High-rate superplasticity in an equiatomic medium-entropy VCoNi alloy enabled through dynamic recrystallization of a duplex microstructure of ordered phases

Abstract

Superplasticity proceeds from fine-grained structures and requires high intrinsic resistance to grain growth at the deformation temperature. Here, we show that a mixture of two kinds of brittle ordered phases enables superplastic behavior through dynamic recrystallization in an equiatomic medium-entropy VCoNi alloy as a model material. The alloy annealed at 900 degrees C exhibits a face-centered-cubic single phase. However, in-depth characterizatin at various length scales reveals that the alloy, when annealed at 800 degrees C, comprises two ordered phases: kappa(Co1.2Ni1.3V) and sigma (Co1.3NiV2.3). As a result of the conventional cold-rolling/annealing process and with the aid of an underlying eutectoid reaction, the alloy exhibits a duplex structure with an average grain size of less than 1 mu m, i.e. microduplex structure. The size, morphology, and crystallographic orientation do not substantially change during static isothermal holding at 800 degrees C, which implies a very high resistance to grain growth. With tensile deformation at 800 degrees C, however, both phases develop into an equiaxed microstructure with low dislocation density and a dramatic change occurs in the crystallographic texture of the K phase. These variations result from dynamic recrystallization (DRX), which leads to superplastic elongations of 330-450% at 700-800 degrees C and at strain rates ranging from 10 to 4 to 10(-2) s(-1). Notably, the superplastic behavior is favorable at the high strain rate due to the enhanced DRX activity, leading to the larger elongation with increasing strain rates. However, deformation-enhanced grain growth occurs concomitantly with dynamic recrystallization; these competitive processes are investigated to elucidate the mechanism of superplasticity in this model material. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.