Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni 3 Al.

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni 3 Al are simulated using molecular dynamics methods. The threshold energy for sputtering, E sp , and adatom creation, E ad , are determined as a function of recoil direction. The study is relevant to FEG STEM (a scanning transmission electron microscope fitted with a field emission gun) experiments on preferential Al sputtering and/or enhancement of the Ni–Al ratio near boundaries. Surfaces intersected by {110} and {111} APBs have minimum E sp of 6.5  eV for an Al atom on the Ni–Al mixed (M) surface, which is close to the value of 6.0  eV for a perfect M surface. High values of E sp of an Al atom generally occur at a large angle to the surface normal and depend strongly on the detailed atomic configuration of the surface. The mean E sp , averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for E ad imply that an electron beam could create more Al adatoms on surfaces intersected by APBs than on those without. The equilibrium, minimum energy structures for a (001) surface intersected by either Σ5[001](210) or Σ25[001](340) symmetric tilt grain boundaries are computed. E sp for surface Al atoms near these GBs increases monotonically with increasing recoil angle to the surface normal, with a minimum value, which is only about 1  eV different from that obtained for a perfect surface. Temperature up to 300  K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for E sp studied here. Possible reasons for this are discussed. [ABSTRACT FROM AUTHOR]