Diffuse γ/γ′ interfaces in the hierarchical dual-phase nanostructure of a Ni-Al-Ti alloy

The nanostructural hierarchy of gamma and gamma' phases is characterized in Ni-8.5Al-5.4Ti (at.%) alloy, after double aging treatment at 1000 and 780 degrees C for 6 and 20 h. Using electron microscopy and atom probe tomography, disordered gamma phase inside ordered gamma' precipitate was detected, which underwent plate-like growth during aging treatment. This unique hierarchical structure enabled the simultaneous study of direct (gamma' precipitates/gamma matrix) and inverse (gamma nanoparticles/gamma' precipitates) order-disorder interfaces in 6 and 20 h double-aged samples. It was found that the compositional interface width, delta, of direct interfaces decreased during aging treatment, whereas the thickness remained almost unaffected for inverse interfaces. The difference in thickening behavior is explained based on the change of the Gibbs-Thomson pressure for gamma plates embedded in gamma' and cuboidal gamma' precipitates surrounded by the gamma matrix. This inference suggests the possibility of prediction and control of delta(t) variations during heat treatment processes based on the current thermodynamic approach.