Interstitials in f.c.c. High Entropy Alloys

Ian Baker

doi:10.3390/met10050695

Abstract

The effects of interstitials on the mechanical properties of single-phase f.c.c. high entropy alloys (HEAs) have been assessed based on a review of the literature. It is found that in nearly all studies, carbon increases the yield strength, in some cases by more than in traditional alloys. This suggests that carbon can be an excellent way to strengthen HEAs. This strength increase is related to the lattice expansion from the carbon. The effects on other mechanical behavior is mixed. Most studies show a slight reduction in ductility due to carbon, but a few show increases in ductility accompanying the yield strength increase. Similarly, some studies show little or modest increases in work-hardening rate (WHR) due to carbon, whereas a few show a substantial increase. These latter effects are due to changes in deformation mode. For both undoped and carbon doped CoCrFeMnNi, the room temperature ductility decreases slightly with decreasing grain size until ~2–5 µm, below which the ductility appears to decrease rapidly. The room temperature WHR also appears to decrease with decreasing grain size in both undoped and carbon-doped CoCrFeMnNi and in nitrogen-doped medium entropy alloy NiCoCr, and, at least for the undoped HEA, shows a sharp decrease at grain sizes <2 µm. Interestingly, carbon has been shown to almost double the Hall–Petch strengthening in CoCrFeMnNi, suggesting the segregation of carbon to the grain boundaries. There have been few studies on the effects of other interstitials such as boron, nitrogen and hydrogen. It is clear that more research is needed on interstitials both to understand their effects on mechanical properties and to optimize their use.

Highlights

The idea of multi-component or multi-element alloys with equal atomic proportions that could be single phase was separately introduced by Cantor et al [1] and Yeh et al [2]
The room temperature work-hardening rate (WHR) appears to decrease with decreasing grain size in both undoped and carbon-doped CoCrFeMnNi and in nitrogen-doped medium entropy alloy NiCoCr, and, at least for the undoped High Entropy Alloys (HEAs), shows a sharp decrease at grain sizes
It is clear that more research is needed on interstitials both to understand their effects on mechanical properties and to optimize their use

Summary

Introduction

The idea of multi-component or multi-element alloys with equal atomic proportions that could be single phase was separately introduced by Cantor et al [1] and Yeh et al [2]. Yeh et al [2] predicted that while single phase at high temperature, T, where the entropy term, T∆Sconf , is more important, HEAs could precipitate second phases or undergo spinodal decomposition at low temperature, where the configurational entropy term is of less importance. Such behavior has been noted in some f.c.c. HEAs such as the so-called Cantor alloy, Fe20 Cr20 Mn20 Ni20 Co20 [3] and

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