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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 Zr 2 AlC and Nb 2 AlC, and particularly their solid solutions (Zr 1- x Nb x) 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 Zr 2 AlC. The stiffness of (Zr 1- x Nb x) 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 (Zr 1- x Nb x) 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 (Zr 1. x Nb x) 2 AlC), melting point, electronic structures, and Vickers hardness were examined. The covalency increases in (Zr 1- x Nb x) 2 AlC as the Nb-content x increases, which may explain the increase in the stiffness of the compositions. Image 1 • (Zr 1- x Nb x) 2 AlC are thermodynamically stable. • Elastic moduli of (Zr 1- x Nb x) 2 AlC increase with Nb content x. • (Zr 0.6 Nb 0.4) 2 AlC composition predicted to be highly thermally conductive. • Covalency in (Zr 1- x Nb x) 2 AlC increases with Nb-content x. [ABSTRACT FROM AUTHOR]

A systematic density functional theory study with two functionals –generalized gradient approximation (GGA) and local density approximation (LDA)–is carried out to explore the structural, electronic, elastic, thermal, vibrational and optical properties of a new 211 MAX phase Nb 2 CuC. To facilitate comparison we also study Nb 2 AlC, the precursor of Nb 2 CuC. The calculated band structures reveal the metallic conductivity of both compounds. The replacement of Al with Cu modifies the band profiles of Nb 2 CuC and consequently leads to its improved physical properties. Considering the position of the Fermi level on the total density of states (DOS), the new compound Nb 2 CuC is structurally less stable than Nb 2 AlC. The total DOS at the Fermi level obtained with GGA is slightly larger than those obtained with LDA. The Nb–C and Nb–A (A = Cu/Al) are covalent bonds, and Nb–Nb bonds lead to antibonding states in both MAX phases. The charge transfer among constituent atoms indicates some ionic character in the chemical bonds of Nb 2 CuC and Nb 2 AlC. Both MAX phases are mechanically and dynamically stable. The Nb 2 CuC is ductile and consequently damage tolerant, whereas Nb 2 AlC is brittle. However, Nb 2 CuC is relatively soft and machinable. In most cases Nb 2 CuC is more elastically anisotropic than Nb 2 AlC, and Nb 2 CuC is expected to be a promising thermal barrier coating material. We propose that Nb 2 CuC is a better coating material for preventing solar heating than Nb 2 AlC, and Nb 2 CuC is expected to be superconductive because its Fermi surface has a nesting nature. Image 1 • Nb 2 CuC is ductile and consequently damage tolerant. • Nb 2 CuC is relatively soft, machinable and elastically more anisotropic than Nb 2 AlC. • Nb 2 CuC is expected to be a promising thermal barrier coating material. • Nb 2 CuC is expected to be a superconductor. [ABSTRACT FROM AUTHOR]