Hydrogen-assisted decohesion associated with nanosized grain boundary κ-carbides in a high-Mn lightweight steel

While age-hardened austenitic high-Mn and high-Al lightweight steels exhibit excellent strength-ductility combinations, their properties are strongly degraded when mechanically loaded under harsh environments, e.g. with the presence of hydrogen (H). The H embrittlement in this type of materials, especially pertaining to the effect of kappa-carbide precipitation, has been scarcely studied. Here we focus on this subject, using a Fe-28.4Mn-8.3Al-1.3C (wt%) steel in different microstructure conditions, namely, solute solution treated and age-hardened. Contrary to the reports that grain boundary (GB) kappa-carbides precipitate only during overaging, site-specific atom probe tomography and scanning transmission electron microscopy (STEM) reveal the existence of nanosized GB kappa-carbides at early stages of aging. We correlate this observation with the deterioration of H embrittlement resistance in aged samples. While H pre-charged solution-treated samples fail by intergranular fracture at depths consistent with the H ingress depth (similar to 20 mu m), age-hardened samples show intergranular fracture features at a much larger depth of above 500 mu m, despite similar amount of H introduced into the material. This difference is explained in terms of the facile H-induced decohesion of GB kappa-carbides/matrix interfaces where H can be continuously supplied through internal short-distance diffusion to the propagating crack tips. The H-associated decohesion mechanisms are supported by a comparison with the fracture behavior in samples loaded under the cryogenic temperature and can be explained based on dislocation pileups and elastic misfit at the GB kappa-carbide/matrix interfaces. The roles of other plasticity-associated H embrittlement mechanisms are also discussed in this work based on careful investigations of the dislocation activities near the H-induced cracks. Possible alloying and microstructure design strategies for the enhancement of the H embrittlement resistance in this alloy family are also suggested. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

This study focuses on the hydrogen embrittlement behavior induced by kappa-carbide precipitation in high-Mn and high-Al lightweight steels. The presence of nanosized grain boundary kappa-carbides at early stages of aging is observed, and this phenomenon is correlated with the deterioration of hydrogen embrittlement resistance. The facile hydrogen-induced decohesion of grain boundary kappa-carbide/matrix interfaces is proposed as the mechanism for intergranular fracture. Other plasticity-associated hydrogen embrittlement mechanisms are also discussed, and possible alloying and microstructure design strategies for enhancing hydrogen embrittlement resistance are suggested.