PSI - Issue 23

Joakim Nordström et al. / Procedia Structural Integrity 23 (2019) 457–462 Joakim Nordström / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 4: Influence of plastic deformation on the twinning and detwinning process during deformation at CT: a) 0.5 mm from the fracture, b) 2.5 mm from the fracture and c) 10 mm from the fracture. 3.4 Influence of deformation twins on damage behavior in necking region The damage behavior in Alloy 625 during tensile testing at RT and at CT has been studied by electron channelling contrast imaging (ECCI) analysis, the images are shown in Figure 5. As already mentioned, deformation twins form continuously during necking at RT. This will contribute to further plastic deformation, i.e. necking elongation and contraction. On the other hand, these newly formed twins will interact with moving dislocations that cause stress concentration and finally local damage (Fig. 5a). At CT, the pre-formed twin boundaries will act as obstacles to stop dislocation movements (Fig. 5b), which will pile-up at twin boundaries (Fig. 5b). This will also cause high stress concentrations at twin boundaries that later will cause a transfer of twin boundaries to large angle grain boundaries. The pile-up dislocations at twin boundaries will also lead to local stress concentrations that cause the damage or crack initiation.

Figure 5: Interaction between twinning and moving dislocations: a) at RT and b) at CT.

3.5 Fracture morphology Fractures of Alloy 625 tested at both RT and at CT show as ductile, it is however more obvious at RT. The fracture at RT shows as typical cup-cone (Fig. 6a-c). In the middle part, large deep equiaxed dimples with small precipitates can be observed (Fig. 6b). At the shear fracture area, large deep shear dimples can be observed (Fig. 6c). Both areas show that this fracture is still tough and ductile. The fracture of Alloy 625 tested at CT shows a rather different morphology, although it is a ductile fracture (Fig. 6 d-f). The dimples are small and shallow and the number of voids in the dimples are drastically reduced compared to at RT. Strain induced martensite has also been reported to change the morphology of the dimples near the fracture (Das, A., S. et al.,2008) The phenomena: deformation twinning and martensite formation are not far from each other, since they are both intimately connected to the stacking fault energy (SFE) and succeed each other on the SFE-ladder. In the middle part, the dimples are small and uneven with many lines from the dimple boundaries (Fig. 6e). In the shear fracture region, river like pattern fracture is shown (Fig. 6f). However, it is not a cleavage fracture, i.e. they consist of dimples with river-like patterns. One explanation can be that these river-lines can be twin boundaries.