Influence of Phases Stability on the Hydrogen Diffusion Coefficient in Ti-6Al-4V

Abstract

For a unique microstructure creation, thermo-hydrogen treatment (THT), using hydrogen as a temporary alloying element within the heat treatment, is applied. This advanced heat treatment requires reliable data about the hydrogen diffusion coefficient (DH) for understanding diffusion kinetics and its effect on the mechanical behavior of the resulted phases. In this research, three different homogeneous microstructures were established for the investigation using different homogenization parameters. After that, the concentration of hydrogen, charged in the half-length of thin titanium rods via electrochemical hydrogenation, is specified. Then, a diffusion annealing heat treatment was carried out at different temperatures, leading to hydrogen diffusion in the hydrogenated specimens. Furthermore, DH was systematically determined using two methods including the explicit finite difference method (EFDM) and Matano technique (MT). For this purpose, Abaqus software was employed for modeling the hydrogen gradient established in the specimens. Additionally, scanning electron microscopy (SEM) was used for the microstructure examination in order to specify the influence of different hydrogen concentrations on the hydrogenated specimens. The experimental outcomes reveal a substantial effect of the β phase stability and grains sizes of the β and α phases on the hydrogen diffusion. Correspondingly, the results confirm that DH was independent of the hydrogen concentration, and obeys an Arrhenius-type temperature dependence. Furthermore, hydrogen diffusion in the α+β titanium alloys Ti-6Al-4V was slower in comparison to the hydrogen diffusion in the metastable β titanium alloys Ti-10V-2Fe-3Al. In conclusion, it was observed that DH is influenced by the previously performed heat treatments that determine the resulted microstructure types, and a slight influence of the volume fraction of the α phase on DH was observed as well.