Changes in microstructures with cold rolling and effects of impurity element concentrations for Mg-Al-Zn alloys

It is generally recognized that impurity atoms affect the phenomena of recrystallization. In the present study, we have prepared a low impurity magnesium alloy, then deformation behavior in cold-rolling and corrosion behavior of the alloy were compared with those of a commercial AZ31B magnesium alloy. The low impurity alloy was prepared with using 5N-Al, 4N-Zn and high purity magnesium, the last one was prepared by a vapor deposition technique. Chemical analyses showed that the composition of the alloy was about Mg-3%Al-1%Zn (in mass%). Slices with about 5 mm thickness were cut from an ingot of the alloy, and hot-rolled into 1 mm at 623 K, then solution heat treated at 673 K for 86.4 ks. The specimens were cold-rolled at room temperature with reduction rates of 0 similar to 80%. For comparison, a commercial AZ31B alloy with 0.8 mm thickness was used. After cold-rolling, foil specimens for transmission electron microscopy were prepared and microstructures were observed. Recrystallization occurred in the AZ31B alloy by 70% cold-rolling at room temperature. Regions with low dislocation density were frequently observed in heavily cold-rolled specimens, which would show recovery phenomena or early stage of recrystallization. Additionally lowering the impurities resulted in improving a corrosion resistance. Corrosion test using a 1 mass%NaCl solution showed that the low impurity magnesium alloy had a superior corrosion resistance compared with that in the AZ31B alloy. In the AZ31B alloy filiform corrosion occurred after a duration time of 27 minutes, while in the low impurity magnesium alloy, filiform corrosion did not occur for 22 hours but general corrosion occurred.