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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Figure 4. The primary cracks at the notch tips of the interrupted tensile specimens, revealed by inverse pole figures from EBSD measurements: a) 22Mn; b) 17Mn_Al.

3.5. Dislocation and twinning evolution at different strain levels Figure 5 reveals ECCI images of the deformation microstructure of the interrupted tensile specimens between the center of each specimen and the region of the notch tip. The Von-Mises strains are indicated on the right corner of the images. Figure 5a) shows the representative images from material 22Mn. In the hydrogen free condition, tangled dislocations (tangled-D) and dilute planar dislocations (planar-D) were observed in the region of 12% strain. As the strain increases to 15%, the dislocations became more aligned taking a planar shape, especially in the region where the primary twins (TW) were observed. At the strain of 21%, more twin lamellae formed inside single grains, which became broadening with the increased strain. At high deformation strains, particularly near the crack tip region, secondary twinning systems were activated. In the hydrogen charged condition, both stacking faults (SF) and twin bundles appeared at lower strain levels. The twins nucleated at an earlier stage, in which the secondary twinning system appeared below the strain of 16%.

Figure 5. Microstructure evolution at different strain levels in the interrupted tensile specimens: a) 22Mn and b) 17Mn_Al.