Ultra-fine Equiaxed Grain Refinement and Mechanical Properties in α+β Type Titanium Alloys Resultant from Processing of Hydrogenation, Hot Working, Heat Treatments, and Dehydrogenation

Abstract

Titanium alloys can be hydrogenated with large amounts of hydrogen and dehydrogenated in vacuum at elevated temperatures. This paper aims at the equiaxed grain refinement and the improvement of mechanical properties of titanium alloys utilizing this phenomenon. Particular attention has been drawn to the control of three different types of dislocation structures introduced by the following treatments:(1) Precipitation of hydrides: On aging of hydrogenated titanium alloys, high-density dislocations are introduced in the interior of hydrides and also in the surrounding region.(2) Formation of cell walls by hot-working: The β transus temperature is decreased by hydrogenation so that the hot-working temperature can be lowered enough to retain the dislocation cell structure due to defomation.(3) Martensitic transformation: On cooling from the β phase region of hydrogenated titanium alloys, high-density dislocations with the uniform distribution are introduced in acicular martensites.The factor (1) gives rise to the localized structure of high-density dislocations which provides with heterogeneous nucleation sites for recrystallization, leading to the grain refinement. The factor (2) gives high-density dislocation cell structures to act as precipitation sites for hydrides, and the factor (3) causes the subdivision of the β-phase grain into fine martensite leaves with variants, in addition to uniformly distributed dislocations in the martensite matrix which serve for the precipitation of hydrides.The combination of these factors such as (3)-(1) and (3)-(2)-(1) results in the grain refinement of the α+β type titanium alloys containing 0.2∼1.0 mass% hydrogen. For example, a Ti-6Al-4V alloy hot-rolled, quenched, aged, and recrystallized as well as dehydrogenated has ultra-fine grains of about 1 μm or less in diameter and has exhibited high strength, high fatique limit and excellent superplastic behavior.