The scientific community has long known about hydrogen embrittlement, the process by which metals become brittle and fracture due to the introduction and subsequent diffusion of hydrogen atoms into the metal. In an environment where hydrogen could become a major source of energy for the future, preventing hydrogen embrittlement has been the subject of significant. While extensive research has been done on the mechanism of hydrogen embrittlement, it was not until recently that much was understood about the evolution of the dislocation structure at higher strain levels. How these microscopic changes modify the local plasticity and develop into macroscopic hydrogen-induced cracking remains an open question.
In working with partners in Japan and the United States, Professor Shuai Wang and his team at the Department of Mechanical and Energy Engineering at Southern University of Science and Technology (SUSTech) finally achieved a breakthrough on evaluating of flow stress based on dislocation structure developed ahead of the fatigue crack in the absence and presence of hydrogen. They published a paper titled “Assessment of The Impact of Hydrogen on The Stress Developed ahead of A Fatigue Crack,” in Acta Materialia (IF=6.036).
They used multiscale characterization methods to access the deformation structure near fatigue crack tips of a low carbon steel. Through a qualitative and quantitative analysis of the type, morphology and characterizing size of dislocation structures, it was found that an elongated cell and labyrinth-type dislocation structures were predominant in the air, and the collective dislocation behavior disappears at about 56μm from the crack. By contrast, in the presence of hydrogen, the dislocation structure is mainly cell type, and it is smaller and mostly equiaxed. The existing of the dislocation structure in hydrogen environment extends to about 104μm from the crack tip. The flow stress ahead of the crack tip in a hydrogen environment is much higher with a wider range, indicating a much larger intense plastic zone than that in air. These results conclude that the dislocation structure evolution process at the crack tip in the hydrogen environment may be similar to that in the air, but the entire dislocation evolution process in the hydrogen environment is promoted.
Another significant contribution is the confirmation of the linear relationship between the fatigue crack tip stress distribution and the distance in different environments. Although this relationship had been predicted theoretically, it is the first time it has been confirmed through experimental results. The research results not only have great significance for the study of hydrogen embrittlement mechanism, but also provide significant reference values for the development of continuum model in predicting the softening and hardening of materials.
Professor Shuai Wang is the first author and corresponding author. Contributing researchers include Dr. Nagao Akihide from the International Institute for Carbon-Neutral Energy Research at Kyushu University, Professor Petros Sofronis from the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign and Professor Ian M. Robertson from the Departments of Engineering Physics and Materials Science and Engineering at the University of Wisconsin-Madison. Southern University of Science and Technology is the affiliation of the corresponding author.
The researchers received funding from National Science Foundation, JFE Steel Corporation, the International Institute for Carbon Neutral Energy Research, and the World Premier International Research Center Initiative (WPI), MEXT, Japan.
Suggested reading (related research): Wang, S., Nygren, K. E., Nagao, A., Sofronis, P. & Robertson, I. M. “On the failure of surface damage to assess the hydrogen-enhanced deformation ahead of crack tip in a cyclically loaded austenitic stainless steel.” Scripta Materialia 166, 102–106 (2019).