Effect of high temperature aging on lattice parameter, lattice strain, γ/γ΄ misfit, microstructure and mechanical properties of Alloy 617

In the present study, the lattice misfit and lattice strain of γ and γ΄ phases in Alloy 617 aged at 700 °C have been studied using XRD and correlated with FESEM and TEM microstructure. Aged Alloy 617 showed the presence of fine uniformly distributed γ´-Ni₃(Al, Ti) precipitates within the grains (γ-matrix) along with carbides predominantly along the grain boundaries. Lattice misfit of γ/γ΄ calculated in aged specimens revealed that misfit is negative for 100 h and 1000 h aged samples and it changes to positive for 4000 h of aging time. The lattice strain was evaluated by Stephen's phenomenological method and it is anisotropic in nature. The reduction in the lattice parameter of the alloy with aging explains the stress relaxation cracking in Alloy 617.