| The continuous fighting against energy consumption and environment degradation, together with increasing shortage of natural resources, is making it indispensable to produce and employ a broad range of light-weight metallic alloys where weight savings directly lower costs and increase efficiencies. To date, magnesium alloys as the lightest structural alloys have been very attractive in a variety of technical applications, especially in automotive, aircraft industries, and electronic sectors. One of the key technology issues hindering the wide use of Mg alloy as a structural material is its poor formability and restricted ductility, owing primarily to its hexagonal close packed (HCP) crystal structure and consequent limitation on number of available slip systems. Therefore, the works on formability and ductility have been the most key research focuses in this Mg alloys field.In this work, we first processed the Mg bars by duo-direction extrusion(DDE), and Magnesium alloy AZ31 was selected as the material for investigation. The mechanical properties of extruded AZ31 and its deformation, failure, static-recrystallization behaviors during and after tension and compression were widely investigated, by the means of Optical microscopy (OM), X-ray diffraction, Scanning electron microscope (SEM), Transmission electron microscopy (TEM). The main results are as follows:For Magnesium alloy AZ31, the grains were refined effectively and the mechanical properties were improved by DDE. With increasing extrusion temperature, grains grew up; microstructures became uneven and mechanical properties were deteriorated. At 400℃, the basal plane {0002} fiber texture was formed along the extrusion direction.Samples extruded at 400℃were tensed and compressed at room temperature and deformation twinning occurred during this process. With increasing strain, the number of twins increased and the twin morphology and mode changed that the thick-lenticular shape{10—12}<101—1> tension twins were generated at onset of plastic deformation and narrow-band {10—11}<101—2> compression twins were formed at a late or final stage of deformation. Under the same strain, compression produced more twins because of the basal plane {0002} fiber texture parallel to the extusion direction. The static-recrystallization took place during annealing. Twins formed during deformation have played an important role for the static-recrystallization and grain refinement.There is an obvious tension/compression asymmetry of magnesium alloy extruded at 400℃during tension and compression at room temperature. Grain size, Al element content and texture must be responsible for this. The smaller the grain size, the higher the Al element content and the more dispersed texture, the weaker tension/compression asymmetry. After heat treatment, solutioned and solution-aged samples acquired the weaker tension/compression asymmetry, which is the combined result of these three aspects.The tensile failure of homogeneous AZ31 is transgranular fracture and microscopic appearance analysis showed that the mechanism was cleavage fracture. Specimen extruded at 400℃was quasi-cleavage fracture. Necking phenomenon occurred during tensile test of heat treatment samples. The failures belonged to ductile fracture and fracture mechanisms were all micropore aggregation fracture. The differences between them is the volume, depth, morphology of dimples; solution-aged sample exhibited the best ductibility because of the"solution softening", which can reduce the critical resolved shear stress (CRSS) in prismatic slip, making prismatic slip easily activated and changing the mode of deformation. The precipitation of the second phase also played an important role for this.
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