Effect Of Aging-Annealing-Aging On Microstructure And Mechanical Properties Of Mg-RE Alloy

Extrusion is a common process for preparing magnesium alloy profiles.After extrusion,the defects in the microstructure of the alloy are greatly reduced,the grain is refined,and the mechanical properties are significantly improved.The deformation ability of magnesium alloy is greatly stimulated,so the microstructure of the alloy is not uniform.The combination of deformation and heat treatment can improve the mechanical properties of magnesium alloy,and the appropriate heat treatment can refine the grain size of magnesium alloy and even the microstructure of the material.In addition,the residual stress in the alloy during machining is greatly reduced,but the tensile strength and ductility of the alloy cannot be optimized simultaneously.Based on the above reasons,this paper proposes a process that combines conventional extruded mg-9Gd-4Y-2Zn-0.5Zr(wt.%)alloy with age-anneal-aging.Optical microscopy(OM),scanning electron microscopy(SEM),electron backscattering diffraction(EBSD),universal tensile testing machine and Vickers Hardness tester were used to study the effects of annealing temperature from 350 ? to 450 ?,annealing time from 1 h to 20 h and secondary aging on the microstructure and mechanical properties of the alloy.The effects of annealing on the microstructure and mechanical properties of two kinds of alloy bars with extrusion ratio(16 and 4)were compared,which provided scientific guidance for the development of Mg-Gd-Y-Zn-Zr alloys with high strength and excellent ductility.The results show that the coarsening of fine grains during annealing is inhibited by the introduction of fine precipitates in primary aging treatment.Annealing refines coarse grains by static recrystallization(SRX)mechanism.The strength of Mg-Gd-Y-Zn-Zr alloy was further improved by the secondary aging treatment.Due to the low annealing temperature and insufficient energy,annealing at 350 ?-410 ? has little effect on the microstructure of the alloy,there are still elongated grains with almost no change in size in the microstructure,and almost no static recrystallization occurs.With the increase of annealing temperature to420 ?-450 ?,the uniformity of recrystallized grains is improved and continuous growth is achieved,while the deformed grains are gradually consumed by them.Compared with other annealing temperature and time,the alloy exhibits uniform microstructure and no obvious change in grain size after annealing at 420 ?�5 h,and the proportion of fine grain increases from 20.9% to 79.4%,which is the best annealing process.Due to the coordinated deformation of uniform microstructure,short rod-shaped long range ordered phase(LPSO)and prismatic ?' relative dislocation and the nailing of grain boundaries,the alloy after aging has excellent strong plasticity,with Yield strength(YS),ultimate tensile strength(UTS)and elongation(EL)were 419 MPa,478 MPa,11.6%,respectively,which increased 47 MPa,45 MPa,4.9%,respectively,compared with the samples after extruding and aging.Small extrusion ratio can not provide enough deformation energy,a large number of initial and coarse deformed grains can not form dynamic recrystallization,even if the alloy is annealed,the recrystallized grains can not consume too many initial deformed grains,and the microstructure of the sample is not uniform.Small lamination in the microstructure of the alloy results in few dislocation and subgrain boundaries,so a large number of grains begin to break and grow.The tensile strength and yield strength of the alloy with extrusion ratio of 4decrease from 394 MPa and 347 MPa to 339 MPa and 249 MPa,respectively,compared with that of the alloy with extrusion ratio of 16.Elongation decreased from 15.7 % to 13.9 %.