The superplasticity improvement of Inconel 718 through grain refinement by large reduction cold rolling and two-stage annealing

Abstract To improve the superplasticity of Inconel 718, cold rolling with 84% reduction, followed by two-stage annealing, was designed to refine and homogenize the grains. Compared with cold rolling and single-stage annealing, a finer grain size of 1.21 μm was obtained and provided better superplasticity (up to 657% elongation) at 950 °C with a strain rate range of 6 × 10-4 to 1 × 10-2 s-1. Sizeable plastic deformation introduces plenty of crystal defects and storage energy, which contributes to the dense and fine distribution of δ phase in the first stage annealing and enhances the driving force for static recrystallization (SRX) in the subsequent high-temperature annealing. The pre-existing δ phase stimulates the nucleation of SRX and hinders the growth of grains. In addition, the second phase particles form a uniform distribution due to dissolution, thereby controlling the difference in grain size. The sufficient pre-precipitation and subsequent uniform dispersion of δ phase achieve a significant grain refinement effect. This paper is expected to serve as a reference for the industrial production of superplastic Inconel 718 sheets.