Effect of microstructural degradation on the hydrogen embrittlement susceptibility and mechanism of a Ni-based single crystalline superalloy

The microstructural deterioration and hydrogen embrittlement (HE) susceptibility of a Ni-based single crystalline superalloy, which has been subject to long-term thermal exposure (LTE) at 1100 °C (for up to 1000 h), has here been investigated. An LTE-induced microstructural evolution was revealed, including coarsening and rafting of the primary γ′ phase, formation of a dislocation network, and precipitation of the secondary γ′ phase and σ phase. The HE index initially decreased, which was primarily due to hydrogen trapping by new traps. However, it was partially recovered after a prolonged LTE as the σ phase coarsening promoted interfacial H accumulation and decohesion. The H atoms were preferentially trapped in the γ matrix, at the γ/γ′ interface, and in the σ phase. These trapped atoms enhanced the localized plasticity via stacking fault formation in non-aged alloys, while promoting <100> super-dislocations in the LTE samples. The HE mechanism in the superalloys, with and without an LTE treatment, was also elucidated. In addition, the effect of elemental segregation at the γ/γ′ interface on the H-induced damage was also analyzed using first-principles calculations.