Hydrogen Embrittlement of a CoNiCr-Based Superalloy: The Role of the Intermetallic Precipitation Phases β and γ′

Abstract Hydrogen embrittlement (HE) presents a critical challenge for the application of superalloys in hydrogen-containing environments, such as future turbine systems. This study investigates the influence of the β phase and γ ′ phase on HE sensitivity in the CoNiCr-based superalloy CoWAlloy6. A tailored heat treatment strategy was applied to vary the β and γ ′ phase fraction while maintaining all other microstructural aspects constant. Tensile tests after high-pressure hydrogen charging (1000 bar, 300 °C) reveal reduced HE susceptibility for CW6+ β /− γ ′ (CoWAlloy6 with higher β and lower γ ′ content), compared to CW6− β /+ γ ′ (CoWAlloy6 with lower β and higher γ ′ content). Thermal desorption spectroscopy measurements confirm that an increased β and the associated decreased γ ′ phase fraction correlates with lower overall hydrogen solubility and slightly reduced diffusion rates. In addition, NanoSIMS mappings demonstrate the highest hydrogen uptake inside β phase precipitates. Consequently, the amount of diffusible hydrogen in the γ / γ ′ compound is lower in CW6+ β /− γ ′, which in turn results in a modest work-hardening behavior. This work-hardening potential is essential for reducing the severity of HE. Furthermore, a slower strain rate during tensile testing increases HE severity, confirming the role of diffusible hydrogen content as a key aspect in the embrittlement mechanism. EBSD analyses of secondary cracks reveal predominantly transgranular cracking, indicating weakened γ / γ ′ interfaces in the presence of hydrogen. The findings suggest that tuning the β and γ ′ content by small modifications in heat treatment can positively affect the alloy’s resistance to hydrogen embrittlement, primarily by reducing the hydrogen solubility through a lower γ ′ fraction and by lowering the amount of diffusible hydrogen in the microstructure through an increased β fraction. Graphical Abstract