Effect of a gradient structure on the mechanical performance of Inconel 718 Ni-based superalloy at elevated temperatures

The effect of a gradient structure (GS) with present precipitates on the tensile properties of Inconel 718 (IN718) alloy at elevated temperatures was studied. A high-density δ-Ni3Nb phase and fine recrystallized γ grains were obtained in the surface gradient layer using an ultrasonic surface-rolling process (USRP) and subsequent heat treatment. The amount of the δ phase in the gradient layer decreased with the layer depth, while the size of the recrystallized γ grains exhibited an opposite trend. Remarkably, the GS did not show a microhardness gradient at room temperature but exhibited excellent strength and plasticity during high-temperature tensile tests. Compared with a uniform sample, the yield strength (YS) and ultimate tensile strength (UTS) of the specimen with a GS increased by 55 and 105 MPa, respectively, and the plasticity increased by 25% in the tensile tests at 650 °C. The δ phase pinned the RANS-scale grain hierarchy in the gradient layer and improved the thermal stability at 650 ℃. This GS promoted the accumulation of high-density geometrically necessary dislocations (GNDs) in the gradient layer during plastic deformation, resulting in additional back-stress strengthening. Therefore, forming a gradient structure with precipitates provides a new approach toward a good combination of strength and ductility at elevated temperatures.