Correspondence between grain refinements and flow softening behaviors at Nimonic 80A superalloy under different strain rates, temperatures and strains

As for a low stacking fault energy alloy, its strain softening degree evaluation and relative mechanism description significantly contribute to deep understanding its hot flow behaviors. An evaluator, decline ratio of flow stress (DRFS) was introduced and computed in this work to quantitatively evaluate the softening degree and its variation of a Nimonic 80A superalloy, and then the dominant softening mechanism was determined as dynamic recrystallization (DRX). The basic stress-strain data for DRFS computation come from a series of isothermal compressions with height reduction of 60% over a wide range of temperature and strain rate. The onsets of DRX were identified from the work hardening rate curves (i.e. theta versus sigma) and their derivative. The results show that the critical onset stress sigma(c) decreases with increasing temperature under a fixed strain rate, while increases with increasing strain rate under a constant temperature. In the softening process with DRX mechanism, grain size evolution and DRFS variation are the intrinsic and extrinsic indicators respectively, and their correspondence under different deformation conditions contributes to understand the relationships between microstructures and macro-mechanical properties. A superimposed map of grain size contour plot over DRFS map was constructed at the true strain 0.916. The results show that with increasing strain rate, the value of DRFS decreases to 0.05 until the strain rate reaching to 1 s(-1) and then increases to 0.18 at low temperature, while the grain size persistently decreases to 8.2 mu m. It indicates that the relationship between grain size evolution and DRFS variation is nonlinear.