Study On Microstructure And Mechanical Properties Of Rolled Mg-Zn-RE-Ca Alloys

In recent years, magnesium and its alloys have potential applications in the automotive, aerospace and communication applications as the lightest structural materials to reduce the weight and further improve the fuel efficiencies. However, due to the low symmetry hexagonal close packed crystal structures and limited number of slip systems at room temperature, strong basal texture usually forms after rolling or extrusion process, which leads to the poor ductility and formability at room temperature. And this severely hinders their wide spread application. In this study, five Mg-Zn-Gd-Ca alloys were developed, the microstructure, texture and mechanical properties were systematically investigated. The main contents in this study are as follows:(1) The as-cast microstructures and the effect of second phase on the grain size were investigated. With increasing alloy content, the grain size decreased. With Ca addition, grain size of the Mg-Zn-Gd-Ca alloys decreased significantly. There were also a few Mg3Zn3Gd2 phases located at grain boundaries after solution heat-treatment.(2) The formability, microstructure, texture and mechanical properties of the rolled sheets were investigated. Compered with other four alloys, Mg-0.1Zn-0.4Gd-0.2Ca alloy have a better formability. With the Ca addition, the Mg-Zn-Gd-Ca alloys exhibited a deformed microstructure, consisting of many twins with them. The room temperature elongation of the Mg-0.1Zn-0.4Gd-0.2Ca alloy is 17.3% and 5.6% along the transverse direction(TD)and the rolling direction(RD), respectively, with yield strength of 228 MPa and 265 MPa. Besides, all of the as-rolled Mg-Zn-Gd-Ca sheets exhibited non-basal textures with three basal poles tilting away from the normal direction.(3) The effect of annealing on the microstructure, texture and mechanical properties of the sheets was investigated. After annealing, the sheets exhibited fine dynamic recrystallized( DRXed) grains and the DRXed grain size increased with increasing annealing temperatures. The grain size increased faster than that of alloy with large content of Ca element. The annealed sheets exhibited weak textures and the titling angle of the peaks is different along the transverse direction and the rolling direction. Compared with rolled sheets, the maximum texture intensity decreases and then increase upon annealing in the texture. After annealing at 325 oC, Mg-0.1Zn-0.4Gd-0.2Ca alloy exhibited a non-basal texture with quadrupling peaks. And the peak intensity of the texture towards transverse direction is larger than the peak intensity toward rolling direction. The yield strength decreased with increasing annealing temperature and lower along the transverse direction than along the rolling direction. The elongation is increased and then decreased. After annealed at 300 oC for 90 min, the Mg-1.5Zn-0.2Gd-0.1Ca alloy exhibited equiaxed grains. The elongation along the RD and TD is 34.1% and 34.9%, respectively with yield strength of 164 MPa and 131 MPa. After annealing at 325 oC, the elongation of Mg-0.1Zn-0.4Gd-0.2Ca sheet is 37.0% and 39.6% along the RD and TD, respectively with yield strength of 147 MPa and 124 MPa.