HELIUM-INDUCED SWELLING IN TUNGSTEN DURING POSTIMPLANTATION ANNEALING

A detailed transmission electron microscopy investigation of the gas porosity has been carried out in two types of tungsten specimens: (1) hot-rolled W, homogeneously implanted with 600 appm He at 325 K and (2) W single crystals, implanted with up to 2 x 10(21) He+ ions/m(2) of 40 keV energy at 325 K, followed by annealing at temperatures between 1273 and 2373 K for 1 h (in case (2) after W vapor deposition onto implanted surface). In both types of specimens monomodal distributions of near-equilibrium bubbles, mostly attached to dislocations have been found for T greater than or equal to 1523 K. This is in contrast to the bubble development in typical fee metals (Ni, Cu) where dislocations are apparently unable to provide sufficient vacancies for the relaxation of the overpressure in the bubbles which leads to spacially separated bubble populations, i.e. large bubbles close to surfaces and some grain boundaries and small bubbles in the bulk. An evaluation of the temperature dependence of the mean bubble radii in W suggests migration and coalescence controlled by surface diffusion as the main coarsening mechanism during annealing.