Elucidating Microstructural Contributions to Hydrogen Embrittlement Susceptibility in Ni-base Alloys

Despite over a century of study, hydrogen-induced premature failure of structural metals continues to negatively impact critical industries spanning the aerospace, marine, transportation, and energy sectors. Efforts to mitigate this deleterious effect are undermined by an incomplete understanding of the microscale processes governing hydrogen-induced degradation. Recent literature has posited that the governing mechanism for hydrogen embrittlement is hydrogen-induced decohesion of grain boundaries principally driven by hydrogen-deformation interactions. However, there is a lack of experiments which quantitatively establish the predominant contribution of hydrogen-deformation interactions to hydrogen-induced intergranular cracking. Additionally, the extension of such mechanisms into more complex, industrially-relevant alloys is hindered by the uncertain contribution of alloy metallurgy to hydrogen-induced degradation. Numerous studies have attempted to ascertain the relative influence of different microstructural features on hydrogen embrittlement susceptibility, with parameters of interest being: grain boundary impurity content, grain size, grain boundary character, yield strength, slip character, and hydrogen-metal interactions. However, isolating the contribution of a particular microstructural feature is complicated by the likelihood that several parameters impact susceptibility, demonstrating the need for a study in which the variation in numerous microstructure features is carefully controlled.

The objective of this dissertation is to address these knowledge gaps by focusing on the following research questions:
• To what extent do hydrogen-deformation interactions govern hydrogen-induced intergranular fracture?
• What influence do microstructural parameters have on hydrogen embrittlement susceptibility?
• What is the role of important metallurgical features in the context of proposed microscale mechanisms governing hydrogen embrittlement?

Towards this end, the dissertation is divided into two separate research thrusts. First, the contribution of hydrogen-enhanced deformation to the conditions required for hydrogen-induced intergranular fracture is assessed using polycrystalline Ni. By quantitatively evaluating the relative contribution of hydrogen-deformation interactions, this study will better inform the extent to which dislocation-based factors like bulk slip morphology may contribute to embrittlement susceptibility in more complex alloys. Second, a series of studies which seek to evaluate the role of microstructural variation on hydrogen environment-assisted cracking susceptibility are completed using a model Ni-base superalloy, Monel K-500. Differences in fracture morphology, crack growth rates, and threshold stress intensity across five engineering-grade heats of Monel K-500 are correlated with observed variations in metallurgy and hydrogen-metal interactions, thereby enabling a broad assessment of the metallurgical features which may contribute to an alloy’s intrinsic resistance to embrittlement. Based on these experiments, targeted heat treatment variations are employed on a single heat of Monel K-500 to produce four aging conditions which result in carefully controlled differences in bulk slip morphology, grain boundary sulfur content, and yield strength. Correlation of these microstructural differences with measured crack growth rates and threshold stress intensities indicates that bulk slip morphology plays a prominent role in hydrogen embrittlement susceptibility in Monel K-500. The effect of hydrogen on bulk slip processes across the aging conditions is then explored through a detailed assessment of work hardening behavior in the presence and absence of hydrogen, thus providing mechanistic insight on how hydrogen may alter deformation processes. In particular, hydrogen is observed to induce the particle shearing-to-looping transition in Monel K-500 at a smaller precipitate size, suggesting that hydrogen perceptibly affects bulk slip processes. Given this importance of bulk slip morphology, as well as the evidence for hydrogen-modified dislocation-precipitate interactions, these results motivate the examination of the deformation proximate to the crack path so as to mechanistically evaluate the contribution of these processes to hydrogen-induced fracture. Towards this end, the dissertation concludes with the development and initial results of a multiple length-scale characterization strategy which couples (1) broad-scale electron backscatter diffraction (EBSD), (2) high-resolution EBSD (HR-EBSD), (3) focused ion beam (FIB) sample preparation, and (4) scanning transmission electron microscopy (STEM) techniques to assess the near-fracture surface deformation.

Considering the practical impact of the current dissertation, results indicate that an over-aged heat treatment can significantly reduce the embrittlement susceptibility of Monel K-500 without compromising strength, thereby providing a possible pathway for reducing the observed heat-to-heat variation in hydrogen-assisted cracking behavior. Additionally, the database of cracking kinetics and microstructural data generated for six material heats of Monel K-500 in this dissertation can be utilized to inform improvements in the Monel K-500 material specification. Considering scientific impacts, the direct assessment of the relative contribution of mobile hydrogen-deformation interactions to intergranular cracking provides critical insights into the operative microscale mechanisms governing embrittlement susceptibility. This work also demonstrates that hydrogen can modify dislocation-precipitate interactions; improved understanding of these interactions could be utilized as the scientific basis for designing hydrogen-resistant precipitation-hardened alloys. Finally, a multiple length-scale characterization strategy was developed to provide a comprehensive evaluation of the deformation proximate to the fracture surface. This approach is expected to provide additional insights into the microscale processes governing hydrogen embrittlement, which can then be utilized to improve current models for hydrogen-assisted cracking.