Stability of L21 (NiM)2TiAl (M=Co, Fe) in high-entropy alloys

The L21 (NiCo)2TiAl phase plays a key role in the excellent mechanics of 3d high-entropy alloys. In this work, we investigate the phase stability and intrinsic mechanical properties of L21 (NixM1-x)2TiAl (M = Co, Fe, x = 0-1) alloys by using ab initio methods. The phonon dispersion relations suggest that the thermodynamic stability of L21 Ni2TiAl, Co2TiAl and Fe2TiAl as well as Y-type NiTiCoAl, NiTiFeAl and CoTiFeAl intermetallic compounds. The formation enthalpy and the energy contribution from vibrational entropy indicated that (NiCo)2TiAl is easier to form with respect to (NiFe)2TiAl and (CoFe)2TiAl. With the increase of the Co content, both the ductility and Young's modulus of L21 (Ni1-xCox)2TiAl (x = 0-1) alloy decrease, while anisotropy become strong. The Co content has no almost effect on the ductility, Young's modulus and anisotropy of the Y-type (Ni1-yCoy)Ti(NiyCo1-y)Al (y = 0-0.5) solid-solution alloys, whereas the (Ni1-yFey)Ti(NiyFe1-y)Al (y = 0-0.5) solid solutions show different ductility and anisotropy, compared with NiTiFeAl intermetallic compound.