Microstructure and Mechanical Properties of In Situ TiB/TiC Particle-Reinforced Ti-5Al-5Mo-5V-3Cr Composites Synthesized by Spark Plasma Sintering
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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 TiC1−x with x ≈ 0.5 was observed. Molybdenum, vanadium, and chromium formed a solid solution in TiB and TiC1−x. Additionally, the titanium content in the matrix particles was markedly reduced, while the aluminum content roughly doubled.
The authors would like to thank the Bundeswehr Research Institute for Materials, Fuels and Lubricants (WIWeB) for its financial support of this project. Special thanks are due to E. Jentsch, Dr. D. Heger, Dr. S. Decker, and G. Bittner for conducting the compression tests, the EPMA and the SEM investigations as well as the measurement of micro-hardness and indentation moduli.
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