Dynamic compression behavior of CoCrFeMnNi high-entropy alloy fabricated by direct energy deposition additive manufacturing

doi:10.1016/j.jallcom.2023.170602

Journal of Alloys and Compounds

Volume 960, 15 October 2023, 170602

https://doi.org/10.1016/j.jallcom.2023.170602 Get rights and content

Highlights

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High strain rate dynamic mechanical behavior of the DED-processed CoCrFeMnNi HEA.
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Advanced performance of the DED-processed HEA and verified compression simulation.
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Temperature and strain distributions by the FEM simulations of high-strain rate compression.
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Specimen region-based correlation of deformed microstructure with simulation results.

Abstract

High-entropy alloys (HEAs) have attracted significant interest in recent years because of their unique physical properties, which make them suitable for usage in extreme environments. The fabrication of HEAs using metal additive manufacturing (MAM) has shown potential for improving their performance. This study investigates the mechanical properties of CoCrFeMnNi HEA manufactured through direct energy deposition using compression tests conducted at various strain rates ranging from quasi-static to dynamic (10⁻⁴ - 4200 s⁻¹). The effect of the strain rate on the mechanical properties was analyzed, and a Johnson-Cook constitutive model was employed to simulate the dynamic compression test using the finite element method. The temperature and strain distributions during high-strain-rate compression were analyzed, reflecting the local temperature rise in the finite element analysis. The comparison of the strain distribution results of the simulation and microstructure after deformation showed that the microstructural behavior was influenced by strain localization, which occurred more prominently at the edge of the material. Because the CoCrFeMnNi HEA had a low stacking fault energy, twin formation was accelerated at high strain rate deformation by easily overcoming the critical twinning stress. The edge region, where strain localization was activated, exhibited a higher twin fraction than the central region (uniform deformation). The partitioned strain in the edge region was absorbed by twin formation, effectively suppressing the formation of the adiabatic shear band. These findings present significant implications for the design and development of MAM-processed HEAs with improved mechanical properties under extreme environments.

Introduction

An addition-based process, a metal additive manufacturing (MAM) process, is evaluated as a novel method with a degree of freedom of shapes. MAM can overcome the limitations of conventional parts design that cannot be fabricated using existing subtraction-based processes [1]. Owing to the advantage of geometric design freedom, MAM is being considered for the production of structures that receive impact (e.g., car crash, shielding for military purposes, and space-debris impact). Among laser-based MAM processes, the direct energy deposition (DED) method has the highest degree of freedom for selecting the deposition material because the amount of powder required for deposition is lower than that of other laser-based MAM methods [2]. During the DED process, powder metal feedstock is sprayed along the line, which is converted from a designed computer-aided design (CAD) structure [3]. Through the layer-by-layer printing procedure, a concentrated laser beam may synchronously melt both the substrate and metal powder, forming a few tens or hundreds of micrometer-sized melt pools along the scanning path [4]. Compared to conventional production techniques, the rapid cooling rate of MAM methods resulting from a high thermal gradient in the melt pool reveals exclusive and unique sub-grain microstructures in the form of cellular structures [5]. According to the literature, the cell structure stemming from insufficient dendrite growth time contributes to strength enhancement by hindering the movement of dislocations on the cell boundary [6].

High-entropy alloys (HEAs) are newly developed advanced materials composed of equal or near-equal atomic fractions of five or more elements, with a configuration entropy higher than 1.5 R (R: gas constant) [7]. Maximizing the solid-solution ability by randomly distributed elements on lattice sites ushers in a new era of physical metallurgy [8]. In particular, various HEAs exhibit excellent mechanical properties in extreme environments of cryogenic temperature [9], high temperature [10], and corrosive environments [11]. Therefore, the extraordinary properties of HEAs confirm their potential for a wide range of industrial applications under extreme operating conditions [12].

