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

Abstract The nanostructural hierarchy of γ and γ′ phases is characterized in Ni-8.5Al-5.4Ti (at.%) alloy, after double aging treatment at 1000 and 780 °C for 6 and 20 h. Using electron microscopy and atom probe tomography, disordered γ phase inside ordered γ′ precipitate was detected, which underwent plate-like growth during aging treatment. This unique hierarchical structure enabled the simultaneous study of direct (γ′ precipitates/γ matrix) and inverse (γ nanoparticles/γ′ precipitates) order-disorder interfaces in 6 and 20 h double-aged samples. It was found that the compositional interface width, δ, 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 γ plates embedded in γ′ and cuboidal γ′ precipitates surrounded by the γ matrix. This inference suggests the possibility of prediction and control of δ ( t ) variations during heat treatment processes based on the current thermodynamic approach.