PSI - Issue 13

Tsubasa Kumamoto et al. / Procedia Structural Integrity 13 (2018) 710–715 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 4. Fractographs of the specimen deformed at an initial strain rate of (a) 10 − 4 s − 1 without hydrogen charging and at initial strain rates of (b, b′) 10 − 4 s − 1 and (c) 10 − 2 s − 1 with hydrogen charging.

4. Conclusions

We investigated the strain rate sensitivity of the micro-damage evolution behavior in a ferrite/martensite dual-phase steel pre-charged with hydrogen. The following conclusions were obtained.  At a strain rate of 10 − 4 s − 1 , the hydrogen pre-charging caused a reduction in elongation and assisted the damage evolution, as observed in the previous study. The damage nucleation rate per strain became twice and the damage arrest regime shortened when hydrogen was introduced. High-resolution observations indicated the acceleration of damage nucleation and the deterioration of the damage arrestability.  The negative effects of hydrogen disappeared when the strain rate was increased from 10 − 4 to 10 − 2 s − 1 ; thus, hydrogen pre-charging did not affect the elongation at 10 − 2 s − 1 . More specifically, the strain rate dependence of the damage arrestability in ferrite is one factor causing the difference in the elongation between the two strain rates with hydrogen.  The strain rate sensitivity is associated with the increase in the damage number density at a given strain and the damage arrestability in ferrite. In particular, the deterioration of the damage arrestability can be caused by a reduction in the critical strain for damage coalescence or propagation owing to localized plasticity in the ferrite matrix at the damage tip whose behavior depends on hydrogen diffusion kinetics. The kinetic effect of hydrogen can explain the strain rate sensitivity of the hydrogen-assisted damage evolution behavior. Acknowledgements This work was financially supported by JSPS KAKENHI (JP16H06365 and JP17H04956) and the Japan Science and Technology Agency (JST) (grant number: 20100113) under Industry-Academia Collaborative R&D Program. Avramovic-Cingara, G., Saleh, C.A.R., Jain, M.K., Wilkinson, D.S., 2009. Void nucleation and growth in dual-phase steel 600 during uniaxial tensile testing. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 40, 3117 – 3127. Davies, R.G., 1981. Hydrogen embrittlement of dual-phase steels. Metall. Trans. A 12, 1667 – 1672. Koyama, M., Tasan, C.C., Akiyama, E., Tsuzaki, K., Raabe, D., 2014. Hydrogen-assisted decohesion and localized plasticity in dual-phase steel. Acta Mater. 70, 174 – 187. Marteau, J., Haddadi, H., Bouvier, S., 2013. Investigation of Strain Heterogeneities Between Grains in Ferritic and Ferritic-Martensitic Steels. Exp. Mech. 53, 427 – 439. Park, K., Nishiyama, M., Nakada, N., Tsuchiyama, T., Takaki, S., 2014. Effect of the martensite distribution on the strain hardening and ductile fracture behaviors in dual-phase steel. Mater. Sci. Eng. A 604, 135 – 141. Senuma, T., 2001. Physical Metallurgy of Modern High Strength Steel Sheets. ISIJ Int. 41, 520 – 532. Tasan, C.C., Hoefnagels, J.P.M., Geers, M.G.D., 2012. Identification of the continuum damage parameter: An experimental challenge in modeling damage evolution. Acta Mater. 60, 3581 – 3589. Uehata, N., Koyama, M., Takagi, S., Tsuzaki, K., 2018. Optical Microscopy-Based Damage Quantification: an Example of Cryogenic Deformation of a Dual-Phase Steel. ISIJ Int. 58, 179 – 185. Yan, D., Tasan, C.C., Raabe, D., 2015. High resolution in situ mapping of microstrain and microstructure evolution reveals damage resistance criteria in dual phase steels. Acta Mater. 96, 399 – 409. References