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

High pressure hydrides of V0.995C0.005 were thermally cycled between beta2- and gamma-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 beta2+gamma region is minimal. This is in contrast to the calculated plateau enthalpies for low pressure (alpha+beta1) mixed phases which showed a noticeable lowering of the values. Thermal cycling between beta2-and gamma-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, 21/2, in the beta2-phase lattice of the thermally cycled hydrides decreased after 778 cycles and the domain sizes increased. However, in the gamma-phase, both the microstrains and the domain sizes decreased after thermal cycling indicating no particle size effect. The dehydrogenated alpha-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 beta2- and gamma-phases) and cold working on absorption/desorption pressures, hydrogen storage capacity, microstrains, long-range strains, and domain sizes of beta2- and gamma-phase hydrides of V0.995C0.005 alloys are presented.