Thermohydrogen processing of titanium alloys

Use of hydrogen as a temporary alloying element in titanium alloys is an at tractive 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 sta tus of the methods and applications of thermohydrogen processing (THP) to t itanium alloys is reviewed. Effects of hydrogen alloying on the phases pres ent, their composition, and the kinetics of phase reactions are considered. The effect of hydrogen on the hot workability, composite- and powder-metal lurgy-product processing, and microstructure modification of wrought and ca st conventional alloys and intermetallics, including production of nanocrys talline structures is discussed. Two recently discovered effects, i.e. hydr ogen-induced softening of alpha titanium and hydrogen-induced hardening of beta titanium are also discussed. Thermohydrogen processing has clear advan tages in the development of improved microstructures and mechanical propert ies. In the case of near net shapes it is the only method for significant m icrostructural modification. It allows energy savings in processing to fina l products by improving the workability. (C) 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserve d.