Titanium matrix composites reinforced with titanium carbide particulates were prepared by a newly developed solid-gas in-situ reaction process. The process consists of mixing titanium and master alloy powders without any reinforcing particles, cold pressing of compacts and reacting the compacts with a carbonaceous atmosphere, methane($CH_4$) in this study, at elevated temperatures to form carbides inside the compacts. The reacted compacts were then sintered in a vacuum and hot-isostatically pressed to obtain fully densified composites.
In chapter 3, process parameters and kinetics of titanium carbide formation in a titanium matrix were investigated. When a titanium powder compact was heated in an atmosphere containing methane, titanium carbides formed at or above 700℃ as a thin layer around the surface of individual titanium particle in the compact. The main process parameters affecting the rate of titanium carbide formation were identified to be the starting powder size, reaction temperature and time. The volume fraction of titanium carbide formed inside the compact increased with increasing the reaction temperature and time, where the temperature had more pronounced effect than the time. More titanium carbides formed with finer starting powder under the same process conditions since more surface area was available for the reaction with the finer powder.
It can be postulated that a titanium carbide layer was initially formed by the reaction between the titanium powder and carbonaceous product decomposed from methane. Once formed, however, the layer became a barrier for the direct contact between titanium and gas. Further growth of the layer was thought to occur by the diffusion of carbon through the already formed carbide layer. The growth rate of the titanium carbide layer or the volume fraction of titanium carbide in the reacted compact was able to be estimated by using Jander's diffusion model.
The titanium carbide layer at the surface of titanium particles broke up into fragments during the following vacuum sintering and rounded into blocky particles with increasing the sintering temperature and time. A minor change in chemistry was also observed after the sintering. The as-reacted titanium carbide was slightly deficient in carbon, but became stoichiometric after the sintering. The chlorine content in the composite was also reduced from over 400ppm in the original powder to less than 50ppm, which is very beneficial for mechanical properties.
Chapter 4 covers the process and resulting mechanical properties of Ti-6Al-4V/TiC composites. Elemental sponge titanium powder and 6Al-4V master alloy powder were mixed and cold compacted in a rectangular die, and reacted with methane. The reacted compacts were then sintered in vacuum and hipped to achieve full densification. Similarly to the Ti/TiC system, titanium carbide layer was formed on the surface of titanium powder during the reaction treatment by a solid-gas reaction. In addition, aluminum and vanadium carbides were found to form around 6Al-4V alloy powder during the reaction. These carbides, however, disappeared after the sintering, presumably due to the reaction with titanium to form thermodynamically more stable titanium carbide at higher temperature. The sintering densified the reacted compacts up to 95% theoretical density or above but full densification could not be obtained by the sintering alone.
After being fully densified by hipping, the Ti-6Al-4V/TiC composites exhibited a higher hardness and elastic modulus than a commercial wrought Ti-6Al-4V product. These properties increased with increasing titanium carbide content. The yield and ultimate tensile strengths were over 25% higher, and the wear resistance was far superior than those of the wrought Ti-6Al-4V alloy. The strengths were even higher than those of Ti-6Al-4V/TiC composites fabricated by other processes. The elongation, however, was reduced to 2% at room temperature with the reinforcement, but increased with increasing test temperature.
Investigation of the room temperature fractured surface revealed that cracks mainly initiated at the interface between titanium carbide and matrix or at the titanium carbide particle boundaries, and propagated through the particles. Therefore, morphological control including the size and distribution of the carbide particles is thought to be critical to improve the ductility at the room temperature.
