Thermohydrogen processing of titanium alloys

Use of hydrogen as a temporary alloying element in titanium alloys is an attractive approach for enhancing processability including working, machining, sintering, compaction, etc., and also for controlling the microstructure and thereby improving final mechanical properties. In this article, the status of the methods and applications of thermohydrogen processing (THP) to titanium alloys is reviewed. Effects of hydrogen alloying on the phases present, their composition, and the kinetics of phase reactions are considered. The effect of hydrogen on the hot workability, composite- and powder-metallurgy-product processing, and microstructure modification of wrought and cast conventional alloys and intermetallics, including production of nanocrystalline structures is discussed. Two recently discovered effects, i.e. hydrogen-induced softening of alpha titanium and hydrogen-induced hardening of beta titanium are also discussed. Thermohydrogen processing has clear advantages in the development of improved microstructures and mechanical properties. In the case of near net shapes it is the only method for significant microstructural modification. It allows energy savings in processing to final products by improving the workability. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.