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