Sequential precipitation behavior of Mg₁₇Al₁₂ and Mg₂Sn in Mg-8Al-2Sn-1Zn alloys

The precipitation behavior of Mg-8Al-2Sn-1Zn (ATZ821) alloy was investigated. Sn addition to the AZ81 alloy greatly influenced the initial precipitation behavior in the following two aspects: (i) the segregation of Sn solutes at the interface of Mg₁₇Al₁₂ via Sn partitioning induced during the growth of Mg₁₇Al₁₂ and (ii) high interfacial energy at the edge-tip of growing Mg₁₇Al₁₂. According to the hardness profile and detailed microstructural analyses of ATZ821 cast samples artificially aged at 453 K for different times, it was confirmed that the initial precipitation of Mg₁₇Al₁₂ was delayed due to Sn partitioning at the interface, and all Mg₂Sn without exception was nucleated at the edge-tip of pre-existing Mg₁₇Al₁₂ precipitates on the basal planes. In addition, a selected area diffraction pattern analysis confirmed that α-Mg, Mg₁₇Al₁₂, and Mg₂Sn had the following orientation relationship: (0001)_α//(110)_Mg17Al12, [12¯10]_α//[1¯11]_Mg17Al12 and (0001)_α//(022)_Mg2Sn, [12¯10]_α//[11¯1]_Mg2Sn.