Atomic-scale observation of hydrogen trap sites in bainite–austenite dual-phase steel by APT

Abstract

It was reported by the thermal desorption analysis (TDA) of hydrogen that bainite–austenite dual-phase steels showed two activation energies of ~30 kJ/mol and ~ 47 kJ/mol under low- and high-fugacity conditions, respectively, and the latter energy corresponded to hydrogen diffusion in austenite; however, the hydrogen trap sites of the former energy were unclear (Sekine et al., 2019, 2018 [ 1 , 2 ]). The hydrogen trap sites under the low-fugacity condition in the steel were investigated by directly observing deuterium-charged specimens using atom probe tomography (APT). The charged deuterium atoms were distributed in carbon-rich regions of the fine irregular structure in the bainite region of the steel. The carbon concentrations in these region were in the range of 12–20 at.% without enrichment of other alloying elements. The diffraction analysis with transmission electron microscopy (TEM) indicated the presence of a substantial amount of fine precipitates of epsilon carbide in the bainite region of the steel. Hence, the hydrogen trap sites were considered to be associated with fine precipitates of epsilon carbide (ε-carbide) with carbon compositions marginally lower than the stoichiometric composition. First principles calculations suggested that the hydrogen trap sites in the steel originated from the carbon vacancies in the surface region of ε-carbide. • Hydrogen trap sites in bainite–austenite dual-phase steel were investigated by APT. • Charged deuterium atoms were observed in carbon-enriched regions in the bainite. • TEM analysis indicated the presence of a large amount of fine ε-carbide precipitates. • The trap energies in ε-carbide were investigated by first principles calculation. • Carbon vacancies in the surface region of ε-carbide are probably the trap sites.