The effect of temporary hydrogenation on the processing and interface of titanium composites

Abstract

Hydrogen as a temporary alloying element in titanium increases its workability at elevated temperatures. It can also be easily extracted from titanium under vacuum conditions. Therefore solid-state consolidation of Ti-based composites may make use of this reversible hydrogenation behaviour of titanium to either reduce the consolidation temperature or shorten the consolidation time at a given temperature, so as to avoid harmful interface reactions. This article reports findings in processing Ti-6Al-4V and Ti-1100 based composites using hydrogen-charged foils and by in situ hydrogenation. Hydrogen-aided processing was found to substantially increase the rate of consolidation at a given temperature, particularly when using a Ti-6Al-4V matrix. This enabled full consolidation at a relatively low temperature without any reactions at the interfaces. In particular, the long TiB needles that have decorated the interfaces in conventionally processed samples were not formed in the hydrogen-treated samples. Both reduced thermal activation at the low temperature and solute hydrogen in the matrix are believed to inhibit boron diffusion through the matrix and so the formation of the borides. Clean and intimate fibre/matrix interface bonding was observed in the hydrogen-treated samples, while conventionally processed samples showed less well bonded interfaces. Owing to the limited period of dehydrogenation after consolidation, hydrogen residuals were not thoroughly removed. Hydrides were found in a boundary β-phase. Further evacuation is required to drive off the excess hydrogen in the processed samples.