Phase stability and weak metallic bonding within ternary‐layered borides CrAlB, Cr2AlB2, Cr3AlB4, and Cr4AlB6

Abstract

AbstractTo explore the potential for use of the Cr–Al–B borides, Cr2AlB2, Cr3AlB4, and Cr4AlB6 as well as hypothetical CrAlB are investigated using density functional theory. In the CrAl(CrB2)n series strong covalent bonding is present between the B and Cr atoms with, significantly, much weaker metallic Cr–Al and B–Al bonds, suggesting similar unusual properties to the MAX phases. The relative stiffness of the weakest and strongest bonds hint at similar unusual properties to the MAX phases with superior damage tolerance expected for hypothetical CrAlB, as evidenced in the lowest Al1–Al2 bond stiffness. The layered nature and metallic bonding are expected to result in high fracture toughness and damage tolerance. Anisotropic compression is demonstrated, with the stiffest axes along the direction of the B–B zigzag‐/hexagonal‐chains and the softest axes determined by an interplay between the soft metallic interlayers and the rigid covalent bonds. In general the elastic moduli in CrAl(CrB2)n increase as a function of n, however, without the price of an increase in density.