface and strain energy of the coherent two-phase mixtures. Precipitate-free or denuded zones have been observed to develop after prolonged ageing apparently resulting from preferential loss of coherency and coarsening of particles in the vicinity of the grain boundaries. This microstructural heterogeneity gives rise to a “discontinuous coarsening” reaction eventually involving the migration of high-angle boundaries. The mechanical strengthening accompanying the formation of the aligned and periodic precipitate morphologies can be accounted for quantitatively in terms of the interaction of dislocations with the internal stress fields associated with the coherent precipitates.