Hysteresis in Vx-Nb1x-H systems: Effect of composition, temperature and cycling

Hysteresis in metal—hydrogen systems is affected by many different factors including temperature, alloy composition and cycling. In an attempt to better understand these phenomena, tests were made on a series of Vx-Nb1−x solid solution alloys (x = 0.1, 0.2 and 0.3) using a Sieverts' type technique to determine PCT relationships. The PC isotherms were determined at temperatures 318, 353 and 373 K and for varying number of hydriding/dehydriding cycles. A large hysteresis effect was observed for x = 0.3 but as the value of x decreases the hysteresis becomes smaller. The Vx-Nb1−x alloys behave like most metal—hydrogen systems in that as the temperature increases, the hyteresis effect as measured by the ratio (Pf/Pd), where Pf is the plateau pressure for hydride formation and Pd is the plateau pressure for hydride decomposition, decreases. The hysteresis effect also becomes smaller at the second and third hydride/dehydriding cycles, but no further change is seen after the third cycle. The hysteresis in these alloys can be explained in terms of strains introduced into the solvent lattice (Nb) by the smaller radii solute atoms (V).