Ruiyao Zhang

Misfit stresses in the γ″-strengthened Ni-base superalloy Inconel 718 were calculated from the measured constrained misfit strain using Eshelby’s inclusion method. The constrained misfit strains of the γ″ precipitates were measured using neutron diffraction at various temperatures with the aid of the stress-induced variant selection method. Eshelby tensor was calculated using the expressions for the case of anisotropic matrix given by Mura. Results show the presence of significant compressive misfit stresses in the γ″ precipitates with an anisotropic distribution, namely 3.0 GPa along the habit plane and 1.7 GPa along the plane normal direction at room temperature, and 2.0 and 1.2 GPa at 664 °C. The decrease in misfit stresses was due to the decrease in stiffness and the different coefficients of thermal expansion of the γ and γ″ phases. The average internal stresses in the γ matrix due to lattice misfit were found to be ~ 329 MPa at room temperature and ~ 186 MPa at 664 °C in tension. The possibility of relieving such high levels of misfit stresses in precipitates by loss of coherency during continued growth of precipitates is also discussed.

Polycrystalline Ni-base superalloy Inconel 718 derives the strengthening mainly from Ni₃Nb γ″ phase. To investigate the evolution of lattice spacing of γ″ precipitates, in situ neutron diffraction experiments were performed during aging heat treatment of two Inconel 718 bar samples at 780 °C for 8 hours. One sample was aged with a negligible applied stress (5 MPa) and the other with an applied tensile stress of 300 MPa. The diffraction data demonstrate three stages of lattice spacing evolution due to compositional and morphological changes during aging. (1) Stage I, the γ lattice spacing decreases isotropically in the first hour of aging as the composition changes. (2) Stage II, the γ lattice spacing decreases and the γ″ lattice spacing increases between 1 and 5 hours of aging, at this stage compositional change on the lattice evolution weakens with aging time and the morphological change becomes comparable. (3) Stage III, as aging proceeds, compositional change is negligible, while morphological change is dominant; the γ lattice spacing decreases in the longitudinal direction but increases in the transverse direction; in contrast, the converse occurs in γ″ lattice spacing evolution.

γ″ diffraction peaks are hard to discern in neutron/X-ray diffraction patterns, hindering studies on the γ″-strengthened superalloys using in-situ diffraction. In this study, we propose a variant selection method to increase the intensity of γ″ peaks and to facilitate accurate fitting. The specific variants of γ″ are controlled by applying a 300 MPa tensile stress during aging at 790 °C for 5 hours. The interaction energy between the applied stress and the transformation strain of each γ″ variant differs, leading to an increase in the amount of the variants with a greater energy reduction at the expense of other variants. The enhanced variants result in greater γ″ peak intensities in neutron diffraction patterns, allowing both the Pawley refinement and single peak fitting to be performed. Lattice parameters of γ″ and γ phases, and lattice misfit between the two phases and volume fraction of γ″ are acquired. The uncertainties associated with the fitting maintain an acceptable level corresponding to 150 microstrains. The proposed variant selection method shows potential for studying the role of γ″ phase in Ni-base superalloys.