Research article
Compositional stability in medium and high-entropy alloys of CoCrFeMnNi system under ion irradiation

https://doi.org/10.1016/j.jallcom.2022.166697Get rights and content

Highlights

  • The irradiation resistance of CoCrNi, CoCrFeNi, and CoCrFeMnNi is higher than that of Ni.

  • Significant Cr segregation occurred in the CoCrNi and CoCrFeNi samples irradiated at 873 K. In contrast, no segregation occurred in CoCrFeMnNi.

  • First-principles calculation results show that the formation rate of Cr-dumbbells is higher in CoCrNi and CoCrFeNi than in CoCrFeMnNi, and that Cr interstitials are more stable in the CoCrNi and CoCrFeNi.

  • Cr segregation is more likely to occur in the CoCrNi and CoCrFeNi.

Abstract

The equiatomic high-entropy alloy (HEA) CoCrFeMnNi not only has excellent mechanical properties but also good irradiation resistance. However, the mechanical properties of some equiatomic medium-entropy alloys (MEAs) are superior to those of CoCrFeMnNi HEA. In this study, the irradiation resistance and changes in composition due to irradiation in CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA are investigated. Thin film samples of the MEAs and HEA and Ni used for comparison were irradiated with up to 1.7 × 1019 ions/m2 of 2.4 MeV Cu ions at 673 and 873 K. The average damage in the observed area was 1 displacement per atom (dpa). No voids were observed in any of the MEA and HEA samples even after irradiation at 873 K; however, large voids were formed in Ni irradiated at 873 K. This indicates that the irradiation resistance of CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA was better than that of Ni. In addition, the formation of stacking fault tetrahedra (SFTs), a type of vacancy cluster, at 873 K was much more pronounced in CoCrNi and CoCrFeNi MEAs than in CoCrFeMnNi HEA. Therefore, the irradiation resistance of CoCrNi and CoCrFeNi MEAs is lower than that of CoCrFeMnNi HEA. Moreover, significant Cr segregation occurred in the CoCrNi and CoCrFeNi MEA samples irradiated at 873 K. In contrast, no segregation occurred in CoCrFeMnNi HEA. First-principles calculation results show that the formation rate of Cr-dumbbells is higher in CoCrNi and CoCrFeNi MEAs than in CoCrFeMnNi HEA, and that Cr interstitials are more stable in the MEAs. Therefore, Cr segregation is more likely to occur in the MEAs. Element segregation may affect the irradiation resistance of the alloys.

Introduction

High-entropy alloys (HEA) are usually alloys that contain five or more elements in equal proportions. A typical HEA with excellent strength and toughness is CoCrFeMnNi, which has been widely studied [1], [2], [3], [4], [5], [6]. CoCrFeMnNi HEA also has good irradiation resistance [7], [8], [9], [10], [11]. Medium-entropy alloys (MEA) are composed of three or four elements in equal proportions, similar to a HEA. Three- (CoCrNi) and four-element MEAs from the CoCrFeNi system are single-phase solid solutions with a face-centered cubic (FCC) structure similar to that of CoCrFeMnNi HEA. CoCrNi and CoCrFeNi MEAs have similar mechanical properties to CoCrFeMnNi HEA [12], [13]. However, fewer studies have been performed on the irradiation resistance of CoCrNi and CoCrFeNi MEAs compared to that of CoCrFeMnNi HEA. The limited research results availably show that the irradiation resistance of CoCrNi MEA is worse than that of CoCrFeMnNi HEA [14]. The cause of the decreased irradiation resistance is not well understood.

In general, irradiation with high-energy particles produces the same numbers of interstitial and vacancy defects. The defects produced by high-energy ions and neutrons are spatially non-uniform because of cascade damage. Moreover, because the migration energy of interstitials in metals is lower than that of vacancies, interstitials can migrate at faster rates. In thin film samples, the escape of interstitials from the surface of the sample, leaves behind many vacancies. Cascade generation during high-energy particle irradiation and vacancy cluster formation under supersaturated vacancy states have therefore been investigated in film samples with thicknesses of several tens of nanometers to about 100 nm [15], [16], [17]. In addition, because the migration of vacancies often leads to the diffusion of solute atoms in alloys, the presence of excess vacancies may promote element segregation and precipitate formation.

In the present study, thin-film samples of CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA were irradiated by heavy ions, and the damage to their microstructures and compositional stability investigated. The differences between the irradiation resistance mechanisms of CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA are discussed.

Section snippets

Sample preparation and ion irradiation

Similar to CoCrFeMnNi HEA fabrication [8], CoCrNi and CoCrFeNi MEAs were fabricated in a vacuum induction furnace. The purity of each element was higher than 99.9 wt%. These HEA and MEAs were homogenized in vacuum at 1473 K for 10 h and 12 h, respectively. Approximately 1 mm-thick plates were cut from the HEA and MEA ingots and rolled to 0.1 mm. Transmission electron microscopy (TEM) samples with a diameter of 3 mm were punched from the 0.1 mm-thick sheets. The TEM samples were electropolished

Damaged microstructure induced by ion irradiation at low temperature of 673 K

Fig. 3 shows bright-field images of the microstructures in the CoCrNi MEA, CoCrFeNi MEA, and CoCrFeMnNi HEA thin films after ion irradiation at 673 K. For comparison, the microstructures induced by irradiation in Ni are also shown in the same figure. As described in Section 2, the amount of damage in the observed region was 1 dpa. The average thickness of the observation area was 130 nm. In irradiated Ni, black loop-like defects marked by red arrows in the figure were formed (in color figure).

Conclusion

Because the stability of an alloy composition affects its irradiation resistance, the composition changes due to the irradiation of CoCrNi and CoCrFeNi MEAs and CoCrFeMnNi HEA were investigated in this study. The irradiation resistance of these alloys was compared to that of Ni.

The thin-film samples of CoCrNi and CoCrFeNi MEAs, CoCrFeMnNi HEA, and Ni were irradiated with up to 1.7 × 1019 ions/m2 of 2.4 MeV Cu ions at 673 and 873 K and the resultant microstructure and composition analyzed. At

CRediT authorship contribution statement

Q. X: Writing, Investigation, Supervision and Conceptualization, Writing - original draft, Writing - review & editing. H. G: Data curation. S. H: Data curation. Z. Z: Investigation, Data curation. H. W: Data curation. M. T: Data curation.

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.

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