Phase stability and mechanical properties of M4AlB4 (m=Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr) from first principles

Abstract: The recent discovery of Cr4AlB4, a laminated ternary metal boride belonging to the family of layered MAB-phases, where the transition metal boride layers are interleaved by an A layer, has spurred theoretical investigation for novel M4AlB4 phases. In this study, first-principles calculations were applied in order to investigate the thermodynamical stability and mechanical properties of M4AB4 where M = Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr while the A layer was kept fixed as Al. The thermodynamical stability calculations validate the recently discovered Cr4AlB4 phase’s stability and suggest the neighboring phase Mo4AlB4 to be stable. Additionally, the phases Mo3AlB4, Mo2AlB2, Ti4AlB4 and Ti2AlB2 indicates phases close to stable with a formation enthalpy within the range of 0 < ∆H < 25 meV per atom compared to competing phases. Hence dynamical stability investigations were carried out, which indicates Mo4AlB4 to be dynamically stable. The stability of Mo4AlB4 does encourage synthesizing attempts to be kept in mind as a future project. Phase stability trends of the 111, 212, 314 and 414 compositions were discovered, where a 212, 314 and 414 composition is seen to be more stable for an M-element with lower electron configuration. Furthermore, the mechanical properties of the 414 compositions were investigated by systematically straining the unit cell in different directions. The bulk-, shear- and Young’s-modulus were derived and are presented, where Ti4AlB4 demonstrates values similar to the commended Ti2AlC MAX-phase. Finally, ductility plots are presented which purposes a linear trend between the elements of group IV, V and VI. Based on the results, further studies with a focus on the temperature and magnetization’s impact on the stability and mechanical properties are suggested.