Quantitative kinetic analysis of γ′ precipitate evolution in a Co–Al–W superalloy during aging heat treatment

Abstract

In this research, the effect of aging-heat treatment on evolution of γ′-precipitates in the Co–Al–W-based superalloy was studied. For this purpose, solution heat treatment was applied followed by water-quenching and then, aging different cycles were carried out at four temperatures of 720, 780, 840 and 900 °C for different holding time (1–48 h). The electron microscope micrographs showed that increasing size of γ′ particles happens with more holding time; while volume fraction stays approximately constant at 0.72. In addition, the γ-γ′ lattice mismatch, as positive values, increased with holding time increase due to simultaneous γ′ shape adaptation. While the γ′ morphology variation was seen to change from spherical to semi-cubic and cubic; the L- and stretched-shapes of γ′-particles with concurrent split of the coarse particles was detected by coarsening, coalescence and splitting phenomena, respectively. The proposed mechanisms for such microstructural evolution were interpreted based on n value as the power term in kinetics equation. As such, the γ′-particles diffusion across the γ/γ′ interface (n∼2), volume-diffusion (n∼3) and coalescence/splitting (n∼7) are key controlled processes at different aging temperature ranges. The characteristics of γ′-precipitates was discussed in detail based on total energy concept in terms of surface (Esur.), elastic strain (Estr..) and interactions of elastic (Eint.) energies and hence, a phenomenological model was proposed for evolution of γ′-particles. Modeling based a physical model and analysis of microhardness experimental revealed that the desired radius of γ′ phase should be between 80 nm and 90 nm respectively; and the optimized aging condition is estimated to be done at 840 °C for 36 h.