PSI - Issue 23

Mattias Calmunger et al. / Procedia Structural Integrity 23 (2019) 215–220 M. Calmunger et al. / Structural Integrity Procedia 00 (2019) 000–000

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a)

100 µm

b)

111

001

101

100 µm

Crack initiation occured from this side Fig. 3. Specimen built in the 90 ◦ tested in LCF at ∆ ε = 1 . 3 %: a) ECCI image and b) EBSD map overlayed on the ECCI image. The melt pool lines have been outlined in b). The build direction is perpendicular to the viewing plane.

3.2. Observations on the fracture surface

Fractography revealed that the fatigue crack propagation portion of the fracture surface did contain striations, as expected. In addition, regions containing striation-like features could also be identified on some specimens, as shown in Fig. 4. These features are not necessarily actual striations; particularly the abrupt change in direction (see Fig. 4 a)) suggests that the features do not necessarily come from a propagating crack front. These striation-like features agree roughly in size with the cellular dendrites (in the order of 1 µ m). Furthermore, at higher magnification, it appears that the sharp change in direction, marked by arrows in Fig. 4 b), are areas where a new bundle of cellular dendrites may have nucleated from an existing one. The cellular dendrites have been shown to influence the fracture surfaces from tensile tests. Zhong (2017) reported that dendrite boundary tearing resulted in “strips” on the fracture surface and Wang et al. (2016) have reported that dimples from ductile fracture agrees in size with the cellular dendrites. To further explore the striations and striation-like features, the underlying microstructure was studied by cutting and polishing the specimens as described in Section 2. Figure 5 shows the fracture surface and corresponding underlying microstructure (i.e. view II and III, see Fig. 1) for a specimen with 0 ◦ build direction tested at ∆ ε = 1 . 3 %. The figure has been produced by overlaying an ECCI image and an EBSD map on secondary electron (SE) images (the SE images captured the edge better). As seen in Fig. 5 a), the microstructure below the fracture surface shows signs of heavy plastic deformation as also seen in the EBSD map in Fig. 5 b). The EBSD did not reveal any correlation between the striations and the underlying grain structure or cellular dendrite boundaries, see Fig. 5 b). The crack propagation, in the displayed region, thus correspond to incremental crack growth via the formation of striations. Figure 6 shows the corresponding results for a specimen with 90 ◦ build direction tested at ∆ ε = 1 . 3 %. The region immediately below the fracture surface is not as heavily deformed as the specimen built in 0 ◦ . Compare the EBSD 3.3. Observations on the cross sectioned fracture surfaces