Features of damage of surface layers of monocrystalline tungsten under the influence of powerful pulsed fluxes of helium ions and plasma

A study of the structural state and microhardness of the surface layer of monocrystalline tungsten samples after exposure to pulsed fluxes of He ions and helium plasma generated in the Plasma Focus facility “Vikhr′” (IMET RAS) has been carried out. Irradiation parameters: plasma power density of 10 W/cm, ions power density 2•10 W/ cm with a duration of exposure to plasma and ions of 100 and 20 ns, respectively; number of pulses– 15; energy of helium ions ~100 keV, plasma temperature ~1 keV. It is shown that beam-plasma exposure in this mode results in surface melting and the appearance of a crystallographically oriented network of microcracks. In contrast to the intergranular cracking typical for polycrystalline samples, in W single crystals the formation of a network of microcracks occurs against the background of plastic flow along active sliding planes under the influence of thermal stresses arising at the cooling stage after crystallization of the melt. A fine-mesh structure with a cell size of 200 nm is formed in the solidified layer. The formation of unopened blisters and craters has been observed due to the release of helium from the surface layers. Using X-ray diffraction analysis it has been shown that irradiation in the implemented mode, accompanied by evaporation of a thin surface layer, helps to reduce distortions of the crystal lattice and structural defects of a technological nature that have arisen in the surface layer of single crystal samples at the stage of their mechanical processing in preparation for experiments. As a result of radiation-thermal melting and directional solidification of the W surface layer, a crystallization texture is formed in it in the direction determined by the crystallographic orientation of the sample surface exposed to radiation and coinciding with the temperature gradient vector. A slight (up to 15%) decrease in microhardness was found in the remelted surface layer of W single crystals, while a spread of H values over the irradiated surface area has been observed.