Properties of Bulk and Sheet Micro-, Submicro-, and Nanocrystalline Alloy 718

Abstract

The superplastic properties of different semi-products out of Alloy 718 with microcrystalline (MC), submicrocrystalline (SMC) and nanocrystalline (NC) structures processed by severe plastic deformation and rolling have been studied. The alloy with SMC and NC structures can exhibit features of low temperature superplasticity. With decreasing the mean grain size of the Alloy 718 the temperature boundary of superplasticity becomes lower, the flow stress reduces and the ductility increases. The studies of the gauge portion of samples have shown that due to low temperature superplastic deformation there occur the following changes in the microstructure, namely, the density of dislocations decreases, non-equilibrium grain boundaries transforms to equilibrium ones and its mean grain size increases slightly. The boundaries of some grains have a banded contrast, which is apt to equilibrium high angle grain boundaries. The studies of the NC structure of Alloy 718 samples from superplastic deformation performed at different levels in various zones allowed revealing deformation bands indicating the occurrence of cooperative grain boundary sliding. The specific feature of the NC structure is a more uniform strain distribution and a less block size of cooperative grains as compared to SMC and MC structures. The comparative analysis of the structure and the mechanical properties of bulk and sheet Alloy 718 after superplastic deformation and heat treatment has been carried out. It has been shown that performance of standard heat treatment provides obtaining mechanical properties corresponding to standard requirements.