Cr-rich nanosize precipitates in a standard heat-treated Inconel 738 superalloy

An analytical transmission electron microscopy study was performed on the nature of nanosize secondary phase precipitates that form in standard heat-treated commercial nickel base superalloy IN 738. In addition to M5B3 borides and M23C6 carbides, the precipitates were found to also consist of particles of M2B-type boride phase, which is previously unreported in the alloy and rarely reported in nickel base superalloys. The M2B borides exhibited a dual crystallographic structure: body centered tetragonal and face centered orthorhombic. Analyses of electron diffraction patterns and high-resolution transmission electron microscopy images indicated that both crystal structures can co-exist in a single M2B particle as an inter-growth with the formation of planar defects. Crystallographic relationship between microconstituent phases in the heat-treated alloy were elucidated and the influence of the borides on resistance to grain boundary liquation cracking during high-temperature material processing, such as laser beam treatment, is discussed.