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

Highlights: • The L2₁ (NiCo)₂TiAl and Y-type (Ni_1-xCo_x)Ti(Ni_xCo_1-x)Al alloys are investigated. • The equimolar Ni-Ti-M-Al alloys are discussed based on the free energy. • The lattice site preference of Co has no effect on the intrinsically elastic properties. 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 (Ni_xM_1-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 (Ni_1-xCo_x)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 (Ni_1-yCo_y)Ti(Ni_yCo_1-y)Al (y = 0–0.5) solid-solution alloys, whereas the (Ni_1-yFe_y)Ti(Ni_yFe_1-y)Al (y = 0–0.5) solid solutions show different ductility and anisotropy, compared with NiTiFeAl intermetallic compound.