Issue 42

A. Brotzu et alii, Frattura ed Integrità Strutturale, 42 (2017) 272-279; DOI: 10.3221/IGF-ESIS.42.29 276 d) rounded or elongated dark particles containing variable percentage of Al, Si and Zn e) biphasic particles characterized by a gray phase that contains Al, Fe and Cu and by a dark phase that contains Al, Si and Zn. Fig. 5 shows the 7075 T7451 microstructure. Like in the previous case, the base material is a 7075 alloy, but the 7075 T7451 alloy has been subjected to a different thermal treatment. SEM/EDS analyses highlighted that in the 7075 T7451 alloy, secondary phases are more homogeneously distributed in the matrix. Considering that heat treatments do not affect type and distribution of secondary phases, differences are due to the manufacturing process. Figure 4: 7075 T6 distribution of the secondary phases. Figure 5: 7075 T7351 distribution of the secondary phases. Intergranular corrosion test A careful analysis of the specimens after the intergranular corrosion tests (ASTM G110 standard) highlighted that the different materials behave in different ways. The 7050 T7451 alloy shows only random pitting (Fig. 6). Fig. 7 shows the deep intergranular corrosion occurring in 2195 T8 alloy. Fig. 8 shows severe intergranular corrosion in 7075 T6 alloy: by comparing this corrosive attack with the previous one it can be seen that in this case there is a more diffused corrosion, but corrosion attack is less deep. 7075 T7351 alloy shows a scattered pitting corrosion. DISCUSSION he fracture of high resistance aluminum alloys can be due to different causes. A cracked component made of an Al 7050 alloy was analyzed in a previous research in order to identify possible causes of crack formation during the manufacturing process. This process was constituted by a material-removal operation followed by a chemical anodization treatment. For that reason it is interesting to highlight the susceptibility of this alloy to intergranular corrosion and to compare it with that of other high resistance alloys subjected to different production processes. From a corrosion point of view it is important to evaluate grain size and distribution as well as distribution of second phases such as intermetallics, precipitates and dispersoids. It is well known in literature that dispersoids and precipitates are characterized by an electrochemical potential that is different from that of the bulk material and then they can be responsible for localized corrosion. Intergranular corrosion is affected also by grain size and distribution: corrosion activity may develop because of some heterogeneity in the grain boundary structure. The study of the specimens after T