Low-energy sputtering events at free surfaces near anti-phase and grain boundaries in Ni3Al

Atomic recoil events on free surfaces orthogonal to two different anti-phase boundaries (APBs) and two grain boundaries (GBs) in Ni3Al 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 Esp 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 Esp, averaged over all recoil directions, reveals that APBs have a small effect on the threshold sputtering. However, the results for Ead 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 Sigma 5[001](210) or Sigma 25[001](340) symmetric tilt grain boundaries are computed. Esp 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 300K has no effect on this result. It is concluded that the experimental observations of preferential sputtering are due to effects beyond those for Esp studied here. Possible reasons for this are discussed.