Transition of deformation behavior and its related microstructure evolution in Nimonic 80A under hot-to-warm working

Abstract

Transitions of deformation behavior and microstructure evolution were observed in Nimonic 80A during hot-to-warm compression from 800°C to 1180°C (0.67Tm–0.91Tm) at a constant strain rate of 1s−1. The relationship between the peak stress (σp) and the deformation temperature was found to be bilinear with the slope of −4.0 in warm working range (800°C–950°C) and −1.0 in hot working range (1100°C–1180°C). The peak stress is much more temperature-dependent in warm working range than in hot working range, which is attributed to the increase of deformation activation energy (Q) from 175kJ/mol to 932kJ/mol. After deformation to a large strain of 1.1, homogeneous grain structures obtained above 1000°C gradually changed into inhomogeneous ones consisting of deformed initial grains as the temperature decreased into warm working range. The dynamic recrystallized grain size (DR) can be related to the peak stress by an empirical power–law function of σpGDRn=K. The grain size exponents (n) are −0.39 and −0.71 for warm compression (800°C–950°C) and hot compression (950°C–1180°C), respectively. These transitions of deformation behavior and microstructure evolution with the temperature variation are associated with the change of the operating dynamic recrystallization (DRX) mechanism. In hot working, new grains form via the bulging of initial grain boundaries (i.e. discontinuous DRX), while in warm working, new grains develop from subgrains via the gradual increase of sub-boundary misorientations into high angle type upon straining (i.e. continuous DRX). There is a temperature range for the transition in Nimonic 80A, which is approximately from 950°C to 1100°C where both DDRX and CDRX take place during the deformation.