Atomistic simulation of hydrogen-assisted ductile-to-brittle transition in alpha-iron

X Xing and MS Yu and WX Chen and H Zhang, COMPUTATIONAL MATERIALS SCIENCE, 127, 211-221 (2017).

DOI: 10.1016/j.commatsci.2016.10.033

Previous atomistic simulations of hydrogen embrittlement in bcc iron have provided atomistic mechanisms for crack propagation and been used to develop predictive models. In order to bridge the nano scale simulations to realistic field operations, in current study molecular dynamics (MD) simulations were first used to predict a hydrogen concentration that corresponds to ductile to brittle transition (DTB). The estimated hydrogen concentration was then used to assess the critical hydrogen concentration that induces DTB transition in experimental specimens. Apparently, the calculated critical hydrogen concentration is much larger than the actual concentration found in pipeline steel under routine operations. Recent experiments found that while the presence of minor cycles (i.e., R = K-min/K-max > 0.9) in underload spectra would only make small contribution to crack propagation in air, they could enhance crack growth significantly in the presence of hydrogen atoms. Therefore, it is reasonable to expect that minor cycles have a significant effect on accumulating hydrogen atoms in the plastic zone and inducing a DTB transition in the aqueous environment where diffusible hydrogen atoms can be generated. Based on this hypothesis, the theoretical critical hydrogen value obtained from atomistic simulation was then used to calculate the number of hydrogen atoms required to saturate the plastic zone. Furthermore, the number of minor cycles required to accumulate hydrogen atoms to cause a DTB transition could be assessed. The prediction based on this model and experimental results are in good agreement. The atomistic mechanism for DTB due to the accumulation of hydrogen during cyclic loading validated in current study implies the importance of the loading spectrum in hydrogen-assisted cracking. (C) 2016 Elsevier B.V. All rights reserved.

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