Early Precipitation Stages of Sigma Phase in Alloy 28 Studied with Scanning Electron Microscopy and Atom Probe Tomography

This study deals with early stages of sigma phase growth in a high end austenitic stainless steel - Alloy 28 (EN 1.4563/UNS N08028). Its precipitation kinetics was followed by a series of heat treatments at 800 degrees C for holding times up to 30 000 s. The samples were studied with high resolution scanning electron microscopy and atom probe tomography. Detailed image analysis of the micrographs made it possible to quantify the growth rate of the precipitates. It was shown that diffusion limited growth along grain boundaries was about 15 times faster than growth perpendicular to a grain face. By combining the image data with quantitative chemical analysis of the phase boundaries, it was possible to estimate diffusion coefficients in the lattice and in the grain boundaries; grain boundary diffusion coefficients were about 250 times those of the lattice.