PSI - Issue 13

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

1456

4

On the other hand, the high temperature peak is associated with the high-energy hydrogen traps, such as twinning lamellae, voids and inclusions. The amounts of hydrogen under each desorption peak were fitted using peak-fitting according to Lorenz method by the software Origin™, as indicated by the dashed curves in Figure 2. The 22Mn contains 0 ppm diffusive and 0.5 ppm trapped hydrogen in the as annealed condition. After charging, the material obtains 26.0 ppm diffusive hydrogen and 5.9 ppm trapped hydrogen. In comparison, 17Mn_Al has 2.4 ppm trapped hydrogen in the as-annealed condition. After charging, it contains 22.6 ppm diffusive and 4.8 ppm trapped hydrogen. 3.3. Interrupted tensile test with notched tensile specimens Figure 3 reveal the tensile stress-displacement curves of as annealed and hydrogen charged specimens from 22Mn and 17Mn_Al. In both materials, the tensile strength of the as annealed material is higher than hydrogen charged ones. The measured potential drop shows the opposite trend, which indicates that the hydrogen charging induced early nucleation of primary cracks. The tensile tests were interrupted when the potential drop reached ~400 mV for both hydrogen uncharged and charged condition. The strain distribution in the interrupted specimens were measured by digital image correlation with GOM optical system. Figure 3c) shows the distribution of Von-Mises strain from notch tip to specimen centre. The strain is homogenous in the centre, whereas the notch tip region has steep strain gradient.

Figure 3. a) Tensile curves and potential-evolutions from the interrupted tensile specimens of 22Mn; b) Tensile curves and potential evolutions from the interrupted tensile specimens of 17Mn_Al; c) Strain distribution in the interrupted tensile specimens. 3.4. Crack propagation in the notched tensile specimens Figure 4 demonstrates the EBSD inverse pole figure maps at the notch tips with respect to the tensile direction. In Figure 4a), the hydrogen-free 22Mn material revealed stepwise crack propagation from the notch tip, which followed a trans - granular dominated crack routine. The crack in the hydrogen charged 22Mn propagated straight along the strain gradient and exact vertical to the tensile axis. It demonstrated an inter-granular dominated propagation path, indicating strong hydrogen degradation of the grain boundaries. In Figure 4b), no obvious cracks were observed in both hydrogen free and hydrogen charged 17Mn_Al although the potential drop also reached 400 mV. The small cracks at the notch tip blunted during further straining the specimens. The material revealed elongated grains along the tensile direction, which were homogenously deformed.