Research On Transformation Behavior Of β-Mg₁₇Al₁₂ Phase During The Solution And Aging Heat Treatment In AZ80 Magnesium Alloy

The effects of solution and precipitate ofβ-Mg₁₇Al₁₂ phase during the heat treatment on the microstructure and mechanical properties of as-cast AZ80 magnesium alloy were systemically studied in this paper by OM, SEM, XRD, EDS, TEM as well as tensile and hardness tests. The results indicated that:Theβ-Mg₁₇Al₁₂ phase discontinuously distributed along the grain boundary was dissolved intoα-Mg matrix through the solution treatment. When the solution temperature was 435℃, theβ-Mg₁₇Al₁₂ phase was dissolved completely with the holding time prolonged, but the size ofα-Mg was coarsening not obviously. The water cooling after solution treatment had more influence on the microstructure of oversaturated solid solution, and the hardness of alloy was dropped gradually along with the holding time prolonged. When the holding time of solution treatment was same, microstructure evolution and hardness changing trend of alloy appeared the same tendency with the solution treatment temperature heighten. The fracture mode of as-cast alloy was intergranular brittle fracture, and the fracture formed at the grain boundary where distributed amount ofβ-Mg₁₇Al₁₂ phase existed as net microstructure. After solution treatment, the tensile strength and elongation enhanced remarkably but the yield strength decreased slightly, and the fractography demonstrated that tensile fracture had the character of cleavage fracture.Due to the different solution treatment craft, the different oversaturatedα-Mg solid solution was obtained, which had the major influence on the mode ofβ-Mg₁₇Al₁₂ phase separating and the hardness of alloy. During the aging treatment, the continuous precipitate appeared ahead of time in the complete solution alloy. The hardness of incomplete solution alloy rose quickly during the initial period of aging treatment, however, the hardness ofcomplete solution alloy was higher than the former with the aging time prolonged.The aging temperature was higher, the amount ofβ-Mg₁₇Al₁₂ precipitate was smaller, but the quantity of continuous precipitate dominated, which resulted in the hardness of alloy lower slightly. The amount ofβ-Mg₁₇Al₁₂ precipitate was increased gradually when the aging time was rising, and the proportion of continuous precipitate was added little by little. Consequently, the hardness, tensile strength and yield strength of alloy were remarkably enhanced and the elongation decreased seriously. The tensile fracture mechanism of alloy after aging treatment was brittle fracture of cleavage fracture.