The Study On High Properties Mg-Al-RE Magnesium Alloys

Magnesium alloy is the lightest practical engineering material, which shows very high specific strength and stiffness, good castability and absorption ability of vibration, excellent machining ability and dimensional stability, has attracted the attention of aviation and automotive industry for a long time. However, its strength and plasticity is lower than aluminum alloy. Furthermore, its poor high-temperature behavior is one of the most important weaknesses for its extensive use. So it's necessary to increase the strength of magnesium alloys at room and high temperatures.Metallographic analysis, energy spectrum analysis, mechanical property test and fracture surfaces observations are used to determine the microstructure and the mechanical behavior of as-cast, hot-extruded and after heat treatment Mg-8Al-1Ca and Mg-8Al-1Zn alloys with different contents of Rare Earth element Gd adding. The study of Mg-8Al-1Zn-xGd alloys is emphasized in this paper.The results show that the microstructures of as-cast Mg-8Al-1Ca-xGd and Mg-8Al-1Zn -xGd alloys is consisted ofα-Mg solid solution, intermetallic compoundβ-Mg₁₇Al₁₂ and Al₂Gd phase; there are Al₂Ca phase located in the grain boundary of Mg-8Al-1Ca-xGd alloys; The average grain sizes of the two series of as-cast alloys are less than 70μm and 100μm.β-Mg₁₇Al₁₂ and Al₂Gd phase in both of the two series alloys approximately dissolved after solution treatment (T4 treatment); After T6 treatment (aging treatment after solution treatment), second-phase dispersedly precipitated in inside of the grain and grain boundary. The ultimate tensile strength and elongation of both of the two series of alloys enhanced after solution treatment; T6 treatment could improve the ultimate tensile strength significantly of Mg-8Al-1Zn-xGd alloys, and that, T4 treatment is more effective for the raise of elongation. After hot-extrusion process, the grain size of Mg-8Al-1Zn-xGd alloys reduced significantly, as a result, the mechanical property increased obviously. Heat treatment shows no help for the improvement of Mg-8Al-1Zn-xGd alloys. The ultimate tensile strength and the yield tensile strength reduced as the test temperature rising, but the elongation increased as the test temperature rising.Fracture surface analysis revealed that: At room temperature, as-cast Mg-8Al-1Ca-xGd alloys show brittle fracture character, the fracture mode changed after T4 treatment, a character of mixed ductile and brittle fracture appeared. The fractography of as-cast, hot-extruded, T4 treated and T6 treated Mg-8Al-1Zn-xGd alloys exhibited mixed fracture characters at room temperatures; At elevated temperatures, as-cast Mg-8Al-1Zn-xGd alloys also shows mixed fracture characters, but the tensile fracture of the hot-extrude alloys occurred in a ductile mode.