Ion irradiation induced grain growth in nanocrystalline Fe and Fe(Zr)

Thin films of nanocrystalline Fe and Fe95Zr5 were prepared by electron-beam evaporation. Grain growth induced by 500 keV Ar+ and Xe+ irradiation was investigated using dark-field electron microscopy and X-ray diffraction. After irradiation with different ions, a large difference in overall grain growth is found for similar amounts of displacement per atom (as estimated from TRIM computer simulations). Rapid grain growth is observed only for 500 keV Xe+ irradiation, which causes dense collision cascades. This observation provides new support for earlier suggestions of radiation induced grain growth being associated with diffusion during the quenching of atomic-collision cascades. In contradiction to earlier assumptions about irradiation induced grain growth, the grain-boundary mobility in nanocrystalline Fe is found not to be proportional to the elastically deposited energy FD. The data appear to be in agreement with a FD2 dependence as suggested from a thermal-spike model recently proposed by Alexander and Was. ((orig.))