Relationship between lattice defects and phase transformation in hydrogenation/dehydrogenation process of the V60Ti25Cr3Fe12 alloy

Abstract

The hydrogen storage capacities of vanadium-based solid solution alloys are determined by both bulk and surface characteristics. In this research, the influence of lattice defects and phase transformation of the ball-milled V60Ti25Cr3Fe12 alloy was investigated systematically. With the elongated ball milling time (0.5 h, 1 h, 2 h and 3 h), both the hydrogenation and dehydrogenation capacities decrease. XRD and TEM methods are applied to make the essential mechanism clearer. The XRD results demonstrate that after full hydrogenation, a large amount of VH2 phase and some V2H phase are observed, and the V2H phase proportion rises with the elongated ball milling time. The alloy particle size is limited in several micrometers, however, the XRD and subsequent TEM results demonstrate that unnegligible amorphous phase is formed in the surface layer (about 5 nm). The PALS and subsequent HRTEM results conformed that some defects (lattice deformation and dislocations/vacancies) appear in the bulk of the milled alloy particles. It is because of the defects and amorphous phase caused by ball milling that the formation of the monohydride is not been suppressed effectively in the hydrogen absorbing and desorbing process, and the defects can trap the hydrogen atoms, leading to the decrease of the hydrogen capacity. In addition, the phase transformation between V2H and VH2 is confined, thus a decrease of effective hydrogenation capacity is observed. As to the cyclic property, the micro-strains cannot be released by refining the particle size of the alloy particles by ball milling, thus, the hydrogen storage cyclic durability is not ameliorated.