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Dynamic precipitation of σ-phase and element partitioning in equiatomic CoCrFeMnNi high-entropy alloy

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Abstract

We studied the development and element partitioning for σ-phase precipitation during hot compression of the CoCrFeMnNi HEA. The σ-phase mainly observed in the necklaced area (NA), where dynamic recrystallization (DRX) occurred but did not occur in the deformed area (DA). The σ-phase increases the hardness and elastic modulus measured by nanoindentation in NA, compared to that of DA. Several defects are transferred by the interface reaction of the crystal grains accompanied by the partitioning of the Cr and Ni elements. This results in the accelerated dynamic precipitation of the Cr-rich σ-phase.

Introduction

Recently, several studies have investigated high-entropy alloys (HEAs). While carbon steels or light alloys usually consist of one major component and, occasionally, up to seven alloying elements, HEAs do not have a main component and consist of more than five elements at relatively equal concentrations [[1], [2], [3]]. The equiatomic CoCrFeMoNi HEA known as the Cantor alloy has the face-centered cubic (FCC) single-phase structure [1]. The alloy shows increased ductility and yield strength as the temperature decreases to that of liquid nitrogen. The temperature also affects other properties of the alloy such as corrosion resistance, fatigue, and wear resistance [[4], [5], [6], [7], [8], [9], [10]].

Grain refinement has shown promise for increasing the strength and ductility of HEAs [6,[11], [12], [13]]. Otto et al. [6] studied the tensile properties of the CoCrFeMnNi HEA between 77 and 1073 K. It was found that the yield strength and ultimate tensile strength increased dramatically as the deformation temperature decreased, and the highest strength was observed for the smallest grain size. Static recrystallization through the annealing process after high strain can help to obtain fine grains. Moreover, the grain size decreases with increasing amount of cold deformation and lower annealing temperature [[14], [15], [16], [17]].

Schuh et al. [4] studied the mechanical properties and microstructure changes during the annealing process after high-pressure torsion (HPT) of the CoCrFeMnNi HEA. They found that the alloy was single-phase FCC for the annealing temperatures above 800 °C, while the σ-phase precipitated upon annealing at temperatures below 800 °C. Using experiments and thermodynamic calculations, Park et al. [18] reported that the Cr-rich σ-phase occurs after HPT and annealing process in the CoCrFeMnNi HEA. The σ-phase formation in the FCC structure can serve as another strengthening mechanism [4,[18], [19], [20], [21], [22]].

Otto et al. [5] observed σ-phases with sizes of approximately 10–20 μm when annealing was performed at 700 °C for 500 days. Pickering et al. [23] observed σ-phases sized approximately 2–5 μm from the microstructure image obtained after annealing for 500 h at the same temperature. Park et al. [18] heat-treated the CoCrFeMnNi HEA at 700 °C after HPT and observed σ-phase particles sized approximately 100 nm after 10 min. After 60 min, the size of the σ-phase was confirmed to be 100–300 nm. Otto et al. found that the composition of the σ-phase was 46.5% Cr-6.6% Ni such that sufficient diffusion of Cr occurred. Park et al. determined that the Cr content of the σ-phase was relatively low at approximately 25% Cr–15% Ni. This implies that the growth of the σ-phase accompanies the diffusion of Cr. Additionally, these studies indicated that σ-phase precipitation was activated due to high deformation followed by annealing, implying that the deformation is related to σ-phase formation. However, the effect of deformation on the dynamic precipitation of σ-phase, which differs from the static precipitation of the σ-phase investigated in previous studies, is unclear. Therefore, this study aims to understand the development and element partitioning for σ-phase precipitation during hot compression of the CoCrFeMnNi HEA.

Section snippets

Experimental procedure

Equiatomic CoCrFeMnNi HEA alloy was prepared and hot deformation was carried out at 700 °C. The alloy was prepared using vacuum induction melting to produce a 60 mm thick ingot (18 kg). For high-temperature rolling, the upper and lower surfaces of the ingot were mechanically polished for 5 mm each to fabricate a 50 mm thick slab. The slab was subjected to a solution treatment at 1150 °C for 2 h and then rolled to a final thickness of 15 mm from 1100 °C to 850 °C, followed by water quenching.

Results and discussion

Fig. 1 shows the microstructure represented using EBSD maps after 70 % compression at 700 °C. After compression, the microstructure was divided into a deformed area (DA) in which the initial grains have a pancake shape and a necklaced area (NA) in which fine equiaxed grains developed along the initial grain boundaries. In the NA, the fine equiaxed grains are mainly surrounded by high-angle grain boundaries (HAGBs). Fig. 1b shows an inverse pole figure (IPF) map of the same region as that shown

Conclusions

Cr-rich σ-phase in the CoCrFeMnNi HEA dynamically occurred during hot compression at 700 °C. The σ-phase formed around the grain boundaries of the initial and DRX grains leads to the increment of the hardness and elastic modulus. During the formation, Cr was absorbed into the σ-phase, while Ni diffused from the σ-phase. The element partitioning was activated by the moving interface of the DRX grains. Therefore, dynamic precipitation of σ-phase has higher kinetics than that of static

CRediT authorship contribution statement

Unhae Lee: Writing - original draft, preparation, Visualization, Investigation. Boris Straumal: Writing - review & editing. Nokeun Park: Project administration, Conceptualization, Methodology, Writing - review & editing, Submission.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2019R1A2C1088535).

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