Study On The Texture Formation Mechanism And Deformation Behavior Of Mg-1Gd Alloy

Strong texture with the c-axis of grains perpendicular to the extrusion direction orrolling plane generally develops in conventional wrought Mg alloys, resulting in limitedformability, poor ductility, strong anisotropy in their mechanical properties, and strongtensile/compression yield asymmetry, which has become a serious problem to its widerange of application. Extensive research has shown that the additions of RE elements atlow alloying levels generally result in significant texture weakening or modification of Mgsheets and extrusions, leading to improved formability and ductility. However, themechanism responsible for the texture weakening is still unclear. In-depth study andexploration of magnesium alloy texture weakening mechanism and deformation behaviorare of great significance for the development of Mg-RE alloys with high formability andthe basic plastic deformation theory of magnesium alloy.In this thesis, a comparative study of the microstructure and texture evolution of pureMg and Mg-1Gd alloy during rolling and annealing process was carried out by usingoptical microscopy (OM), scanning electron microscopy (SEM) and electron backscatterdiffraction (EBSD), and the mechanism for texture weakening by Gd additions wasrevealed. Secondly, the effect of annealing temperature and time on the microstructure andtexture evolution of Mg-1Gd alloy was investigated by the indirect extrusion, waterquenching and annealing experiment, the formation mechanism of21RE fibre textureof Mg-1Gd alloy was discussed, and the effect of extrusion temperature on themicrostructure and texture evolution of the as-extruded and annealed Mg-1Gd alloy wasalso analyzed. Finally, the deformation behavior of Mg-1Gd alloy was investigated. Byroom-temperature tensile test, the deformation mechanisms and texture evolution duringtensile deformation of pure Mg and Mg-1Gd alloy sheets were studied. Meanwhile, themicrostructure and texture evolution during compression in Mg-1Gd alloy with an initial 21RE fibre texture were studied, and the deformation mechanism was also discussed.From these investigations, the following conclusions can be drawn.The texture evolution of Mg-1Gd alloy is strongly dependent on strain path under hotdeformation conditions. A large amount of secondary twins and deformation bands formduring hot rolling at300oC of Mg-1Gd sheet, and may serve as favorable nucleation sitesfor SRX during annealing at400oC. The recrystallized grains at bands/twins in Mg-1Gdalloy display a relatively wide spread of orientations, leading to a further weakening oftexture. The texture of Mg-1Gd sheet is tilted slightly away from the basal texture.Mg-1Gd alloys extruded at a temperature between300oC and450oC show a partiallyrecrystallized microstructure consisting of a large amount of dynamically recrystallizedgrains and large deformed grains elongated in the ED. With increasing extrusiontemperature, the dynamically recrystallized grain size is increased, and the100texturecomponent is weakened gradually. With increasing extrusion temperature, the basal polesare inclined to the ED due to the activation of c+a slip system. The recrystallized grainsand large elongated grains in the as-extruded Mg-1Gd sample are oriented mainly with100orientation parallel to the extrusion direction although it is found that21grainsare nucleated at shear bands.The addition of Gd has a significant effect on the recrystallization, grain growthbehavior and texture evolution of Mg sheet and bar during static annealing. Duringannealing at400oC for5min, a fully recrystallized microstructure is formed in both pureMg and Mg-1Gd alloy sheets. Pure Mg sheet shows a strong basal texture, whereasMg-1Gd alloy sheet shows a weak basal texture. With increasing annealing time, graingrowth is limited in both sheets. The microstructure and texture of the extruded Mg-1Gdalloy change significantly during annealing. With increasing annealing temperature, the21RE-texture component strengthens gradually at the expense of the100component, which can be ascribed to the preferred growth of21grains. The drivingforce for the preferred grain growth during recrystallization is the difference in storedenergy between the21grains and the100grains. During short-time (≤40min)isothermal annealing, the21grains grow faster than other grains due to a sizeadvantage, leading to the strengthening of21texture component. Abnormal growth of100grains occurs upon long-time (>40min) annealing due to a texture effect. The fiber texture observed in extruded Mg-1Gd alloys has a correlation with theextrusion temperature. When extruded at350oC, a large amount of shear band areobserved in Mg-1Gd alloy. During subsequent annealing at500oC/10min,21grainshave grown up preferentially, which leads to the strengthening of21fiber texture.However, when extruded at300oC, although lots of shear band can be observed, the grainsremain to be fine recrystallized grains after an anneal at500oC/10min, resulting in a weak21fiber texture. After extrusion at a temperature higher than400oC, themicrostructure tends to be more homogeneous with less shear band and a higherproportion of c+a slip, which results in the weakening of21texture component.The deformation behavior during uniaxial tension was studied. The results show thatthe ultimate tensile strength (UTS), yield strength (YS) and elongation to failure of theannealed pure Mg sheet are167.6MPa,69.5MPa and5.4%, respectively. By contrast, theroom-temperature UTS and YS of annealed Mg-1Gd alloy sheet are193.5MPa and129.9MPa, with the elongation dramatically increasing to15.6%, indicating an enhancedcombination of strength and ductility. The improvement in strength is related to the solidsolution strengthening of Gd in magnesium and fine grain strengthening, whereas theincrease in ductility is mainly due to the texture weakening and grain refinement. Aweaker texture component is formed after tensile test in the TD. This texture componentarises from a larger number of extension twins caused by the compressive stress in the TDduring tensile. In the Mg-1Gd alloy, however, grains are elongated along the tensiledirection after tensile, showing good plasticity. In addition, significant texture changetakes place, more grains are aligned with basal planes parallel to the tensile direction,which is mainly attributed to the activations of basal slip and extension twinning.The deformation behavior during uniaxial compression was also studied. Since theextruded Mg-1Gd alloy shows a typical21RE fibre texture (21∥ED), extensiontwinning and basal slip are the dominant deformation modes in the early stage ofcompressive deformation, and are responsible for the mechanical behavior and textureevolution. With increasing strain, the texture is changed from the21into the0001fibre texture component (0001∥ED) mainly due to the gradual rotation of c-axestowards the compressive direction caused by basal slip. By contrast, extension twinning isthe main deformation mode during compression along extrusion direction in conventional extruded Mg alloys(AZ31, AM30) with strong basal texture ((0001)//ED), leading to a86°reorientation of grains and therefore a sudden change in texture from (0001) basal textureinto0001fibre texture.