Observation of interstitial loops in He+ irradiated W by conductive atomic force microscopy

Polycrystalline tungsten (W) has been irradiated with a low-energy (30–830 eV) He+ beam at the W surface temperature of 523–1073 K up to a fluence of 1.0 × 1025/m2. Measurements by non-destructive conductive atomic force microscopy show the existence of nanometer-sized interstitial loops in He+-implanted layer. The size and distribution of interstitial loops are significantly affected by He+ energy and fluence, and W surface temperature. The distribution of interstitial loops becomes orientated in one certain direction after being irradiated at a relatively high fluence or W surface temperature. The cascading slipping of W atoms along one certain dense-packed face has been proposed to explain the ordered arrangement of interstitial loops at elevated temperature. Analysis indicates that the continuous growth of unstable nanometer-sized interstitial loops can result in the surface exfoliation of W materials.