Dehydrogenation and consolidation of titanium hydride powder using powder metallurgy routes

Abstract

TiH2 powder has been trialled as an alternative to Ti-powder to fabricate titanium-based products. To enhance the utilization of TiH2 powder, further understanding of the dehydrogenation mechanism of TiH2, the effect of hydrogen on dehydrogenated and equal channel angular pressed (ECAP) samples and on the fabrication of titanium matrix composites (TMC) is critical.

This research work analysed the phase transformation steps of TiH2 to Ti using high-temperature X-Ray Diffraction with the dehydrogenation step occurring under both isothermal and non-isothermal conditions. Crystallographic data was obtained through Rietveld analysis. Results showed that, with increasing temperature expansion occurred, which was negated by the phase transformations due to dehydrogenation. The sequence of TiH2 phase transformations were: delta−>delta’−>beta−>beta’−>alpha−>alpha’-phase. Further confirmation of the mechanistic steps was obtained through thermogravimetric analysis, transmission electron microscopy and selected area electron diffraction studies. The dehydrogenation reaction of compacted TiH2 powder was investigated in terms of four parameters, namely, heating rates, compaction pressures, temperatures and times, in order to optimize the dehydrogenation process. The hydrogen loss and bulk density decreased with increasing heating rate while the density and hardness improved with the increasing in temperature and time.

TMCs were fabricated using TiH2 powder containing SiC or TiB2 as the reinforcement. In samples with SiC, when the sintering temperature was increased, the density, hardness and the reaction layer (TixSiyCz layer) were found to increase. However, the density and hardness decreased when TiB2 powder content increased. Hardness values were affected by the H2 content in the matrix and the reinforcement.

Back-pressure ECAP on sintered TiH2 samples was carried out at 590°C using route C, where the sample is rotated 180° with focus on the hydrogen effect. Density of the ECAP samples reduced when the hydrogen content was increased. The presence of hydrogen improved the hardness and tensile strength, but the ductility was lowered. Dehydrogenation is very crucial step for TiH2 powder because of the phase transition from delta−>beta−>alpha-titanium, which is a critical step in the fabrication of TMC and titanium alloys from TiH2 powder.