PNNL

Abstract

Surface self-diffusion studies on metals under elevated reaction conditions are limited due to challenges inherent to surface analyses. Here, quantitative and mechanistic insight into thermally induced atomic transport processes in bcc a-iron at the sub-nanometer level was achieved using isotopic tracer techniques coupled with in situ atom probe tomography (APT) capabilities. Specifically, using a reactor directly connected to the APT, needle-shaped specimens were fabricated from epitaxial thin films with an embedded 57Fe tracer layer were annealed in Ar at 500°C and 350°C for 1 h. Furthermore, the tracer had been positioned at various depths in the APT specimen by field evaporation, enabling targeted and simultaneous analysis of lattice, subsurface, and surface diffusion. 57Fe concentration profiles revealed that lattice self-diffusion occurs at 500°C on the order of ~7 – 9 monolayers, while lattice diffusion is not resolvable at 350°C. Considerable surface transport was, however, observed at both conditions, where atomic transport over the specimen surface led to the formation of a thin (