Recently, attempts to apply DED to HEA materials have increased because the reinforcing effect of laser-based MAM is evident in HEA materials [13], [14]. To date, the mechanical properties under quasi-static (low strain rate) conditions have been verified. The mechanical properties under various conditions must be analyzed to consider actual applications. The structural design and manufacturing process must always be prepared for possible impact. Understanding the dynamic behavior of materials is the first step toward user safety. The superior mechanical properties of HEAs fabricated using the MAM process require verification of the mechanical properties at high strain rates for application in various extreme situations. Despite some attempts to investigate the mechanical properties at high strain rates, no information has been reported on the differences in microstructure due to differences in strain distribution during dynamic (high strain rate) compression tests.

In this study, quasi-static and dynamic compression tests were conducted on the DED-processed CoCrFeMnNi HEA. The excellent mechanical properties of the DED-processed HEA were discussed. In addition, the finite element method (FEM) simulation was employed to analyze the deformation considering local heat generation and local strain distribution during high-strain-rate adiabatic compression. The reliability of the simulation was determined by comparing the dynamic behaviors in the simulation and the experiment. Microstructural analyses were conducted based on the correlation between strain distributions on simulation and deformed microstructure. The findings of this study represent a valuable foundation for further investigations into high-strain rate deformation and simulations of HEA fabricated using the MAM process.

Section snippets

Material and specimen fabrication

The morphologies of the gas-atomized equiatomic CoCrFeMnNi powders and their size distributions are shown in Fig. 1(a). As shown, spherical-shaped powders with a size distribution of $d_{10}$ = 53.8 µm, $d_{50}$ = 83.0 µm, and $d_{90}$ = 126 µm are used for the DED process. 28 × 23 × 6 mm³ cuboid CoCrFeMnNi HEA block is built using DED equipment (MX-lab, InssTek Co., Republic of Korea) with a laser power of 240 W, beam size of 0.4 mm, hatch spacing of 0.3 mm, layer height of 0.1 mm, scanning speed of

Microstructure of the as-built CoCrFeMnNi HEA

The density of the DED-processed CoCrFeMnNi HEA was ∼7.85 g/cm³, measured using the Archimedes method. The relative density was 98.84%. The synchrotron XRD pattern of the as-built specimen shown in Fig. 2 indicates a face-centered crystal (FCC) single-phase structure with a lattice parameter of 0.3593 nm. As a result of the rapid cooling rate during the DED process, a relatively high dislocation density of approximately 4.78 × 10¹⁴ m⁻² was obtained for the as-built CoCrFeMnNi HEA. During the

Conclusions

A CoCrFeMnNi HEA was successfully fabricated using the DED process. The quasi-static and dynamic compression properties along the build direction were evaluated. Significant strengths were obtained at each strain rate, which are presumed to be due to the sub-grain structure (cell structure). A Johnson-Cook constitutive model was fitted by quasi-static tests in which the temperature was assumed to be constant at each location inside the specimen, and the FEM simulation of dynamic compression was

CRediT authorship contribution statement

Soung Yeoul Ahn: Conceptualization, Data curation, Investigation, Formal analysis, Methodology, Validation, Visualization, Writing – original draft. Dong Geun Kim: Conceptualization, Investigation, Writing – original draft, Writing – review & editing. Jeong Ah Lee: Investigation, Validation, Formal analysis. Eun Seong Kim: Investigation, Visulalization. Sang Guk Jeong: Investigation, Visulalization. Rae Eon Kim: Investigation. Jungho Choe: Investigation. Soon-Jik Hong: Investigation. Pham Quang:

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

The present study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (2022R1A5A1030054, 2021R1A2C3006662). Eun Seong Kim is also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2022R1A6A3A13073830). The authors appreciate the Pohang Accelerator Laboratory (Pohang, Republic of Korea) for the synchrotron 8D beamline. We would also like to thank Farahnaz

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¹: These authors contributed equally to this work.

View full text

Dynamic compression behavior of CoCrFeMnNi high-entropy alloy fabricated by direct energy deposition additive manufacturing

Highlights

Abstract

Introduction

Section snippets

Material and specimen fabrication

Microstructure of the as-built CoCrFeMnNi HEA

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgments

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