Hydriding and structural characteristics of thermally cycled and cold-worked V–0.5 at.%C alloy

High pressure hydrides of V 0.995C 0.005 were thermally cycled between β 2- and γ-phases hydrides for potential use in cryocoolers/heat pumps for space applications. The effect of addition of carbon to vanadium, on the plateau enthalpies of the high pressure β 2 + γ region is minimal. This is in contrast to the calculated plateau enthalpies for low pressure (α + β 1) mixed phases which showed a noticeable lowering of the values. Thermal cycling between β 2-and γ-phase hydrides increased the absorption pressures but desorption pressure did not change significantly and the free energy loss due to hysteresis also increased. Hydriding of the alloy with prior cold-work increased the pressure hysteresis significantly and lowered the hydrogen capacity. In contrast to the alloy without any prior straining (as-cast), desorption pressure of the alloy with prior cold-work also decreased significantly. Microstrains, 〈 ɛ 2〉 1/2, in the β 2-phase lattice of the thermally cycled hydrides decreased after 778 cycles and the domain sizes increased. However, in the γ-phase, both the microstrains and the domain sizes decreased after thermal cycling indicating no particle size effect. The dehydrogenated α-phase after 778 thermal cycles also showed residual microstrains in the lattice, similar to those observed in intermetallic hydrides. The effect of thermal cycling (up to 4000 cycles between β 2- and γ-phases) and cold working on absorption/desorption pressures, hydrogen storage capacity, microstrains, long-range strains, and domain sizes of β 2- and γ-phase hydrides of V 0.995C 0.005 alloys are presented.