Effect of Grain Size on the Superplastic Behavior of a Nanostructured Nickel-Based Superalloy

Abstract

The present paper deals with the review of earlier studies and original investigation of microcrystalline (MC) and nanostructured (NS) nickel based INCONEL® alloy 718 processed by severe plastic deformation (SPD) via multiple isothermal forging (MIF). The alloy with a mean grain size of 1 µm - 80 nm has been studied in terms of its thermal stability, superplastic and mechanical properties. It was established that the NS state with the 80 nm grain size can be considered as thermally stable up to the temperature 600°C (0.56Tm). The increase of annealing temperature beyond 600°C causes static recrystallization. Investigations of mechanical properties at room temperature have shown that the decrease of a mean grain size provides the enhancement of strength and reduction of plasticity. Thus, the alloy with a grain size of 80 nm has shown the ultimate strength - 1920 MPa and ductility - 4.8%. Mechanical properties of the NS state of the alloy after annealing are discussed. It has been established that alloy 718 with a grain size of 80 nm displays superplasticity at 600°C and a strain rate of 1.510-4s-1. The values of relative elongation and strain rate sensitivity coefficient m are 350% and 0.37, respectively.