Image shows the localization of hydrogen atoms at dislocations, grain boundaries in steel, as well as at the interface between carbide precipitates and the broader steel matrix. Image B, a 2-D slice from its 3-D visualization (Image A), shows the locations of hydrogen (red) at dislocations with carbon decorations (blue), and likewise for the case of grain boundaries in Images C and D.
Photo of Yi-Sheng Chen with the special atom probe instrument used to carry out the work.A ‘hydrogen economy’ promises a low-carbon future but is hampered by the fact that hydrogen drastically reduces the strength of steel, which is needed for pipework and high-pressure storage. This phenomenon, which is called embrittlement, is thought to involve the accumulation of hydrogen at defects in steel.
“Whilst the phenomenon of hydrogen embrittlement has been known for more than a century, the exact origin and effective solutions are yet to be found,” explains Julie Cairney of the Australian Centre for Microscopy and Microanalysis at The University of Sydney.
Together with colleagues at CITIC Metal, the University of Science and Technology Beijing, Shanghai Jiao Tong University, and Microscopy Solutions, Cairney has found a way to determine exactly where hydrogen is trapped in steel, confirming the origins of the phenomenon and opening the way to the design of embrittlement-resistant steels [Chen et al., Science 367 (2020) 171 https://science.sciencemag.org/content/367/6474/171.abstract].
“Our work focused on observing the behavior of hydrogen in steels at the atomic scale,” say Cairney and Yi-Sheng Chen, first author of the study. “By determining precisely which microstructural features within steels interact with hydrogen and are responsible for fracture initiation, we sought to provide a more sophisticated insight into this problem.”
Hydrogen is thought to accumulate at defects and grain boundaries, leading to intergranular failure or enhanced dislocation activity, which allows cracks to grow and propagate. To determine whether this theory holds true in practice, the team turned to atom probe tomography (APT). The technique ablates the surface of a sample, detecting the atoms that are driven off, to generate a three-dimensional map of the positions of atoms within the structure to near atomic resolution.
“Coupled with a custom-developed sample preparation technique, we set out to understand the specific mechanisms that lead to hydrogen embrittlement of steel, as well as to highlight a tangible pathway to solve this problem,” says Cairney.
The researchers used an isotope of hydrogen, deuterium, to give a more unambiguous signal and a customized cryogenic sample-transfer so that the samples can be cooled to very low temperatures very quickly.
“This allows us to ‘freeze’ the hydrogen in place prior to APT observation, ensuring that the measured location is a true reflection of the hydrogen location without significant movement due to diffusion,” explain Cairney and Chen.
Not only did APT confirm that hydrogen accumulates at dislocations and grain boundaries, it also revealed that hydrogen collects at the surface of carbide precipitates present in the steel matrix.
“This exciting result demonstrates that carbide precipitates can be utilized to trap damaging hydrogen, providing a clear design pathway to create new materials that are highly resistant to hydrogen embrittlement,” says Cairney.
The research was funded by the Australian Research Council and CITIC Metal, and conducted using instruments and technical assistance provided by Microscopy Australia at the Australian Centre for Microscopy & Microanalysis at the University of Sydney, a facility funded by the University, and New South Wales and Australian Federal Governments.