PSI - Issue 23

I. Bacaicoa et al. / Procedia Structural Integrity 23 (2019) 33–38 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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3. Results and discussion

Fig. 3 shows the results of the fatigue tests conducted with cast specimens containing 2.2% iron. The large dispersion in the results is associated to the variation of the casting process parameter that led to different cooling rates.

Fig. 3. Statistical distributions of the fatigue cycles with 2.28% iron contents.

Fig. 4 show the fracture surface of a fatigue specimen that failed after just 22 cycles with a stress amplitude of 100 MPa. The cause of the failure was surface crack initiation and rapid crack propagation along the Fe-rich inclusions located between the surface and a large shrinkage pore. This is an example that shows that the cast skin in the alloy with 2.5% Fe contains ß -Al 5 FeSi inclusions that lead to rapid crack initiation, unlike the alloy with 0.6% Fe content that present a cast skin free of defects.

Fig. 4. Fracture surface of a specimen with 2.28% Fe failed at 100 MPa and 22 load cycles: a) global view of the fracture surface; b) Fe-rich inclusions located in the sub-surface.

The same fractured specimens was analyzed by micro computed-tomography in order to segment the three dimensional morphology of the shrinkage pores located beneath the fracture surface as well as the Fe-rich inclusions located between the dendritic arms of the shrinkage pores (see Fig. 5). Fig. 5 shows the high volume fraction and size of the shrinkage pores that interacted with the surface crack that caused the failure. It can be observed that the shrinkage pore presents large ß -Al 5 FeSi particles between the dendritic arms in zone near the fracture surface. This suggests that iron content apart from promoting rapid surface crack initiation due to the presence at the cast skin, it also accelerates the crack propagation along the Fe-rich inclusions located between the dendritic arms.