Microstructure and Mechanical Properties of In Situ TiB/TiC Particle-Reinforced Ti-5Al-5Mo-5V-3Cr Composites Synthesized by Spark Plasma Sintering

In situ TiB and TiC particle-reinforced titanium matrix composites (TMCs) based on a near-β Ti-5Al-5Mo-5V-3Cr alloy (Ti-5553) reacting chemically with B₄C were processed by spark plasma sintering (SPS). The influence of powder milling parameters (low-energy mixing or high-energy milling) on the chemical reaction behavior between the matrix and the B₄C particles during sintering was investigated. Taking the microstructure into account, characterization of the particle strengthening effect was carried out under compressive loading conditions. High-energy milling resulted in a significantly higher degree of B₄C conversion during sintering. This was attributed to plastic deformation of the initial matrix powder and more homogeneous distribution of the B₄C particles accompanied by a significant reduction in cluster formation. In comparison to the unreinforced Ti-5553 matrix, the hardness, stiffness, and compressive strength of the TMCs were successfully increased due to particle reinforcement. The powder milling treatment improved these properties further—a phenomenon directly associated with the higher degree of B₄C conversion. Instead of the expected formation of stoichiometric TiC, the formation of nonstoichiometric TiC_1−x with x ≈ 0.5 was observed. Molybdenum, vanadium, and chromium formed a solid solution in TiB and TiC_1−x. Additionally, the titanium content in the matrix particles was markedly reduced, while the aluminum content roughly doubled.