HYDROGEN BUBBLES IN METALS

First a brief introduction to hydrogen bubbles in metals and their technological importance is presented. The review concentrates on those areas of bubble formation where the distinct chemical character of hydrogen is important. A distinction is made between nucleation and subsequent cavity growth mechanisms. Both homogeneous and heterogenous nucleation are given as possibilities. Three cavity growth mechanisms are delineated: plastic deformation near a surface referred to as ''blistering'', plastic deformation by loop punching, and the collection of vacancy-hydrogen complexes. Several factors are considered which can determine the particular mechanism which applies, such as the source of the hydrogen and its concentration, solubility and diffusivity and the concentration of defects in the metal. A preemptive reaction to bubble formation is the one leading to metal hydride formation. This reaction must be considered both for predicting the formation of bubbles and for experimental observations. The possibility of plastic deformation near a free surface can lead to blistering, and this must be considered. The current understanding of loop punching processes is presented and applied to hydrogen bubble formation. Vacancy clustering may also lead to void formation which may then absorb further hydrogen-vacancy complexes. Illustrative examples of hydrogen filled cavities in selected metals and alloys are discussed in detail.