(Nb_x, Zr_1-x)₄AlC₃ MAX Phase Solid Solutions: Processing, Mechanical Properties, and Density Functional Theory Calculations

The solubility of zirconium (Zr) in the Nb₄AlC₃ host lattice was investigated by combining the experimental synthesis of (Nb_x, Zr_1-x)₄AlC₃ solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH_0.89, ZrH₂, Al, and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum of 18.5% of the Nb content in the host lattice) in Nb₄AlC₃ observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb_0.85, Zr_0.15)₄AlC₃ was mapped using atom probe tomography. The hardness, Young's modulus, and fracture toughness at room temperature as well as the high-temperature flexural strength and E-modulus of (Nb_0.85, Zr_0.15)₄AlC₃ were investigated and compared to those of pure Nb₄AlC₃. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 ± 0.1 MPa/m^1/2 for pure Nb₄AlC₃ to 10.1 ± 0.3 MPa/m^1/2 for the (Nb_0.85, Zr_0.15)₄AlC₃ solid solution.