Development of Magnesium Alloys with High Performance

Abstract

In Mg-Al-Zn and Mg-Al-Mn alloys containing 2.0~6.0mass%Al and 0~1.5mass%Zn, grain refinement in the as-rolled (F) specimens containing large amount of Al and Zn are achieved by both dynamic recrystallization and dynamic precipitation during hot rolling and leads to high strength and high ductility at room temperature. At high temperatures, the tensile strength of the investigated alloys is almost the same, while the elongation of the F-specimens increases with increasing Al and Zn contents, leading to 150% in Mg-4.5%Al-1.5%Zn alloy. High Al and Zn contents alloys significantly accumulate large working strain in grain interiors, and involve large amounts of high angle grain boundaries and fine spherical precipitates, which can become the nucleation sites for recrystallization. Therefore, dynamic recrystallization in such alloys occurs at small strain region during tensile test. This dynamic recrystallization causes reduction of flow stress and large elongation by grain boundary sliding at high temperatures. Furthermore, .fine recrystallized grains contributes to deformation in normal direction, resulting in isotropic deformation behavior. Authors attempt to improve proof stress and its anisotropic property of Mg-Al-Zn wrought alloys by grain size and precipitates controls utilizing dynamic recrystallization and dynamic precipitation during hot extrusion. In the alloy specimens extruded at lower temperatures increasing Al and Zn contents enhance dynamic recrystallization and dynamic precipitation, resulting in grain refinement and large amount of Mg17Al12 precipitates. As a result, the extruded Mg-9%Al-1%Zn alloy specimen shows high tensile strength of 370MPa, 0.2% tensile proof stress of 240MPa and moderate elongation of 20%, which are almost same as standard values of tensile properties of T5-treated 6N01 Al extruded alloy. Furthermore, a ratio of compressive proof stress to tensile proof stress of the as-extruded specimen improves up to a higher ratio of 0.9 than that of Mg-3%Al-1%Zn alloy specimen with no precipitation, 0.5, due to prevention of tensile twin, which easily occurs during compressive deformation even under a low applied stress perpendicular to the extrusion direction, by dynamic precipitation of Mg17Al12 phase.