Interstitially carbon-alloyed refractory high-entropy alloys with a body-centered cubic structure

The introduction of carbon interstitials into high-entropy alloys (HEAs) provides an effective way to improve their properties. However, all such HEA systems explored so far are limited to those with the face-centered-cubic (fcc) structure. Here we report the structural, mechanical and physical properties of the refractory (Nb0.375Ta0.25Mo0.125W0.125Re0.125)(100-x)C-x HEAs over a wide x range of 0 <= x <= 20. It is found that, whereas the starting HEA (x = 0) is composed of a major body-centered-cubic (bcc) phase with significant impurities, the bcc phase fraction increases with the C concentration and achieves almost 100% at x = 20. Moreover, the increase of C content x results in an expansion of the bcc lattice, an enhancement of the microhardness, an increase in residual resistivity and a small variation of density of states at the Fermi level. All these features are consistent with the expectation that carbon atoms occupy the interstitial site. For x >= 11.1, the X-ray photoelectron spectroscopy indicates the bond formation between the carbon and metal atoms, suggesting that some carbon atoms may also reside in the lattice site. In addition, a semiquantitative analysis shows that the enhanced mixing entropy caused by carbon addition plays a key role in stabilizing the (nearly) single solid-solution phase. Our study not only provides the first series of carbon interstitial HEAs with a bcc structure, but also helps to better understand the alloying behavior of carbon in refractory HEAs.