An important aspect in describing the microstructure of non-heat treatable Al wrought alloys is their constitution in the form of dissolved alloy elements or precipitated second-phase particles. These characteristic structural constituents, which practically can be subsumed as microchemistry, have an impact on the physical and, in particular, on the mechanical properties of Al materials. The present work examines the formation of constituent phases during solidification as well as the microchemical changes during a subsequent homogenization annealing of the alloy AA 3103 (AlMn1). The forming phases were statistically analyzed in a scanning electron microscope regarding their size, shape, area density and chemical composition. The results were matched with complementary characterization methods to explain the microchemical changes during solidification and the subsequent homogenization.
Intergranular corrosion refers to a selective corrosion attack at the grain boundaries of polycrystalline materials. In engineering Al-Mg alloys of the AA 5xxx series it is caused by the formation of chemicallyless noble β-Al8Mg5 phases along the grain boundaries. The sensitivity of a material to intergranular corrosion can be assessed based on the mass losses in the so-called nitric acid mass loss test (NAMLT) according to ASTM G67. However, a detailed investigation of the underlying corrosion mechanisms requires that the β phases are made directly visible in the microstructure by metallographic methods. In the present work, the NAMLT mass losses of three AA 5xxx alloys with different Mg contents are compared with the results from two different etching methods. On the one hand, the alloys are etched in diluted phosphoric acid, a substance routinely used to examine the grain boundary occupancy in AA 5xxx materials. On the other hand, a newer etching method using a dilute ammonium persulfate solution is tested which etches the β-Al8Mg5 phases in the microstructure in a way that they can be examined at higher magnifications; even examinations in the scanning electron microscope are possible.
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