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 °C exhibits a face-centered-cubic single phase. However, in-depth characterization at various length scales reveals that the alloy, when annealed at 800 °C, comprises two ordered phases: κ (Co1.2Ni1.3V) and σ (Co1.5NiV2.5). 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 μm, i.e. microduplex structure. The size, morphology, and crystallographic orientation do not substantially change during static isothermal holding at 800 °C, which implies a very high resistance to grain growth. With tensile deformation at 800 °C, however, both phases develop into an equiaxed microstructure with low dislocation density and a dramatic change occurs in the crystallographic texture of the κ phase. These variations result from dynamic recrystallization (DRX), which leads to superplastic elongations of 330–450% at 700–800 °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.
Bibliographical noteFunding Information:
This work was supported by the Korea University Grant for the first author, by the Future Material Discovery Project of the National Research Foundation of Korea (NRF-2016M3D1A1023383), by the Fundamental Research Project of the National Research Foundation of Korea (NRF-2019R1F1A1057687), and by Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (P0002019, The Competency Development Program for Industry Specialist). S.S. Sohn is grateful to the kind support of the Alexander von Humboldt Stiftung (AvH, Alexander von Humboldt Foundation, ). A. Kwiatkowski da Silva is grateful to the Brazilian National Research Council (Conselho Nacional de Pesquisas, CNPQ) for the PhD scholarship through the “Science without Borders” Project (203077/2014–8).
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- Dynamic grain coarsening
- Dynamic recrystallization
- Multi-principal element alloy
- Ordered phase
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys