ULTRA-FINE EQUIAXED GRAIN-REFINEMENT AND IMPROVEMENT OF MECHANICAL-PROPERTIES OF ALPHA+BETA TYPE TITANIUM-ALLOYS BY HYDROGENATION, HOT-WORKING, HEAT-TREATMENT AND DEHYDROGENATION

Titanium alloys absorb large amounts of hydrogen and it is desorbed in vacuum at elevated temperatures. This paper describes equiaxed grain refinement and the improvement of mechanical properties of titanium alloys utilizing this phenomenon. Particular attention has been paid to the control of three different types of dislocation structures introduced by the following treatments. (1) Precipitation of hydrides: As hydrogenated titanium alloys undergo aging, high-density dislocations are introduced in the interior of hydrides as well as in the surrounding region. (2) Formation of cell walls by hot working: The beta transus temperature is lowered by hydrogenation so that the hot-working temperature can be lowered enough to retain the dislocation cell structure formed during the deformation. (3) Martensitic transformation: On cooling from the beta-phase region of hydrogenated titanium alloys, uniformly distributed high-density dislocations are introduced in acicular martensites. A combination of these factors such as (3) + (1) or (3) + (2) + (1) results in the grain refinement of alpha + beta type titanium alloys containing 0.2 approximately 1.0 mass% hydrogen. In a Ti-6Al-4V alloy with 0.2 mass% hydrogen, ultra-fine grain sizes of 1 mum or less, have been obtained. It presents a high tensile strength, a high fatigue strength, and an excellent superplastic formability.