First-principles investigation on the chemical bonding, elastic properties and ideal strengths of MoAlB and WAlB nanolaminated MAB phases

The present work deals with first-principles calculations on chemical bonding characteristics, elastic properties, and ideal strengths under both shear and tensile deformations of MoAlB and WAlB nanolaminated MAB phases. Diverse chemical bonds were found in MoAlB and WAlB, including strong BB covalent bonds, AlB covalent-ionic bonds, MoB bonds and metallic MoAl and AlAl bonds. Elastic properties of MoAlB and WAlB are not highly anisotropic with shear modulus of (0 1 0)[1 0 0], (0 1 0)[0 0 1], (0 0 1)[1 0 0] similar to each other. Nevertheless, ideal strengths display significant anisotropy. The results show that ideal shear strength τm of basal plane shears are lower than those of non-basal plane shears, while ideal tensile strength σm of the basal plane is lower than those of non-basal planes. In addition, ideal tensile strength of the basal plane is also much lower than ideal shear strengths of basal plane shears. The basal plane cleavage and shear failure both take place along the weakly bonded AlAl layers. Moreover, the basal plane shear failure is found accompanied by remarkable expansion of the AlAl layers, which reduces the resistance against basal plane shears. It suggests that MoAlB and WAlB tend to cleavage along the AlAl layer or dislocations tend to multiply and slip in the AlAl layer, which is similar to MAX phases.