Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 225 Figure 8: Correlation between mass loss and material hardness. As aforementioned, the strengthening of the Al matrix after heat treatment improves its cavitation erosion resistance [14, 19-20] and, consequently, a low degree of plastic deformation is present on the surface of AlSi9 T6 sample (Fig. 9c) where, at this stage, it is not possible to distinguish between different phases. Figure 9: Eroded surfaces after 2 minutes of cavitation test for a) AlSi3, b) AlSi9, c) AlSi9 T6 alloys. Mg Al Si 1 1.44 96.63 1.93 2 95.52 4.48 3 0.81 85.68 13.51 4 80.59 19.41 5 98.14 1.86 Table 6: EDS analysis (wt. %) for SEM images in Fig. 9. After 5 min of testing, the morphology of eroded surfaces of AlSi3 and AlSi9 alloys (Fig. 10a-b) are similar and a corrugation of the eutectic phase is now visible for AlSi9 alloy. On the other hand, after 5 min, the heat-treated sample (Fig. 10c) shows only a slight undulation of the surface. Nevertheless, in comparison with the previous analysis performed after shorter exposure time, some pits appeared due to the repetition of micro-jets. The slower erosion of AlSi9 T6 sample is consistent also with the higher incubation period, recorded from mass loss measurements (Tab. 5). The influence of microstructure can be no longer observed after 30 min of cavitation exposure (Fig. 11). In fact, at this stage, the erosion mechanism was already at an advanced stage and all the investigated alloys exhibited a certain mass loss (Fig. 5-6). The corrugation of the eroded surfaces appears uniform and with a morphology that resembles that of ductile fracture [19].