A theory was proposed for the formation of vacancy supersaturations, amounting to 10 to 20at%, in metal-H alloys. The H was assumed to decrease the formation energy of a vacancy by cluster formation, and the configurational entropy of the system at high H concentrations. A formula was derived for the vacancy concentration and was found to give excellent descriptions of experimental results on Ni-H and Mo-H alloys which were produced under high H pressures. Some of the consequences of vacancy supersaturation were solubility enhancement, the formation of defect structures and voids, and the enhancement of metal-atom diffusion.

Super-Abundant Vacancy Formation and its Consequences in Metal-Hydrogen Alloys Y.Fukai, Y.Kurokawa, H.Hiraoka: Journal of the Japan Institute of Metals, 1997, 61[8], 663-70