Helium ion irradiation-induced microstructure evolution on the surfaces of thin nickel foils

Polycrystalline nickel foil (sample I) with thickness of ∼950 nm was irradiated with 0.5-1.2 MeV helium ions at room temperature. Another piece (sample II) with same thickness was mounted behind to receive the irradiation of transmitted helium ions (∼0.026-0.537 MeV). Morphology evolutions on irradiated surfaces were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results show that the open cracks, which were located mainly at grain boundaries, occurred on the surfaces of both irradiated samples. Interestingly, ripple patterns were observed to be regularly arranged on the front surface of sample I. The compressive stress resulting in the sliding on close-packed (1 1 1) planes was regarded as the origin of the ripple formation. Moreover, protrusion islands and its surrounding microstructures were observed on the front surface of sample II. The mass transport driven by the lateral stress generated in the helium ion irradiation were discussed as possible reasons.