Characterization and Modeling of Deformation Mechanisms in Ni-Base Superalloy 718

Although the relationships between processing and the resulting properties are relatively well known for alloy 718, a better understanding of the deformation mechanisms activated across its usable temperature range is needed to create more mechanistically accurate property models. In this work, direct atomic-scale imaging with high angle annular dark field scanning transmission electron microscopy (STEM) has been complemented by phase field modeling informed by generalized stacking fault surface calculations using density functional theory. This coupled experiment/modeling approach has shed light on the complex shearing processes occurring in alloy 718 following a standard commercial heat treatment, which produces both monolithic γ and γ″ particles, as well as composite particles. Deformation at room temperature occurs through complex shearing of γ″ into intrinsic stacking fault configurations that were restricted to the precipitates. Exploration of possible shearing sequences with the aide of phase-field dislocation dynamics has revealed that precipitate shearing by motion of coupled 1/2<110> dislocations of non-parallel Burgers vectors on the {111} glide plane is the dominant deformation mechanism at lower temperature. A “fast-acting” yield strength model is discussed which takes into account microstructure variations and deformation mechanism transitions. Deformation at higher temperature (427 and 649 °C) has revealed a distinct transition in deformation modes, including stacking faults extending into the matrix, as well as microtwinning. The possible origin of the temperature and rate dependence of the stacking fault and microtwinning modes and temperature will be discussed.