Formation of superabundant vacancies in M-H alloys and some of its consequences: a review

Superabundant vacancies (SAVs) are the vacancies of M atoms formed in M-H alloys, of concentrations as large as < or approx. 30 at.%. After presenting some results of SAV formation as revealed by X-ray diffraction (XRD) at high temperatures and high hydrogen pressures, its mechanism in terms of vacancy-hydrogen (Vac-H) cluster formation is described, including the underlying information of Vac-H interactions. One of the most important conclusions of the theory is that defect structures containing SAVs are in fact the most stable structure of M-H alloys, and therefore SAVs should be formed whenever the kinetics allow. It is shown subsequently that SAVs can be formed in the process of electrodeposition. Some of the consequences of SAV formation including the enhancement of M-atom diffusion and creep are described, and its possible implication for hydrogen embrittlement of steels is mentioned