On spinodal decomposition in Cu–34at.% Ta thin films – An atom probe tomography and transmission electron microscopy study

In metals and alloys, phase separation takes place either by nucleation and growth or by spinodal decomposition. Here, transmission electron microscopy (TEM) and atom probe tomography (APT) are combined in an effort to study phase separation in an immiscible Cu–34at.% Ta thin film alloy on a nanometer scale. Upon annealing at 400–600°C the initially X-ray amorphous metastable alloy phase separates and crystallizes into a Cu-rich phase with fcc structure and a Ta-rich phase with β-Ta structure. While the fcc phase crystallizes already upon 10min annealing at 400°C, first indications of a crystalline β-Ta phase are observed after 30min annealing at 400°C, but this phase does not become fully crystalline at temperatures below 600°C. Proximity histograms of the APT data show that phase separation is taking place predominantly by diffusion of Cu, which is in accordance with the known relative diffusivities of the materials. An increase in the amplitude of the composition fluctuation and a decrease in the width of the interface between Cu-rich and Ta-rich regions are observed in the proximity histograms as phase separation progresses. These results suggest spinodal decomposition as the mechanism of phase separation.