Back stress evolution as a function of prestrain in a nickel-based superalloy

Back stress, an internal stress linked to dislocation interactions with microstructural features, plays a crucial role in governing the Bauschinger effect in metals. Although the deformation behaviour of IN718 has been widely investigated, the influence of prestrain applied on back stress evolution in aged IN718 remains insufficiently explored. In this study, two complementary approaches are employed: a macroscopic method, defining back stress as half the stress difference between prestrain and reversed yield, and a microscopic method, estimating back stress through geometrically necessary dislocation (GND) density derived from electron backscatter diffraction (EBSD). Both approaches yield consistent results, demonstrating that back stress increases with prestrain, although the rate of evolution differs between low and high prestrain regimes. The change in the rate is likely associated with the evolution of microstructural features during deformation. EBSD analysis reveals substantial microstructural modifications, including an increase in GND density, elevated KAM values, and a gradual loss or transition in character of Σ3 boundaries. Since Σ3 boundaries usually act as efficient dislocation transmitters and accommodate strain, their degradation increases resistance to dislocation transmission, thereby amplifying the evolution of back stress. Incorporating these microstructural observations into a dislocation-based flow stress model enables a direct linkage between the macroscopic mechanical response and the underlying microstructural variations. These findings highlight the critical role of Σ3 boundaries in controlling the development of back stress, providing insights into the interplay among prestrain, microstructure, and mechanical behaviour in aged IN718.