The influence of Σ3 grain boundary and δ phase competition mechanisms on size dependence and micro-deformation behavior of Inconel 718 foil

In this study, the grain boundary structure and microstructure of Inconel 718 foils were optimized by adjusting the finished annealing process. The close relationship between the grain size dependence and the mechanical properties was analyzed to achieve a synergistic enhancement of the strength and plasticity. The result shows that saturated δ phase depletes the deformation storage energy and restricts the migration of grain boundaries to hinder the formation of Σ3 grain boundaries. The main competition between the weakening effect due to free surface grains and the strengthening effect induced by internal grain confinement affects the mechanical properties of the foils. The reduced constraint of free surface leads to dislocation escape, making intergranular interactions and coordination weaker, which is reflected in reduced flow stress and work hardening capacity. For 0.08 mm samples annealed at 1000 °C, the increased number of internal grains is beneficial to the activation of multiple slip systems and the coordinated deformation. Further, the suitable δ phase interacts with dislocations and Σ3 grain boundaries with low excessive energies to resist intergranular cracking, achieving a tensile strength of 846 MPa and an elongation of 20.3 %. This study provides a new idea for the microstructure optimization and mechanical property improvement of foils.