Study On Structure And Mechanical Properties Of Wrought Mg-Gd-Y-Nd-Zn(Zr) Alloy System

Recently,due to the emphasis on material light design, reducing energy consumptionand environment protection, Mg alloys are becoming the ideal substitute materials in thehigh-technology fields and civil producing industries. Meanwhile, the research anddevelopment of wrought Mg alloys are attracting more and more attentions. Rare earth (RE)elements are one of the vigorous alloying elements in Mg alloys. The addition of RE elementsin Mg can refine the structure, and can improve the oxidation resistance and creep resistanceas well that expands the service temperature range of Mg applications. Moreover, RE alloyingelements can produce weaker textures in wrought Mg alloys by changing the recrystallizationkinetics during thermo-mechanical processing, which is benefit to reduce the plasticanisotropy and enhance the comprehensive properties of wrought Mg alloys. In thisdissertation, the Mg–Gd–Y–Nd–Zn(Zr) alloy system was chosen as research object, andprocessed by hot extrusion, hot rolling and free forging technologies. The influences ofdifferent processing parameters on the microstructure, texture, mechanical properties andage-hardening behavior of the wrought alloys were systematically investigated. And theaction mechanisms of RE alloying elements and Mg–RE compounds on the microstructureand mechanical properties of these alloys were discussed.Firstly, the Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy was processed by hot extrusiontechnology, and the extruded alloy sheets and bars were produced, respectively. Resultsshowed that this alloy possesses good formability. During thermal-mechanical processing,dynamic recrystallization took place and the structure of wrought alloys was greatly refined.The grain size distributions on different planes of extruded alloy sheets and bars were uniform.The extruded alloy exhibited remarkable age-hardening response from200℃to300℃. TEMobservation of the peak-aged alloy indicated that there are plenty of β′precipitates in α-Mgmatrix. The β′phase precipitated on the {1120} prismatic planes, which has a bco structureand exhibits semi-coherent structure with α-Mg matrix, could effectively block the basal dislocation slip. The results of tensile tests showed that these β′precipitates played animportant role in the improvement of room temperature and high temperature strengthes ofthe peak-aged alloy. Texture analysis of the extruded alloy revealed that the basal planes ofmost grains are distributed parallel to the extrusion direction. Due to the effect of texture, theas-extruded and peak-aged alloys exhibited almost equivalent asymmetry ratios with theCYS/TYS along extrusion direction both lower than1at room temperature. With increasingtemperature, the influence of texture reduced. The relative magnitude between CYS and TYSreversed, and the ratio of CYS/TYS increased higher than1. This was attributed to the effectof microstructure (i.e. β′secondary phase or solute atom clusters) of the alloy. Meanwhile,different interactions between the dislocations and β′secondary phase or solute atom clustersled to the different asymmetry ratios of the as-extruded and peak-aged alloys.Secondly, the Mg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy was processed by free forgingtechnology. Results showed that grains of the as-forged alloy were greatly refined and thegrain size distributions were similar on the planes parallel and vertical to the drawingdirection. The results of tensile tests revealed that strengths of the alloy specimens along thedrawing direction were higher than those of the alloy specimens vertical to the drawingdirection; while, the tensile stress–strain curves of the alloy specimens vertical to the drawingdirection exhibited remarkable strain hardening after yielding and possessed higherelongations.Furthermore, twinning and dynamic precipitation upon hot compression of theMg–8Gd–2Y–1Nd–0.3Zn–0.6Zr alloy was systematically investigated. Results revealed thatdeformation twinning was an important supplementary deformation mechanism to——accommodate plastic strain of the alloy, especially the {1011}-{1012} double twinning.Dynamic precipitation occurred extensively during hot compression, which hindered or evensuppressed dynamic recrystallization of the alloy. The morphology of β′precipitates obtainedby dynamic precipitation differed from that of the same phase produced by T6heat treatment.But their orientation relation ship with Mg matrix did not change.The Mg6.5Gd1.3Nd0.7Y0.3Zn alloy was processed by hot extrusion and rollingtechnologies. Extruded alloy sheets with thicknesses of2mm,8mm, and rolled alloy sheetwith a thickness of3mm were produced, respectively. Results showed that this alloypossesses good formability. All alloy sheets exhibited fine equiaxed grains, and coarsesecondary phases in the matrix had been crushed into small particles. In the extruded alloysheets, the crushed particles showed a banded structure along extrusion direction; while in therolled alloy sheet, the distribution of crushed particles was more dispersive. Texture analysis revealed that both the extruded and rolled alloy sheets showed a weak <1010>ED fiber texture,and the {0001} basal poles were split in the transverse direction of the sheet, exhibitingtwo-peak intensity maxima. The RE elements both in Mg solid solution and Mg–REcompounds contributed to the texture weakening of alloy sheets by affecting dynamicrecrystallization behavior during thermal-mechanical processing. The results of tensile testsindicated that the extruded and rolled alloy sheets exhibited different tensile planaranisotropies along the ED/RD and TD. It was influenced by both the texture and thedistribution of secondary phase particles.By means of microstructural analysis, the influences of RE alloying elements andthermo-mechanical processing on the strengthening of mechanical properties ofMg–Gd–Y–Nd–Zn(Zr) alloy system were discussed. Solid-solution strengthening,grain-refining strengthening and precipitation strengthening were considered to be the mainstrengthening mechanisms for the RE alloying elements, and among them the precipitationstrengthening took the greatest strengthening effect. After hot extrusion, free forging or hotrolling processing, the grain-refining strengthening was enhanced, and the deformationstrengthening emerged as well.