Effect of hydrogen on mechanical properties of β-titanium alloys

Conflicting opinions exist in the literature on the manner in which hydrogen influences the mechanical properties of beta-titanium alloys. This can be attributed to the beta-stabilizing effect of hydrogen in these materials leading to major changes in the microstructure as a result of hydrogen charging. The resulting (extrinsic) effect of hydrogen on the mechanical properties can possibly cover up the direct (intrinsic) influences. On the basis of experimentally determined thermodynamic and kinetic data regarding the interaction of hydrogen with beta-titanium alloys, hydrogen concentrations Of Lip to 8 at.% were established in three commercial alloys by means of hydrogen charging from the gas phase. In order to separate intrinsic and extrinsic effects the charging was carried out during one step of the two-step heat treatment typical of metastable beta-titanium alloys, while the other step was performed in vacuum. The results on the single-phase beta condition represent the intrinsic hydrogen effect. Monotonic and cyclic strength increase at the expense of ductility with increasing hydrogen concentration. The brittle to ductile transition temperature shifts to higher values and the fatigue crack propagation threshold value decreases. The microstructure of the metastable, usually two-phase beta-titanium alloys is strongly affected by hydrogen, although the extent of this effect depends not only on the hydrogen concentration but also on the temperature of charging. This microstructural influence (extrinsic effect) changes. the mechanical properties in the opposite direction as compared to the intrinsic hydrogen effect.