Issue 42

A. Brotzu et alii, Frattura ed Integrità Strutturale, 42 (2017) 272-279; DOI: 10.3221/IGF-ESIS.42.29 275 Figs. 2-5 show SEM micrographs of the studied alloys. SEM/EDS observation allows to study composition and distribution of secondary phases present in the alloy microstructures. The solubility of almost all elements in solid aluminum is very low. For this reason aluminum alloy microstructures are characterized by the presence of secondary phases (usually intermetallic compounds) dispersed in an aluminum matrix. Generally speaking secondary phases can be divided into three main groups:  fine intermetallic compounds like CuAl 2 , Mg 2 Si etc. that are responsible for the heat treatment material hardening. They are finely dispersed in the aluminum grains and cannot be observed by means of SEM.  fine dispersoids segregated at the grain boundaries, usually formed by elements added as grain refiners.  coarse intermetallic particles that can be easily observed by SEM and analyzed by EDS. They are usually aligned along the deformation direction and they do not dissolve during heat treatment processes. Fig. 2 shows the 7050 T7451 microstucture. It is characterized by a great quantity of coarse intermetallic compounds aligned along the rolling direction. In the area close to these particles microcavities and microcraks can be often observed. EDS analyses allowed to identify three different types of intermetallic compounds: a) Al 2 CuMg and CuAl 2 coarse rounded bright particles b) Al 7 Cu 2 Fe elongated bright particles c) Mg 2 Si dark rounded particles A fine dispersion of bright particles has been observed at the grain boundary. Microvoids and microcraks are usually found close to the b type particles. Fig. 3 shows the 2195 T8 microstructures. This alloy has a higher quantity of intermetallic particles in comparison with the previous one. Also in this case secondary phases are aligned along the rolling direction. They consist of two different coarse particles identified by means of EDS: a) Al 7 Cu 2 Fe compact bright particles b) AL 2 Cu fragmented bright particles. Figure 2: 7050 T7351, distribution of the secondary phases. Figure 3: 2195 T8 distribution of the secondary phases. Fig. 4 shows the 7075 T6 microstructure. A wide variety and quantity of coarse intermetallic phases has been identified: a) Al 7 Cu 2 Fe bright and irregular intermetallic phases b) gray irregular particles containing variable quantities of Al, Fe, Zn and Cu c) Al 2 Cu rounded bright intermetallic phases