Effects of sintering parameters on the hardness and microstructures of bulk bimodal titanium

Spark plasma sintering was used to condense commercially pure titanium powder into bulk samples. The effects of sintering temperature, holding time, pressure and heating rate on the density, chemical constitution, microstructure and hardness were systematically investigated. The results indicate that (1) applying high pressure and increasing the sintering temperature accelerate the densification process significantly, while the heating rate and holding time show less important effects. (2) The as-sintered samples have bimodal grain size distributions (consisting of both micro-sized grains and nanocrystalline grains). (3) The as-sintered samples exhibit extremely high hardness compared to as-cast pure titanium and nanocrystalline/ultrafine-grained pure titanium fabricated by other methods. (4) A self-cleaning effect exists during the sintering process, resulted from oxide decomposition.