Phase stability and physical properties of (Zr1-xNbx)2AlC MAX phases

In this study, density functional theory (DFT) calculations were performed to investigate the key structural, elastic, mechanical, thermal, and electronic properties of the MAX phases of Zr2AlC and Nb2AlC, and particularly their solid solutions (Zr1-xNbx)(2)AlC, which are found to be thermodynamically stable. The partial inclusion of Nb in the M-site improves the bond strength and the hardness of Zr2AlC. The stiffness of (Zr1-xNbx)(2)AlC increases with the Nb-content x, and thus improves its ability to resist the shear deformation. With large negative Cauchy pressure, all of the compositions of (Zr1-xNbx)(2)AlC are predicted to exhibit directional covalent bonding. The composition with x = 0.2 is expected to be more brittle than the other compositions. The other properties including the anisotropy in elasticity, average sound velocity, Debye temperature (predicted to be highest for the composition with x = 0.4 in (Zr1-xNbx)(2)AlC, melting point, electronic structures, and Vickers hardness were examined. The covalency increases in (Zr1-xNbx)(2)AlC as the Nb-content x increases, which may explain the increase in the stiffness of the compositions.