Sputtering-induced nanometre hole formation in Ni3Al under intense electron beam irradiation

The irradiation damage of polycrystalline Ni₃Al thin foils of stoichiometric composition by a stationary nanoscale 200?keV field emission gun (FEG) electron probe in a FEI Tecnai F20 (S)TEM has been investigated. At current densities greater than 10⁷?A/m², nanometre holes are produced quickly with both ?001? and ?110? incident electron beam directions. EDX spectra from the irradiated volume have been collected simultaneously during the hole forming process. From the EDX results, preferential surface sputtering of aluminium from Ni₃Al has been demonstrated. To understand the underlying physical process of sputtering, modelling based on a combination of molecular dynamics and Monte Carlo simulation has been performed. It appears to reproduce faithfully the overall film sputtering and hole formation processes, but is not capable of predicting the detailed geometry of the hole. It predicts that the sputtering cross-section of Al atoms is much higher than that of Ni atoms, resulting in a very small concentration of Al at the surface. This, together with the increase of surface area during hole formation, explains the preferential Al loss observed from the specimen. Calculated sputtering rates agree well with experiment, and are of the order of magnitude of 10^?8?atoms/electron.