This study investigated the characteristics of hydrogen-related crack propagation in low carbon lath martensite steel through orientation analysis using electron backscattering diffraction. The orientation analysis revealed that the hydrogen-related fracture surface consisted of the {011}M facets, which were also parallel to the block boundaries or lath boundaries in the lath martensite structure. In addition, micro-cracks were observed on or in the vicinity of the prior austenite grain boundaries. On the basis of the experimental results, we proposed that hydrogen enhanced local plastic deformation occurred in the vicinity of prior austenite grain boundaries. The mechanism of hydrogen-related fracture is characterized by the formation of micro-cracks around prior austenite grain boundaries and subsequent crack propagation along block boundaries or lath boundaries.

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Hydrogen-related fracture behaviors in low-carbon (Fe-0.1wtpctC) and medium-carbon (Fe-0.4wtpctC) martensitic steels were characterized through crystallographic orientation analysis using electron backscattering diffraction. The martensitic steels with lower strength (Fe-0.1C specimen or Fe-0.4C specimen tempered at higher temperature) exhibited transgranular fracture, where fractured surfaces consisted of dimples and quasi-cleavage patterns. Crystallographic orientation analysis revealed that several of the micro-cracks that formed around the prior austenite grain boundaries propagated along {011} planes. In contrast, fracture surface morphologies of the martensitic steels with higher strength (Fe-0.4C specimen tempered at lower temperature) appeared to be intergranular-like. Crystallographic orientation analysis demonstrated that, on a microscopic level, the fracture surfaces comprised the facets parallel to {011} planes. These results suggest that the hydrogen-related fractures in martensitic steels with higher strength are not exactly intergranular at the prior austenite grain boundaries, but they are transgranular fractures propagated along {011} planes close to the prior austenite grain boundaries. A description of the mechanism of hydrogen-related fracture is proposed based on the results.

Anaerobic methane-oxidizing archaea (ANME) are known to play an important role in methane flux, especially in marine sediments. The 16S rRNA genes of ANME have been detected in terrestrial freshwater subsurfaces. However, it is unclear whether ANME are actively involved in methane oxidation in these environments. To address this issue, Holocene sediments in the subsurface of the Kanto Plain in Japan were collected for biogeochemical and molecular analysis. The potential activity of the anaerobic oxidation of methane (AOM) (0.38-3.54 nmol cm⁻³ day⁻¹) was detected in sediment slurry incubation experiments with a (13) CH(4) tracer. Higher AOM activity was observed in low-salinity treatment compared with high-salinity condition (20‰), which supports the adaptation of ANME in freshwater habitats. The 16S rRNA sequence analysis clearly revealed the presence of a distinct subgroup of ANME-1, designated ANME-1a-FW. Phylogenetic analysis of the mcrA genes also implied the presence of the distinct subgroup in ANME-1. ANME-1a-FW was found to be the most dominant active group in the archaeal communities on the basis of 16S rRNA analysis (75.0-93.8% of total archaeal 16S rRNA clones). Sulfate-reducing bacteria previously known as the syntrophic bacterial partners of ANME-1 was not detected. Our results showed that ANME-1a-FW is adapted to freshwater habitats and is responsible for AOM in terrestrial freshwater subsurface environments.

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