Effect Of Extrusion And Heat Treatment On Microstructure And Properties Of Rare Earth Magnesium Alloy Containing LPSO Phase

Magnesium and its alloys,as the lightest metal structural parts,have excellent properties and broad development prospects.However,due to their mechanical properties and other reasons,their development is limited to a certain extent.Therefore,it is necessary to study magnesium alloys with high strength.In recent years,scholars have found that rare earth Mg alloys with long-period stacked ordered structure(LPSO)phases have excellent mechanical properties,which make up for the shortage of commercial Mg alloys and have broad practical application prospects.On the basis of previous studies,a rare earth magnesium alloy containing LPSO phases were designed and prepared.The atomic ratio of Er/Zn in the alloy is 3/2,and the specific chemical composition of the alloy is Mg-1Gd-0.75-Er-0.5Zn-0.18Zr(at.%).Firstly,the microstructure changes of as-cast,homogenized and T4 treated alloys were studied.Then the homogenized alloys were hot extruded to further study the influence of extrusion process on the microstructure and properties of the alloy.Finally,the extruded alloys were annealed to explore the influence of annealing process on the microstructure and properties of the alloy,and the best annealing process was summarized.In this thesis,the Mg-1Gd-0.75-Er-0.5Zn-0.18Zr(at.%)alloy was successfully prepared by conventional casting method.It was found that the LPSO structure phases were formed in the alloy in the as-cast state,homogenization and T4 treatment.After homogenization at 500?for 12h,the bulk LPSO phases precipitated along grain boundaries in the alloy decrease significantly and are converted to lamellar LPSO phases during heat treatment.After solution treatment of different time,the eutectic structure Mg₃RE formed during solidification decrease with the increase of solution time,and the optimal solution process is 500?×12h.After hot extrusion,the morphology of LPSO phases in the alloy change.At the edge of the alloy near the die,the morphology of LPSO phases are mainly long fiber,while at the core of the alloy,the morphology are mainly short fiber and block.With the increase of extrusion temperature,the deformation mechanism changes from kinking to sliding.In general,the dynamic recrystallization degree increases with the increase of extrusion temperature.The dynamic recrystallization mechanisms are continuous dynamic recrystallization mechanism and PSN mechanism,and the dynamic recrystallization can weaken the texture to some extent.The mechanical properties of the alloy extruded at 300?are the best.The tensile strength,yield strength and elongation after fracture of the alloy are 340MPa,308MPa and 4.64%,respectively.After annealing,the change of deformation mechanism leads to different degrees of static recrystallization.In the alloy without complete static recrystallization,the morphology of as extruded LPSO phases are maintained,and the degree of recrystallization decreases with the increase of extrusion temperature,which is related to the deformation mechanism of as extruded alloy.The main texture in the region without static recrystallization is{0001}<10(?)0>//ED.When static recrystallization begins,the rare earth texture with<2(?)1>//ED grows preferably in the region without static recrystallization.Compared with the extruded alloy,the strength decreases and the elongation increases obviously after annealing.The results show that the best properties are obtained after extrusion at 400?and annealing at 500?for 1 h,with yield strength of 155 MPa and elongation of 9.48%.