TOKYO—One of the many ways to reduce the energy required for transportation is to make vehicles lighter. High-strength (HS) steels are perfect materials for this purpose, as their higher weight-to-strength ratio allows for the use of less metal to achieve a similar structural integrity. Many automobile companies believe HS steels will be essential for building cars in the future.
However, for this to become a reality, a glaring problem must be solved. When HS steel is exposed to water or hydrogen, a phenomenon known as hydrogen embrittlement occurs. Hydrogen atoms diffuse into the lattice structure of the material and progressively weaken it, making it more prone to failure under mechanical stress.
Scientists have observed that hydrogen embrittlement in HS steel often leads to intergranular (IG) fractures, which occur along the grain boundaries of the crystalline lattice. Unfortunately, the underlying mechanisms behind this particular type of fracture are difficult to study in isolation in HS steel because other types of fracture tend to occur alongside it.
Now, researchers at Sophia University in Japan have conducted detailed experiments on fractures induced on embrittled HS steel specimens. Their findings shed light on the types of defects found in the crystalline structure of fractured HS steel at the atomic scale. This knowledge will help engineers understand hydrogen embrittlement better and possibly pave the way to methods to suppress it.
Thanks to an ingenious alternative to the conventional tensile tests used to assess the mechanical properties of materials, the researchers managed to produce almost-pure IG fractures on embrittled HS steel samples. In turn, this enabled them to study these fractures with unprecedented detail.
In conventional tensile tests for metals, a dog-bone-shaped sample is put under increasing tension until it breaks. This causes multiple types of fracture besides IG fractures, such as quasi-cleave fractures, dimples and shear lips. To prevent this, the researchers came up with an original mechanical test involving repeated loading and unloading of the sample during hydrogen charging.
“Our load reduction test was designed to progressively reduce the material’s ultimate tensile strength (UTS). We achieved this by repeatedly removing the load applied to the specimen immediately after the tensile stress reached the UTS…and then reapplying it,” explains Kenichi Takai, Ph.D., professor of materials science at Sophia University and lead researcher on the project.
As confirmed by scanning electron microscopy (SEM) images, the proposed load reduction test successfully produced pure IG fractures. The team believes this happens because, after each unloading step, hydrogen atoms are given enough time to fill up the new cracks generated in the material to keep advancing the fracture exclusively along the grain boundaries.
To gain insight into the lattice defects present right below the fracture, the researchers carefully extracted small pieces of the broken sample very close to the fracture surface and used them for lower-temperature thermal desorption spectroscopy (L-TDS). This technique involves observing the rate of desorption of a gas (hydrogen, in this case) from the material at different temperatures, which in turn provides information about the number and types of defects present in it.
“L-TDS enabled us to distinguish hydrogen trapping sites on the atomic scale,” says Takai. “Obtaining such basic knowledge about the lattice defects formed in the local area just below an IG fracture surface will provide important clues to understand and potentially suppress hydrogen embrittlement in HS steel.”
In a final set of experiments, the team performed various analyses on SEM images to determine whether the formation of the vacancies and vacancy clusters observed via L-TDS involved plasticity. These analyses revealed that some vacancies coalesced into nano-voids, and that the martensite laths and blocks in the regions around these voids were heavily distorted and difficult to tell apart. This suggests that local plastic deformation occurs right below the IG fracture caused by hydrogen embrittlement.Overall, the findings of this study will help materials scientists understand hydrogen embrittlement in HS steel better. With luck, this will pave the way to new methods to suppress it and enable the safe use of HS steel in hydrogen-powered vehicles.