Characterization of titanium-6% aluminum-4% vanadium/titanium carbide particulate reinforced metal matrix composites consolidated by sintering and thermomechanical processing

TiC reinforcement particles were incorporated into a Ti-6 %Al-4%V matrix and processed by two powder metallurgy techniques, namely elevated temperature pressureless sintering and hot deformation-assisted sintering (also known as hot pressing). For these composites, processing by sintering alone necessitated high temperatures (>1500°C) for near-complete density consolidation, whilst the conditions for temperature and hold time were reduced (i.e. 1000°C and 1/2 hour) through deformation-assisted sintering. During high temperature processing in the absence of deformation, considerable coarsening of the lamellar matrix microstructure occurred. The interfacial reaction between the reinforcement and matrix was characterized by in situ neutron diffraction sintering studies at temperatures between 1100°C and 1350°C. Initial reaction occurred by carbon diffusion from the TiC particle to the titanium alloy, as evidenced through the increase in the lattice parameter of the matrix phase with holding time at the various sintering temperatures. Beyond the carbon solubility limit of the matrix phase, a stable stoichiometric phase formed as shown by the appearance of distinct peaks in the neutron diffraction patterns. Room temperature lattice parameter measurement gave a value of 4.290 Å with a fractional occupancy of carbon of 0.45 ± 0.04, which corresponds to a stoichiometry of Ti₂C. For the various isothermal sintering temperatures, change in the Ti₂C volume fraction with hold time was determined and growth of this interfacial phase was reasoned to occur by carbon diffusion from the TiC particles, through the reaction zone and to the Ti-6%Al-4%V alloy. Transformation of the entire TiC particle to Ti₂C occurred in the composites sintered at 1500°C. For the composites processed by sintering only, the mechanical properties determined by shear punch testing indicated that the strength and ductility are limited at low temperature sintering because of high porosity, and at high temperature sintering because of the interfacial phase. To improve the properties, the composites need to be processed to high densities at low temperatures using deformation-assisted sintering. The interfacial layer thickness in the deformed composites processed to near-complete densities at temperatures between 1000°C and 1200°C was less than 0.3 μm. Moreover microstructural modification of the initial lamellar structure was possible and a refined bimodal microstructure (5 μm alpha plate size) was obtained in the composites deformed just below the beta transus temperature (∼1017°C). A combination of low porosity, small interfacial reaction thickness, and microstructural refinement increased the strength and ductility close to that typical of these composites. Subsequent heat treatment of these composites at 1100°C rapidly increased the interfacial reaction layer and degraded the mechanical properties. Hence further processing to increase the density should involve low temperature mechanical working techniques to minimize additional interaction of the matrix and reinforcement and maximize the properties of the composite.