Effect of gradient-structure versus uniform nanostructure on hydrogen storage of Ti-V-Cr alloys: Investigation using ultrasonic SMAT and HPT processes

Lattice defects can have contradicting effects on the hydrogen storage behavior of titanium-vanadium-chromium alloys: they may facilitate the surface activation, or they may deteriorate the hydriding/dehydriding reversibility. In this study, two types of microstructure containing different structural defects were investigated to gain further insights on the impact of lattice defects on the hydrogen storage performance of beta Ti-V-Cr alloys: (i) a gradient-structure with high density of surface defects processed by ultrasonic surface mechanical attrition treatment (SMAT), and (ii) a uniform structure containing highly-strained nanograins processed by high-pressure torsion (HPT). Because of the effect of surface lattice defects on initial activation, both the SMAT- and HPT-processed materials readily absorbed hydrogen at room temperature. However, while the SMAT-processed samples showed good hydrogen storage reversibility, the HPT-processed materials exhibited poor reversibility because of the effect of bulk defects on hindering the hydrogen transport to/from the hydride. The results clearly demonstrate that the engineering of structural defects on the surface is an effective approach to achieve both easy activation and good reversibility.