Synthesis, chemical bonding, and mechanical properties of Ti–Nb–Hf ternary solid solution MAXs

MAX phase ceramics have garnered widespread attention owing to their numerous exceptional functionalities from their emerging new phases. M-site element regulation in MAXs stands as one of the core strategies for new phase exploration and performance optimization. However, challenges lie in the compositional design and kinetic regulation to bypass the multiple intermediate phases in acquiring new MAX phases with high purity for target properties. Herein, two new MAX phases with a Ti, Nb and Hf solid solution in the M-site, namely Ti_1−xNb_1−xHf_2xAlC (2x = 0.2 and 0.4), are synthesized using the spark plasma sintering method. Density functional theory (DFT) simulations are employed to study their electronic structures, bonding status of different M–C and M–A bonds, and the corresponding elastic properties. The calculations indicate that in the solid solution MAX phase, the bond strengths follow the order of Nb–C > Hf–C > Ti–C and Nb–Al > Hf–Al > Ti–Al. The corresponding bulk modulus, shear modulus, and Young's modulus are calculated to be 165.55 GPa, 122.76 GPa, and 295.28 GPa, respectively. The mechanical properties of the as-prepared samples are investigated on microscopic and macroscopic scales as well. Specifically, Ti_0.9Nb_0.9Hf_0.2AlC exhibits leading compressive strength of 1574.77 ± 31.15 MPa and fracture toughness of 7.14 ± 0.05 MPa m^1/2 among the reported MAX phases. This work highlights the superiority of M-site composition regulation in boosting the mechanical properties of MAX phase ceramics.