Soft MAX phases with boron substitution: A computational prediction

With a goal to improve upon the mechanical properties of the MAX phase, materials of high technological interest, we explore boron substitution in these compounds. Employing first-principles density functional theory (DFT) calculations, combined with continuum modeling to access the core structure of dislocations, we investigate the effect of boron-substitution on plastic deformation properties of a typical MAX phase compound, ${\mathrm{V}}_{2}\mathrm{AlC}$. Our $T=0$ K results show that, due to the differential nature of chemical bonding between V and B compared to that between V and C, both V-Al and V-B basal slip planes get activated in boron-substituted compounds, compared to only V-Al basal slip in the parent compound. This, in turn, makes the boron compounds significantly more ductile compared to their carbide counterpart ${\mathrm{V}}_{2}\mathrm{AlC}$. The computation of temperature-dependent free energies of stable and unstable stacking faults further reveals the important and interesting role of thermal fluctuations in the deformation behavior of the boride compounds at elevated temperature. This suggests a change in the microscopic mechanism of plastic deformation upon varying temperature condition in the proposed boron compounds. Our study should motivate future exploration of boron-based MAX compounds.