Low angle grain boundaries and re-segregation in single crystal Ni-base superalloys

Low angle grain boundaries (LAGBs) form during directional solidification of Ni-base single crystal superalloys and extend over distances of one to several average dendrite spacings (mm scale). Their influence on the superalloys' behavior has often been overlooked. In the present work we use the high-resolution rotation vector base line electron back scatter diffraction (RVB-EBSD) technique to locate LAGBs, and focused ion beam (FIB) micromachining to prepare specimens for investigations by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). STEM confirms that LAGBs consists of fine dislocation networks. APT provides experimental evidence for the segregation of Rhenium (Re) to these LAGBs in the as-cast state. RVB-EBSD results show that during a multi-step post cast heat treatment, irregular shaped LAGBs straighten out and move over distances of the order of several 10 mu m. Considering this LAGB movement, and in the light of previous reports (He et al., 2020), we propose that Re segregates to LAGBs during Bridgman processing, redissolves during the high temperature (> 1300 degrees C) solution phase, allowing LAGB migration, and then resegregates during the medium temperature (< 1000 degrees C) precipitation phase of the multi-step heat treatment, which immobilizes the LAGBs during creep loading in the 1000 degrees C temperature range. The finding of the present study shades a new light on the behavior of Re and LAGBs in Ni-base single crystal superalloys.