Microstructural evolution during transient plastic phase processing of titanium carbide-titanium boride composites

Transient plastic phase processing is a form of reactive hot pressing for fabricating fully dense ceramic-ceramic composites at relative low homologous temperatures. In this study, this technique has been used on two powder mixtures-4:1 Ti/B4C and 1:1 TiC0.5/TiB2, which are equivalent in terms of elemental compositions-to produce fully dense titanium carbide-titanium boride composites. The composites formed in each case are comprised of the same final phases-TiCx, TiB2, and Ti3B4, in roughly the same volume fractions-but exhibit distinctly different grain morphologies. Ti3B4 phase nucleates and grows as platelets for the 4:1 Ti/B4C starting composition but as equiaxed grains for the 1:1 TiC0.5/TiB2 composition. TiB has been identified as an intermediate phase in the ''platelet'' composition and appears to be important to the development of the Ti3B4 platelets. X-ray diffractometry and scanning electron microscopy results indicate that the evolution of the microstructure is governed by the diffusion of boron and carbon, rather than titanium. In addition, the faster diffusion of carbon, relative to boron, is instrumental in the microstructural evolution of the platelet composite. The produced composites possess >99% density and good mechanical properties. The higher strength and toughness of the platelet composite are believed to be due to the platelet morphology of the Ti3B4 phase.