Site preference and elastic properties of L21-Ni2TiAl doped with refractory metal elements from first principles

Abstract

Refractory high entropy alloys (RHEAs) are attracting wide attention due to their excellent high-temperature strength. L21-Ni2TiAl was found as a strengthening phase in high entropy alloys (HEAs) which significantly improves room and elevated-temperature mechanical properties. To explore the potential application in RHEAs, we address the calculations of 7 refractory elements (i.e., V, Cr, Zr, Nb, Mo, Hf, and Ta) doping from first principles on the site preference, elastic properties and bonding effect of Ni2TiAl. The results show that the antisite defects is more stable than the vacancy defects. Elements Cr and Mo prefer the Ni-sites, while V, Zr, Nb, Hf, and Ta prefer the Ti-sites. Moreover, all the doping elements lead to a lattice shrinkage, among which V cause the largest distortion. For the normalized shear modulus (G/G0), all the elements increase except Nb, of which Hf rank the top. While for the normalized bulk modulus (B/B0), all the elements slightly increase except Hf. Furthermore, Nb is the only one to improve the toughness but has little impact on hardness (Hv), then Hf improves hardness the most. Additionally, all the doping elements decrease the anisotropy of Young's modulus for Ni2TiAl, of which Nb has the largest anisotropy, leading to a weak resistance to deformation. Notably, the density of states (DOS) shows that V and Cr can effectively improve the bonding stability and strength of Ni2TiAl. Overall, the alloying of Nb, V, and Cr may synergistically improve the stability and toughness of Ni2TiAl, which is expected to develop novel RHEAs.