Abstract:

When metals are exposed to hydrogen (H)-rich environments, they may lose ductility and fail catastrophically without showing any sign of damage. This problem is known as hydrogen embrittlement (HE). Over the last few decades, extensive research on HE has shown that hydrogen-assisted cracking is a microstructure-sensitive phenomenon. The causes behind crack initiation, however, remain elusive to this day. We used in situ tensile straining experiments to investigate the role of H in crack initiation in a widely used structural material, nickel-based alloy 725. In our experiments, we varied different hydrogen charging conditions, which are representative of a range of applications: electrochemical charging and high temperature thermal charging. We found fundamentally different crack initiation mechanisms in both scenarios. In electrochemically charged samples, cracks initiate at regions which experience localized slip if the hydrogen content is low, or more ubiquitously at higher concentrations, but always on the sample surface. Conversely, thermal charging leads to a predominance of secondary cracks in the sample interior. We ascribe these differences to the variance in internal hydrogen distribution and, possibly, to the degree of confinement on crack tip plasticity. Comprehending these mechanisms is pivotal for making reliable lifetime predictions of components operating in H-rich environments, and paves the way for future development of novel, HE-resistant alloys.

Short Bio:

Mengying Liu is an Assistant Professor of Engineering at Washington and Lee University. Her primary research interests focus on environmental degradation of metallic materials. This includes specific projects on hydrogen embrittlement of Ni-based alloys and localized corrosion of pure Ni. Liu employs techniques like scanning electron microscopy and digital image correlation to explore the structure-properties relationship of these materials. She holds a Ph.D. in Materials Science and Engineering from Texas A&M University and a bachelor’s degree in engineering from Tianjin University in China. Liu is an awardee from the American Association of University Women, known for her passion for mentoring female students and fostering their growth, both academically and personally.