Interstitial alloying and the stacking fault energies of CrMnFeCoNi high entropy alloys

Interstitial alloying can improve the mechanical properties of high-entropy alloys (HEAs). In some cases, the interstitial-alloying impact is very different from those in conventional alloys. We investigate the effect of interstitial alloying in the CrMnFeCoNi HEA, particularly focusing on the solution energies and the impact on the stacking fault energy, based on first-principles calculations. Our results clarity, e.g., that the interstitial solution energy in HEAs is no longer a single value but shows a substantial distribution due to the dependence on local chemical environments.

Schematic of the interstitial alloying in HEAs. Gray-scaled circles indicate the interstitial sites, and thicker-colored ones indicate that they are more likely occupied.

Interstitial alloying is one of the typical approaches to improve the mechanical properties of high entropy alloys (HEAs) as well as traditional alloys. In experiments, carbon increases the ultimate strength of CrMnFeCoNi-based HEAs while keeping good ductility [1]. More interestingly, hydrogen is found to improve both the strength and the ductility of CrMnFeCoNi [2], unlike the traditional alloys that show hydrogen embrittlement. While these observations motivate us to design the mechanical properties of HEAs by interstitial alloying, from the atomistic viewpoints, little is so far known for the impact of interstitial alloying in HEAs. Particularly, in HEAs, each interstitial site has distinct local chemical environment, which is however difficult to access only by experiments.

For face-centered cubic (fcc) alloys, their ductility as well as their deformation behavior is closely connected with their stacking fault energies (SFEs), as well known for e.g. high Mn steels. For the design of mechanical properties of HEAs by interstitial solid solutions, it is therefore essential to elucidate its impact on SFEs.

We have studied the impact of interstitial alloying in CrMnFeCoNi, a prototypical HEA based on 3d transition elements, based on first-principles calculations using supercell models. Particularly, to reveal the dependence of local chemical environment, we compute the solution energies of thousands of interstitial sites. For carbon solutions, we actually found that the solution energy depends on the local chemical environment. This means that, in HEAs, the solution energy is no longer a single value but actually shows a distribution. Considering the average solution energies, carbon atoms are found to stabilize the fcc phase more than the hcp phase, indicating that the SFE becomes higher by the carbon solution.

References

[1] Z. Li, C. C. Tasan, H. Springer, B. Gault, and D. Raabe, Sci. Rep. 7, 40704 (2017).

[2] H. Luo, Z. Li, and D. Raabe, Sci. Rep. 7, 9892 (2017).

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