| In this thesis microstructure and texture evolution during hot extrusion ofcommercial AZ31magnesium alloy was investigated. Microstructure evolution wasobserved by optical microscope (OM) and electron backscatter diffraction (EBSD)technology equipped in a thermal field emission scanning electron microscopy (SEM).Local crystallographic orientations were measured by EBSD technology and themacro-textures were examined by X-ray diffraction (XRD) technology. Influence ofinitial microstructure (initial grain size, initial texture) and deformation conditions(extrusion ratio) on microstructure and texture of hot-extruded AZ31magnesium alloyrod were discussed in detail. Effect of initial texture on deformation mechanisms anddynamic recrystallization (DRX) behavior of AZ31magnesium alloy during hot tensionand extrusion was specifically studied. Microstructure and texture evolution during theannealing of AZ31magnesium alloy extruded rods with different DRX volumefractions was examined. Based on the experimental results and discussions, the mainconclusions are as follows:①The partially DRX hot-extruded microstructure of AZ31magnesium alloy rodis usually a heterogeneous structure consisted of fine equiaxed recrystallized grains,large elongated deformed grains and coarse recrystallized grains with different volumefractions. The three kinds of grains with different morphologies have their ownorientations. The texture of the large elongated deformed grains strongly centers at <101ˉ0>, while coarse recrystallized grains present a different texture component with <11ˉ20> parallel to extrusion direction (ED). The texture of the recrystallized grains is muchmore randomness than that of the deformed grains. Extrusion ratio has a significantinfluence on the microstructure and texture of AZ31magnesium alloy hot-extruded rod.With the increasing of extrusion ratio, the volume fraction of deformed grains decreasedand the volume fraction of recrystallized grains increased, which made the maximumintensity of fiber texture decrease. Therefore, in AZ31magnesium alloy hot-extrudedrods, two strong textures could be formed depending on the extrusion parameters: afiber-type texture with the <101ˉ0> directions of the crystallites align parallel to ED andthe <101ˉ0>-<112ˉ0> double fiber texture parallel to ED. The influence of extrusion ratioon DRX grain size of hot-extruded microstructure is much more complicated. Strainrate and actual exit temperature both increased in the meantime as extrusion ratio increased, however, the influences of these two factors on DRX grain size are opposite.DRX grain size of AZ31Mg alloy is mainly dependent on the Z-value of thedeformation process.②Initial grain size has a great influence on the microstructure and texture ofAZ31magnesium alloy hot-extruded rod. With the decreasing of the initial grain size,the recrystallization volume fraction of the hot-extruded microstructure increased andtexture weakened, since at finer grain sizes, DRX proceeded at a greater rate because ofthe higher density of grain boundary nucleation sites.③Initial texture has a significant influence on the deformation mechanisms andDRX behavior of AZ31magnesium alloy during thermo-mechanical process, therebyaffecting the final microstructure and texture of the products of thermal processing.1) Initial texture has great effect on twinning behavior of AZ31magnesium alloyduring thermo-mechanical process. During uniaxial tension at300℃, lots of {10ˉ12}extension twins were observed in ND specimen and only small amounts of {101ˉ2}extension twins were observed in ND45specimen, while no twinning was observed inRD specimen. During direct extrusion at300℃, hardly any twins were found in theinterrupted extruded sample of extruded and annealed (EA) rod while {101ˉ2} extensiontwin was observed to be very active in the interruoted extruded sample of rolled andannealed (RA) rod.2) Initial texture has significant effect on DRX behavior of AZ31magnesium alloyduring thermo-mechanical process. During uniaxial tension at300℃, if the c-axis isparallel to the tensile axis (TA), as seen in ND specimen, extensive DRX process wasobserved, and a highest volume fraction of DRX together with a smallest grain size wasformed after45%tension strain before fracture. When the c-axis is inclining45oorperpendicular to the TA, as seen in ND45or RD specimen, DRX occurred at a slow rateresulting with a low volume fraction of DRX compared to ND specimen. During directextrusion at300℃, DRX in the RA rod was retarded at initial stage of extrusioncompared with that in the EA rod3) Initial texture does affect DRX grain size at low and medium strain, but when itcomes to high strain at which a fully DRX structure is obtained, the influence of initialtexture on DRX grain size is gradually disappearing.4) The effect of initial texture on the orientation of DRX grains is connected withdeformation mode. The different deformation mechanisms and DRX behaviors causedby the different initial textures in ND, ND45and RD specimens resulted in different final textures in the three tensile samples during uniaxial tension at300℃. However, theinitial texture had no significant effect on final fully DRX extrusion microstructureduring direct extrusion at300℃.④DRX volume fraction of hot-extruded rod has significant effect on themicrostructure and texture evolution during the annealing. For the partially DRXhot-extruded rod, the texture transition from the <101ˉ0> to the <112ˉ0> component aswell as the preferential growth of grains having the <112ˉ0> texture component wereobserved during the annealing. For the fully DRX hot-extruded rod, it was a normalgrain growth process, no obvious change on texture was observed during the annealing,⑤DRX is a texture weakening process during extrusion. Although the texture ofthe partially DRX hot-extruded rod can weaken by static recrystallization, the texture ofthe extruded and annealed rod is still a little strong due to the preferential growth ofgrains having the <11ˉ20> texture component during the annealing. |
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