Processing and mechanical properties of bulk nanostructured nickel-based alloys

The present paper deals with investigation of nanocrystalline (NC) nickel-based alloys processed by severe plastic deformation via multi-axis forging and high-pressure torsion. The nickel-based alloys Ni–20%Cr, PDS and Inconel 718 with a mean grain size of 50–80 nm have been studied in terms of their thermal stability and mechanical properties. The NC structure is characterized by higher internal stress and microhardness than a coarse-grained alloy. It was established that the NC structure of Ni–20%Cr alloy can be considered as thermally stable up to 500 °C (0.46 Tm), which is lower than the temperature threshold for the retention of NC structure in Inconel 718 (580 °C = 0.5 Tm). The increase of annealing temperature of these alloys up to 600 °C causes static recrystallization and transformation of a NC structure into a submicrocrystalline one. It has been established that Inconel 718 with the NC structure displays superplasticity at 600 °C under a strain rate of 1.5 × 10−4 s−1. The values of relative elongation and strain rate sensitivity coefficient m are 350% and 0.37, respectively.