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.

Electron transmission microscopy and diffraction have revealed directly the fine-scale precipitation processes and the mechanism of formation of the periodic microstructures which characterize the decomposition of Cu-Ti “sideband” alloys. The initial stages of decomposition involve the development of «100» compositional waves giving rise to the well-known sideband or satellite reflections along the «100» directions. This «wave-like» clustering producing a triaxially modulated structure involves the formation of two disordered phases; ordering does not accompany the initial clustering. Prolonged aging produces a periodic array of interpenetrating rods along the «100» directions of the matrix. During aging the periodic structures coarsen according to a t1/3 law. The experimental activation energy for the coarsening of the modulated structures is approximately 48 kcal/ mole. Maximum strength is associated with the formation of the transition phase β’ which forms byin situ transformation of the titanium-rich regions; this transformation is not accompanied by a loss of coherency in the modulated structures at the aging temperatures studied in this investigation. It is suggested that these periodic structures arise from spinodal decomposition.