Exploring the correlation between variants selection and applied stress in IN718 through transmission electron microscopic analysis

Abstract

Both tensile and rupture strength of Inconel 718 (IN718) are influenced by the morphology and volume fraction of γ′′ precipitates. This study investigated the preferrable coarsening behaviors of γ′′ precipitation affected by applied stress during aging. Neutron diffraction reveals the clear presence of {004}, {204}and {116} peaks of the γ′′ precipitates in the specimens aged under 300 MPa, compared to the one without stress. Scanning Electron Microscopy and Electron Backscatter Diffraction analyses have demonstrated that the stress applied promote a selective alignment of the γ′′ precipitates, to the detriment of alternative variants. The growth of preferential variants is correlated with the angle between the stress direction and the orientation of face-centered cubic nickel matrix. Crystals adhere to this pattern with misorientation angles within 15° for the {100}γ, {110}γ, {111}γ, and {311}γ planes. Transmission electron microscopy observations coupled with growth free energy calculations have elucidated that the central region of the variants experiences tensile stress exerted by the matrix under applied stress. Concurrently, the energetic barrier at the variant tips is notably reduced. This energetic disparity accounts for the observed preferential growth of variants along their axial direction.