Hydrogen embrittlement in a metastable high Mn TWIP-assisted steel: Correlation between grain size and hydrogen-enhanced ε-martensite

The correlation between hydrogen-enhanced deformation-induced epsilon-martensite and hydrogen embrittlement at two different grain sizes has been investigated in a metastable high Mn steel. The variation in the fraction of epsilon-martensite was detected to be 34% for coarse grain (CG) and 10% in (FG) after hydrogen charging. The significant difference in epsilon-martensite fraction between CG and FG was attributed to a higher concentration of hydrogen per unit area (C-H(gb)) of high-angle boundaries (theta > 15(degrees)) in CG compared with FG one, which caused a local decrease of stacking fault energy (SFE) at the vicinity of these boundaries in favor of martensitic trans-formation. Intergranular cracks were identified as the primary form of hydrogen damages, occurring more frequently than transgranular features in both alloys. The drastic hydrogen embrittlement in CG was attributed firstly to higher hydrogen concentration per grain boundary unit area and secondly to the more severe hydrogen cracking in CG than FG resulting from explosive hydrogen-enhanced martensite formation. In the FG , defor-mation twins and single-variant epsilon-martensite were the primary mechanisms triggering intergranular crack nucleation. However, they played a dual role in crack propagation, either arresting or facilitating crack growth. They were capable of arresting the crack if suitably aligned with the direction of crack propagation and the tensile axis. Conversely, in CG structure, the intense impingement of multiple-variant epsilon-martensite grains with each other and with grain boundaries can create severe microstructural stress concentration zones, promoting severe hydrogen-induced cracking.