Recently synthesized (Zr1-xTix)2AlC (0 ≤ x ≤ 1) solid solutions: Theoretical study of the effects of M mixing on physical properties

The effects of M atomic species mixing on the structural, elastic, electronic, and thermodynamic properties of newly synthesized MAX phase (Zr1-xTix)2AlC (0 ≤ x ≤ 1) solid solutions have been studied for the first time by means of density functional theory (DFT) based first principles calculations. The lattice constants are in good accord with the experimental results and found to decrease with Ti contents. The elastic constants, Cij, and the other polycrystalline elastic moduli have been calculated. The elastic constants satisfy the mechanical stability conditions of these solid solutions. The constants, C11, C33 and C44, are found to increase with Ti contents up to x = 0.67, thereafter they decrease slightly. A reverse trend is roughly followed by C12 and C13. The elastic moduli are also found to increase up to x = 0.67, beyond which these moduli go down slightly. Pugh's ratio and Poisson's ratio both confirm the brittleness of (Zr1-xTix)2AlC. Different anisotropy factors revealed the mechanically anisotropic character of these solid solutions. A non-vanishing value of the electronic energy density of states (EDOS) at the Fermi level suggests that (Zr1-xTix)2AlC are metallic in nature. A mixture of covalent, ionic and metallic bonding has been found from the electronic structure with dominant covalent bonding due to hybridization of Zr-4d states and C-2p states. This is also supported by the charge density distribution. The variation of elastic stiffness and elastic parameters with x is seen to be correlated with the partial DOS (PDOS) and charge density distribution. The calculated Debye temperature and minimum thermal conductivity are found to increase with Ti contents, while melting temperature is the highest for x = 0.67. The solid solution with x = 0.67 shows improved mechanical and thermal properties compared to that of the two end members Zr2AlC and Ti2AlC.