Solid-state reaction mediated microstructural evolution in a spark plasma sintered in situ Ti–B4C composite

Abstract

A novel porosity-free in situ Ti–B4C composite was fabricated via spark plasma sintering (SPS). Detailed analysis of the phase evolution, coupling results from XRM, XRD and SEM-EBSD-EDS, indicate that primarily TiB2 precipitates formed due to the in-situ reactions between the boron-carbide and titanium powders. The precipitation of TiB2 resulted in the formation of a graphitic C-rich thin layer circumscribing the partially reacted B4C particles. Further progression of the in-situ reaction leads to the out-diffusion of the excess carbon from the B4C particles, across the graphite and TiB2 phases, forming TiC upon reacting with the titanium matrix. Therefore, the final microstructure primarily consisted of TiB2, TiC, and partially reacted B4C phases, with small amounts of TiB, α-Ti, and graphite. Furthermore, the microstructure in these SPS processed in situ composites appeared to be the product of a solid-state Ti–B4C diffusion couple, substantially different from their fusion-based additively manufactured counterparts. Nano-indentation tests revealed a remarkably high average hardness of ∼25 GPa for this SPS-processed Ti–B4C composite and comparable (with literature) phase-specific hardness and modulus values for the constituent TiB2, TiC, and B4C phases.