Origin of the trade-off relationship between tensile strength and hydrogen embrittlement resistance

To achieve widespread application of high-strength steels, such as martensitic steels, understanding the origin of the trade-off relationship between tensile strength and hydrogen embrittlement resistance is crucial. This study aimed to clarify the origin of the trade-off from a perspective of sensitivity to the hydrogen-induced intergranular fracture, which mainly occurs along prior austenite grain boundaries (PAGBs) among several types of high-angle boundaries in martensitic structures. Applying a digital image correlation (DIC)-based stress mapping technique, the distribution of the local stress during tensile loading was correlated with the martensitic structures; the stress concentration behaviors at PAGBs were compared among three as-quenched martensitic steels with different tensile strengths. The tensile strength, ranging from 1.25 to 1.65 GPa, was controlled by changing carbon composition and austenitization temperature (and resultant PAG size) in Fe-3Mn-C (wt.%) alloys. The intergranular fracture preferentially occurring at PAGBs is due to the low plastic relaxation ability of PAGBs relative to other substructure boundaries (within PAGs) and the resultant stress concentration at PAGBs. Additionally, it was demonstrated that as plastic relaxation at PAGBs crystallographically becomes more difficult relative to within PAGs, the consequent stress concentration at PAGBs increases, leading to more severe hydrogen embrittlement. The results demonstrated both the origin of the trade-off (i.e., carbon composition) and a strategy for breaking the trade-off (i.e., PAG refinement), and both can be understood in terms of the shared factors: sensitivity to the intergranular fracture, strongly depending on the local stress concentration at PAGBs related to variant selection behavior in the martensitic transformation.