| dc.description.abstract | MAX phases are a rapidly expanding family of atomically-layered carbides and nitrides known for their characteristically high temperature stability, excellent thermal shock resistance, good mechanical properties, and easy machinability. Approximately 70 pure MAX phases of different compositions and fewer than 100 solid solution alloys on M, A, or X sublattices have been successfully synthesized and characterized thus far. MAX phase solid solutions have gained particular interest recently as alloying of MAX phases facilitates tailoring of properties to particular applications and behaviors, yet the number of synthesized solid solutions is alarmingly small.
This work reports the first successful dedicated syntheses of new solid solutions of Ti₂AlC with bismuth and antimony, showing that maximum solubility of Bi and Sb on A sites are approximately 40at% and 70at%, respectively, and that substitution of Al with larger elements such as Bi and Sb leads to anisotropic stretching of the crystal cells. In this study, bulk, dense, predominantly phase-pure samples Ti₂(Al₁₋ₓBiₓ)C with x ranging from 0 to 0.4 were synthesized under argon at 1400˚C via Pulsed Electric Current Sintering from elemental powders. Maximum solubility of Bi in Ti₂AlC was found to be around 40% (x = 0.4), since all attempts to synthesize Ti₂(Al₁₋ₓBiₓ)C with x > 0.4 resulted in the phase decomposition of Ti₂(Al₁₋ₓBiₓ)C. Substitution results in the formation of vacancies on Al-site and anisotropic stretching of the a lattice parameter from 3.06Å for x = 0 to 3.11Å for x = 0.4 with little effect on the c lattice parameter, as corroborated by X-Ray Diffraction and High-Resolution Scanning Transmission Electron Microscopy. Increase in bismuth concentration results in an hardening of bulk Ti₂(Al₁₋ₓBiₓ)C phase without altering the elastic moduli.
Aluminum is also substituted with Sb in Ti₂AlC MAX up 70at%, observed in bulk, dense, phase-pure samples synthesized via Pulsed Electric Current Sintering. Incorporation of antimony results in anisotropic stretching of the a lattice parameter from 3.06Å to 3.13Å with corresponding shrinking of the c lattice parameter from 13.67Å to 13.53Å. In contrast with other large A element substitution (i.e. bismuth), antimony exhibits large thermodynamic preference toward formation of Ti₂Sb, resulting in more Ti₃(Al₁₋ₓSbₓ)C₂ formed. | en |
