Hydrogen concentration dependence of phase transformation and microstructure modification in metastable titanium alloy β-21S

The present study aims at revealing the relationships between hydrogen concentration and phase structure, as well as microstructure modification in the β-metastable β-21S titanium alloy. The β-bcc phase can accommodate a large number of interstitial atoms, and hydrogenation by means of molecular hydrogen gas was employed in the present work. The phase structure as well as the microstructure of this alloy was found to be strongly dependent on hydrogen concentration. At lower hydrogen concentration ( H/M ≤ 0.300), the microstructure consisting of the single β-phase revealed that the interstitially dissolved hydrogen atoms expanded the bcc lattice and inhibited the decomposition of the β phase upon cooling. The introduction of hydrogen beyond H/M = 0.300 was found to generate a large amount of internal stresses in the microstructure inducing the formation of metastable phases α'' in the form of lamellae and ω in the form of nanoparticles. The generation of the nanosized ω-phase was presumed to relax the strain caused by the volume expansion (2.28%) from the hydrogen-containing β phase to the α'' martensite.