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Irregular Helium Bubbles In Niobium Alloys

Helium bubbles have been produced in niobium‐1% Zr by ion implantation and their growth has been studied by transmission electron microscopy of annealed specimens. Bubbles are normally faceted and develop from tetrakaidodecahedra at short times to cuboids bounded by {100} faces after long annealing periods. This behaviour, together with the observation of steps on the faces of many bubbles, is accounted for in terms of competition between the conditions necessary for thermodynamic equilibrium and the kinetics of bubble motion. The larger bubbles must contain an excess gas pressure.