New Delhi: Hydrogen embrittlement could put the brakes on the safe use of high-strength (HS) steel in hydrogen-powered vehicles, according to a new study. Upon exposing HS steel to rainwater (H2O) or hydrogen, the diffusion of hydrogen atoms into into its lattice structure creates intergranular (IG) fractures and progressively weakens it, making it more prone to mechanical failure.
A research team at Sophia University, Tokyo, Japan made studying these fractures in isolation possible, initially difficult because other types of fracture tend to occur alongside of it. They describe their findings in the journal Scripta Materialia.
By devising an alternative to the conventional tensile tests, the scientists were able to produce pure-IG fractures on embrittled HS steel samples, enabling them to study these fractures in unprecedented detail.
Conventional tensile tests in metals involve placing a dog-bone-shaped sample under increasing tension until it breaks, which causes multiple types of fracture besides IG fractures, such as quasi-cleave fractures, dimples, and shear lips.
However, the test developed by the scientists at Sophia involved 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 under hydrogen charging and the re-applying it,” said Kenichi Takai, co-author of the study.
The proposed load reduction test successfully produced pure IG fractures, confirmed by scanning electron microscopy (SEM) images.
According to the scientists, 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 understand the lattice defects present right below the fracture, lower-temperature thermal desorption spectroscopy (L-TDS) was used on small pieces of broken samples extracted from very close to the fracture surface.
L-TDS observes the rate of desorption of a gas from the material at different temperatures, in turn providing information about the number and types of defects present in it.
“L-TDS enabled us to distinguish hydrogen trapping sites on the atomic scale,” said 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,” said Takai.
The scientists performed further experiments to analyse the SEM images for the role of plasticity in the formation of vacancies.
Analyses revealed that local plastic deformation occurs right below the IG fracture caused by hydrogen embrittlement.
HS steels are the perfect candidate materials for making vehicles lighter to achieve energy reduction in transportation.
However, the phenomenon of hydrogen embrittlement needs to be understood better and suppressed before HS steel can be used in hydrogen-powered vehicles.
