PSI - Issue 23

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

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M. Calmunger et al. / Structural Integrity Procedia 00 (2019) 000–000

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a)

b)

20 µm

3 µm

Fig. 4. Fracture surface after fatigue failure at room temperature with ∆ ε = 1 . 3 %, R = 0, 90 ◦ building direction. a) The substructure of the AM material is visible on the fracture surface which could be mistaken as striations. b) Area where a packet of subgrains could have nucleated from an existing one.

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b) Fracture sur face, view II

a) Fracture sur face, view II

Cross section, view III

Cross section, view III

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30 µm

30 µm

Fig. 5. Fracture surface and corresponding underlying microstructure for a specimen built in 0 ◦ and tested in LCF at ∆ ε = 1 . 3 %: a) overlayed ECCI image and b) overlayed EBSD map. The white arrows points out the heavy plastic deformation and striations respectively (black arrows show how the fracture surface relates to the cross section).

maps in Fig. 5 b) and Fig. 6 b), where the latter show less distorted crystal structure, confirming a lower degree of plastic deformation at the subsurface region, cf. Lundberg et al. (2017). As seen in Fig. 6 a), the cellular dendrite structure is visible just below the fracture surface. Furthermore, the longitudinal axis of the columnar dendrites roughly align with the striations indicating that the fracture might have occurred, to some degree, by interdendritical tearing, see e.g. Zhong (2017). As shown by Lindstro¨m et al. (2018), the 90 ◦ build direction gave longer fatigue lives than 0 ◦ . A possible explanation may be that the crack growth increment, locally, is forced to comply to the dendrite size. As shown by Zhong (2017), the dendrite size influences the material strength through the Hall-Petch e ff ect. It is possible that interdendritic boundaries may retard crack growth resulting in a, at least locally, slower crack growth rate in the 90 ◦ build direction. As seen in Fig. 6 a), the striation spacing immediately above the cellular dendritic structure is locally very small.

4. Conclusions

In this paper, the fracture appearance and crack propagation of an AM nickel-based superalloy, similar to Hastelloy X, subjected to LCF loading at room temperature have been investigated. The following could be concluded: