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 2: Grain orientations and geometry analysed with EBSD. IPF maps over: a) the as received microstructure, b) the microstructure from the homogeneous deformation zone at RT, c) the microstructure from the homogeneous deformation zone at CT. 3.3 Influence of deformation on twinning and detwinning process Figure 3 shows the influence of plastic deformation on the twinning process in Alloy 625 during the tensile testing at RT. In the homogeneous plastic deformation range (Fig. 3c, 10 mm from the fracture), the deformation occurred mainly due dislocation slip, but small amounts of twins have also formed (red points and lines). In the necking region (Fig. 3b, 2.5 mm from the fracture), more deformation twins can be observed. Near the fracture (Fig. 3a), a significant increase of the deformation twin density can be observed (red lines). These observations show that occurrence of necking during deformation at RT is accompanied by the formation of deformation twins. This can explain a larger necking elongation for this alloy during the tensile testing at RT.

Figure 3: Influence of plastic deformation on the twinning and detwinning process during deformation at RT: a) 0.5 mm from the fracture, b) 2.5 mm from the fracture and c) 10 mm from the fracture.

Deformation behavior at CT is quite different (Figure 4). In the homogeneous deformation range (Fig. 4c, 10 mm from the fracture), the deformation twin density is already high. This indicates that deformation twinning is the main deformation mechanism in the region. Near the necking (Fig. 4b, 2.5 mm from the fracture), the density of deformation twins changes relatively small. This means that deformation by dislocation slip can become dominant. Some red lines have changed color and transferred to black. This indicates that some twin boundaries have transferred into large angle grain boundary (Chai G, 2016). Near the fracture, black lines (large angle grain boundaries) becomes dominant. At this stage, twining rate is now very small, and the detwinning rate becomes higher. Most deformation twin boundaries have transferred into large angle grain boundaries.

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