Sophia University Researchers Reveal Nature of Fractures by Hydrogen
Hydrogen embrittlement weakens the material, making it more likely to fail under mechanical stress, and is a major challenge for the hydrogen automobile industry. High-strength steels have a higher weight-to-strength ratio, making them ideal materials for lightweight vehicles, but their exposure to hydrogen negatively affects their mechanical properties.
Scientists have discovered a way to study intergranular fractures caused by hydrogen embrittlement in high-strength steel. A research team from Sophia University, Japan, led by Professor Kenichi Takai, has developed a new mechanical test that allows for the study of intergranular (IG) fractures in high-strength (HS) steel caused by hydrogen embrittlement. Hydrogen embrittlement occurs when hydrogen atoms diffuse into the material's lattice structure, weakening it and making it more susceptible to failure under mechanical stress. IG fractures, which occur along the grain boundaries of the crystalline lattice, are particularly difficult to study in isolation due to other types of fracture that tend to occur alongside them.
The team's innovative mechanical test involves repeated loading and unloading of the HS steel sample during hydrogen charging. This test progressively reduces the material's ultimate tensile strength (UTS) and produces almost-pure IG fractures. The researchers believe that this occurs because hydrogen atoms are given enough time to fill up new cracks generated in the material, allowing the fracture to advance exclusively along the grain boundaries.
To gain insight into the lattice defects present below the fracture, the team used lower-temperature thermal desorption spectroscopy (L-TDS). This technique involves observing the rate of desorption of a gas (hydrogen) from the material at different temperatures, providing information about the number and types of defects present in it. The researchers were able to distinguish hydrogen trapping sites on the atomic scale, providing important clues to understand and potentially suppress hydrogen embrittlement in HS steel.
Further analyses of scanning electron microscopy (SEM) images showed that local plastic deformation occurs right below the IG fracture caused by hydrogen embrittlement. The findings of this study will help materials scientists understand hydrogen embrittlement in HS steel better, paving the way for new methods to suppress it and enabling the safe use of HS steel in hydrogen-powered vehicles.
To investigate this issue, researchers, including Professor Kenichi Takai from Sophia University in Japan, devised a load reduction test to produce pure intergranular fractures in embrittled high-strength steel samples. They also used thermal desorption spectroscopy to study lattice defects below the fracture surface.
The study's findings could aid in the development of methods to suppress hydrogen embrittlement and enable the safe use of high-strength steel in hydrogen-powered vehicles. The research was supported by the Japan Society for the Promotion of Science, New Energy and Industrial Technology Development Organization, and The Iron and Steel Institute of Japan.
Sophia University, established in 1913 in Tokyo, is a prestigious Jesuit-affiliated university that offers education in various fields, including engineering and applied sciences.
Kenichi Takai is an accomplished researcher with a strong background in materials science and engineering. Prior to joining Sophia University, he conducted research at Nippon Telegraph and Telephone Corporation for almost a decade. At Sophia University, he is a full Professor in the Department of Engineering and Applied Sciences at the Faculty of Science and Technology. His research focuses on hydrogen embrittlement in metals, with a particular interest in understanding the hydrogen trapping sites in metals and developing high-strength steels with high resistance to hydrogen embrittlement.