Quantitative precipitate classification and grain boundary property control in Co/Ni-base superalloys

TL;DR

This study employs a correlative approach combining EBSD and EDS to accurately classify precipitates and assess grain boundary properties in novel Co/Ni-base superalloys, revealing phases that differ from thermodynamic predictions.

Contribution

It introduces a combined structural and chemical fingerprinting method for precise phase identification in superalloys, surpassing traditional chemical-only techniques.

Findings

Principal carbides are Mo and W rich M6C phases.

An M2B boride phase is identified at higher B levels.

Grain boundary serratability is improved without pinning precipitates.

Abstract

A correlative approach is employed to simultaneously assess structure and chemistry of (carbide and boride) precipitates in a set of novel Co/Ni-base superalloys. Structure is derived from electron backscatter diffraction (EBSD) with pattern template matching, and chemistry obtained with energy dispersive X-ray spectroscopy (EDS). It is found that the principal carbide in these alloys is Mo and W rich with the M6C structure. An M2B boride, also exhibiting Mo and W segregation is observed at B levels above approximately 0.085 at.%. These phases are challenging to distinguish in an SEM with chemical information (EDS or backscatter Z-contrast) alone, without the structural information provided by EBSD. Only correlative chemical and structural fingerprinting is necessary and sufficient to fully define a phase. The identified phases are dissimilar to those predicted using ThermoCalc. We additionally perform an assessment of the grain boundary serratability in these alloys, and observe that significant amplitude is only obtained in the absence of pinning intergranular precipitates.