PSI - Issue 13

Xiaofei Guo et al. / Procedia Structural Integrity 13 (2018) 1453–1459 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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The primary crack at the notch tip nucleated at a much lower strain (epsilon=38%) compared to the hydrogen free condition (epsilon=58%). Furthermore, with the presence of diffusive hydrogen, multiple (tertiary and quaternary) twinning systems were frequently observed. In comparison, the hydrogen free 22Mn reveals only primary and secondary twinning systems at the maximum deformation strain. Figure 5b) shows the microstructure features from 17Mn_Al. The twins nucleated at higher deformation strains, in which the primary twins showed up when the strain reached 18%. More tangled dislocations were observed, which formed dislocation cells (D-cells). With the presence of diffusive hydrogen, the 17Mn_Al material also exhibited more planar dislocations (strain level of 14%) and larger volume of deformation twins (strain level of 17-18%) at the comparable deformation strain. The secondary twinning system also appeared in lower strain. The observed phenomenon indicated that the Al free grade showed earlier twinning nucleation and broadening than the Al-alloyed 17Mn_Al, although according to thermodynamic calculation they have almost the same stacking fault energy. The charged hydrogen led to dislocation planarity, facilitated the formation of stacking faults and deformation twins at lower strains. Accordingly, the twinning broadening and multiple twinning systems also appeared at earlier stages. The EBSD investigation at the primary cracks near the crack tip region clarified the crack nucleation mechanisms in two high manganese steels with different chemistry and hydrogen concentration. The Al alloyed grade showed more ductile fracture mode and higher fracture strain in both hydrogen free and hydrogen charged conditions. In the Al free 22Mn grade, hydrogen was observed effectively degrading the grain boundary properties, leading the trans granular dominated fracture mode to inter-granular dominated fracture mode. The crack propagation path also changes from stepwise routine to a straight strain gradient driven routine. It indicated that, without the presence of diffusive hydrogen, crack propagation was both grain orientation and strain gradient dependent. With diffusive hydrogen, crack propagation was more strain gradient dependent. The grain boundary energy from 22Mn is much more affected by hydrogen than that from the 17Mn_Al. In combination with the ECCI characterization of the microstructures, hydrogen induced dislocation planarity and early formation of stacking faults, deformation twinning, twinning broadening and multiple twinning systems had been revealed in the Al free 22Mn material. The Al alloying postponed the formation of stacking fault energy and deformation twinning to larger deformation strains. The twin lamellae in the Al alloyed grades were very thin compared to the 22Mn, which may due to the late nucleation of deformation twins. The interaction of thin twin bundles with grain boundaries would lead to less lattice distortion and less stress concentration at the grain boundaries. In addition, less twinning systems in both Al alloyed or hydrogen free 22Mn specimens contributed to less stress localization, which rendered the material degradation. This work investigated the influences of hydrogen on dislocation and twinning behaviors in high manganese steels Fe-22Mn-0.6C and Fe-17Mn-1.5Al-0.6C steels. Diffusive hydrogen at the level of 22-26 ppm had a conspicuous effect on promoting dislocation planarity, initiating early formation of stacking faults, deformation twinning in both materials. Additionally, hydrogen facilitated twinning broadening and activated multiple twinning systems in Fe-22Mn-0.6C steel. In comparison, hydrogen does not obviously affect the microstructure evolution with respect to the twinning thickness and the amount of twinning systems in Fe-17Mn-1.5Al-0.6C grade. Eventually, the interactions between deformation twins and grain boundaries lead to stronger grain boundary decohesion in Fe-22Mn-0.6C grade. The Al added grade revealed a postponed nucleation of deformation twins, delayed onset of the secondary twinning system and developed finer twinning lamellae in comparison to the Al free grades. These observed features contributed to the improved resistance to hydrogen induced cracking in this Al-alloyed TWIP grade. 4. Discussion 5. Conclusion

Acknowledgements

The authors gratefully acknowledge the financial support of Deutsche Forschungsgemeinschaft (DFG) within the Collaborative Research Centre (SFB) 761 “St eel - ab initio; Quantum mechanics guided design of new Fe based