Composition: 4.1 Mg; 0.45 Mn; 0.10 Cr; 0.34 Fe; 0.24 Si; 0.03 Cu; Al bal.

Processing: Forging, rolling, annealing (430 ºC - 8h) and water quenching.

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

5xxx (Al-Mg) alloys are the strongest non-heat treatable aluminium alloys and they show good weldability and corrosion resistance. This is the reason why these alloys are often used for marine applications. In the non-etched specimen, two types of intermetallics aligned in the rolling direction can be differentiated: Al₃Fe (gray) and Mg₂Si (dark). Etching does not reveal the equiaxial α-Al grains that are present in this alloy. The reason is twofold: (i) annealing and quenching prevents the massive precipitation of  Al₃Mg₂ at the grain boundaries, and (ii) the selected etchant does not attack the grain boundaries.  5xxx alloys are not recommeded for applications where moderate temperatures (50-200ºC) are sustained for long times due to risk of sensitization: precipitation of anodic Al₃Mg₂ at the grain boundaries increases the risk of intergranular and stress corrosion phenomena.

Although hardly visible, this alloy also contains dispersoids (Al₆(Cr,Mn), Al₁₈Mg₃Cr₂), which are used to control the grain size. 

Al-Mg diagram

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