Impact of high pressure torsion processing on helium ion irradiation resistance of molybdenum


Impact of high pressure torsion processing on helium ion irradiation resistance of molybdenum

Krawczyńska, A. T.; Ciupiński, Ł.; Gloc, M.; Setman, D.; Spychalski, M.; Suchecki, P.; Adamczyk-Cieślak, B.; Liedke, M. O.; Butterling, M.; Wagner, A.; Hirschmann, E.; Petersson, P.

The microstructure of molybdenum mirrors was refined by high pressure torsion. Already after one rotation microhardness significantly increased from 231 for the as-received mirror to 542 HV0.2. The increase of number of rotations to five caused further slight increase of microhardness to 558 HV0.2. The higher microhardness values correspond well with the grain refinement as the grain size decreased with the increase of the deformation degree down to 480 and 110 nm, respectively for 1 and 5 rotations. Subsequently, refined mirrors and a reference micrograined one were irradiated by He ions to the dose of 8x1016/cm 2 to simulate the effect of plasma exposure on diagnostic mirrors to be applied in D-T fusion devices. Irradiations were followed by reflectivity measurements in the 300-2400 nm range with a dual beam spectrometer. It was noticed that irradiation caused a slight decrease in total reflectivity of the micrograined mirror, whereas that of high-pressure torsion-processed samples decreases by an additional 2.5%. Nanohardness measurements, detailed microscopy observations using focused ion beam and scanning transmission electron microscope as well as positron annihilation spectroscopy investigations were performed to elucidate that cause of those changes. Based on the results, it is postulated that the nanocracks created at grain boundaries during irradiation in the optically active layer are responsible for lower reflectivity of high-pressure torsion-processed mirrors.

Keywords: nanomaterials; ion irradiation; electron microscopy; vacancy; severe plastic deformation; positron annihilation spectroscopy; PALS

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Publ.-Id: 32045