Issue 43

M. Tocci et alii, Frattura ed Integrità Strutturale, 43 (2018) 218-230; DOI: 10.3221/IGF-ESIS.43.17 221 Figure 2 : Typical microstructure of a) AlSi3, b) AlSi9, c) AlSi9 T6 alloys. Analogously to AlSi9 alloy, AlSi9CuFe alloy is characterized by a dendritic microstructure surrounded by eutectic phase in as-cast condition (Fig. 3a), while the heat treatment causes the spheroidization of Si particles (Fig. 3b). Figure 3 : Typical microstructure of a) AlSi9CuFe, b) AlSi9CuFe T6 alloys and c) , d) details of intermetallic particles by SEM analysis. For this alloy, analyses at higher magnification were carried out by means of SEM in order to identify the presence and chemical composition of intermetallic particles, due to the addition of Cu and Fe. As shown in Fig. 4, in fact, several intermetallic compounds with various morphologies (Chinese-script, acicular) were detected. Analyses by EDS probe (Tab. 2) confirmed the presence of significant content of Fe in the composition of most of these coarse particles, while Cu-based intermetallic phases (without Fe) were less abundant and with a finer size. Image analysis allowed identifying an average area fraction of intermetallic phases equal to 2.5 ± 0.2 %. Since intermetallic particles are mainly Fe-based, no significant effect of heat treatment was detected in terms of area fraction [24]. Figure 4 : Details of intermetallic particles by SEM analysis for a) AlSi9CuFe, b) AlSi9CuFe T6 alloys. In order to complete the microstructural characterization of the alloys, calculated values of SDAS and grain size are shown in Tab. 3. It appears that the alloys exhibit comparable values of SDAS, even though slightly lower for AlSi9CuFe, while significant differences were detected in terms of grain size. In fact, AlSi9CuFe alloy is characterized by an average grain size more than double the value measured for the other alloys. This is likely due to the decrease in solidus temperature from 577 °C for AlSi9 alloy to 535 °C for AlSi9CuFe alloy, because of Cu and Fe addition. This means that the solidification