Composition: Al-5.6%Zn-2.5%Mg-1.6%Cu-0.2%Fe-0.2%Si-0.05%Ti-0.02%Mn-0.2%Cr-0.15% Other

Processing: Extrusion and T73 temper. 

Etching: 30-120 s inmersion in mixture of acids (HCl+HF+HNO₃).

The alloy 7075 (Al-Zn-Mg-Cu) is widely used in the aeronautic industry due to its high specific strength. The polished surface without etching reveals two types of intermetallic compounds that are aligned in the extrusion direction. Light gray particles with polygonal shapes correspond to Fe-rich intermetallics, mostly Al₇Cu₂Fe. Small clusters of globular particles with a dark tonality correspond to the Mg₂Si intermetallic. After etching, it becomes more difficult to distinguish the intermetallics due to their dissolution/detachment, but the elongated grains are revealed. The lack of brightness after etching is associated with the presence of dispersoids (Al₁₈Cr₂Mg₃) and small precipitates that form after the T73 treatment (solution heat treated, quenched and overaged). The latter precipitates provide strength and hardness to the alloy and their composition is close to MgZn₂. In case of dispersoids, their main effect is related to the grain size. They act as obstacles during recovery, recrystallization and grain growth, which explains the elongated shape of the grains.

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