Hydrogen as a temporary alloying element in titanium alloys: thermohydrogen processing

Thermohydrogen processing is a technique in which hydrogen is used as a temporary alloying element in titanium alloys to control the microstructure and improve the final mechanical properties. Thermohydrogen processing can also be used to enhance the processability/fabricability of titanium products including sintering, compaction, machining, and hot working (forging, rolling, superplastic forming, etc.). In the case of near net shapes, such as castings and powder metallurgy products, thermohydrogen processing is the only method available for significant microstructural modifications and consequent enhancement in mechanical properties. This paper reviews the status of the methods and applications of thermohydrogen processing to titanium alloys. Principles of thermohydrogen processing, based on the hydrogen induced alterations of the phase compositions and the kinetics of phase reactions in hydrogenated titanium alloys, are overviewed. Stable and metastable phase diagrams of several titanium alloys containing hydrogen are also reviewed. Different applications of thermohydrogen processing for titanium alloys, such as processing, forming, microstructural modifications and improvement in mechanical properties are also discussed.