Ultra-Fine-Scale Observation of Trapped Hydrogen using Atom Probe Tomography

Hydrogen embrittlement poses a significant challenge to the durability of structural steels, a crucial factor in the advancement of the hydrogen economy. A potential remedy involves integrating hydrogen traps into the microstructure of steel. The efficacy of this approach is greatly influenced by the capacity of these hydrogen traps.<br/><br/>Theoretical models suggest carbon vacancies within metal carbide precipitates exhibit notable effectiveness as hydrogen traps in ferritic steels. In contrast, conventional structural defect traps such as dislocations and iron lattice vacancies have limited trapping energies and uncertain impact on countering hydrogen embrittlement. Therefore, enhancing the population of carbon vacancies within metal carbides holds significance. By introducing abundant metal carbides into steels, which can be feasibly achieved, the overall hydrogen trapping capacity can be substantially increased.<br/><br/>In order to validate this concept of material design, we produced two variants of titanium-microalloyed ferrite steels. The first variant contained stoichiometric titanium carbides (TiC) lacking carbon vacancies, serving as a reference. The second variant was an experimental steel infused with a small quantity of molybdenum (Mo). This addition led to the formation of Ti-Mo carbides, characterized by substitutional Mo atoms alongside carbon vacancies within the same TiC lattice.<br/><br/>We then used atom probe tomography to experimentally observe the hydrogen trapping behaviors of both the reference and Ti-Mo carbides. The outcomes demonstrate a shift in the hydrogen trapping mechanism within the metal carbide precipitates upon Mo addition. This alteration allows hydrogen to access the carbon vacancy traps within the carbide structure, causing a substantial escalation of trapping capacity. The identification of the influence of carbon vacancies on hydrogen trapping, achieved through a straightforward Mo addition, holds valuable implications for the development of carbide-strengthened steels with enhanced compatibility for hydrogen applications.