Formation of modulated structures in copper-nickel-iron alloys

Abstract

By means of X-ray diffraction, the formation of modulated structures with periodic variations in composition due to a simple interchange of atoms has been studied in copper-nickel-iron alloys as a function of composition, temperature and time. The results are compared with two models suggested for this type of “exchange transformation”. The reaction characteristics seem to favor a kinetic model rather than the Guinier model. The activation energies for the transformation, evaluated at the first detectable stage and for a later stage, show that both these stages are diffusion-controlled inside as well as outside the spinodal. This indicates that the spinodal is of no real significance with regard to the nucleation process. The variation with temperature and composition of the first wavelength to form in the modulated structure is in agreement with the kinetic model.