| Magnesium as a kind of lightweight material is the lightest structure material forthe practical application. Due to abundant and conducive to recycling, magnesium andits alloys have broad application prospects in the automobile industry, aerospaceindustry and national defense military and other fields. Quasicrystal, which was foundin rapid solidification of aluminum-manganese alloys firstly, is a new kind ofsecondary phase and has new atom structure with rotational symmetry but do not havetranslational periodicity. Due to the special atom structure, quasicrystal has manyunique properties which crystalline phases do not possess.In this study, in order to fabricated the Mg-Zn-Gd alloy with different volumefraction of quasicrystal, three alloys with different alloying element contents but withconstant Zn/Gd atomic percentage of6were designed. They are Mg-1.5Zn-0.25Gd，Mg-2.4Zn-0.4Gd and Mg-3.6Zn-0.6Gd （at.%）. Firstly, the microstructure andmechanical properties of the as-cast samples and the samples after T4treatment at400℃for8hours were studied so as to analyze the effect of alloying element content on themicrostructure and mechanical properties of these samples. And then, the hotcompression deformation behavior of the as-cast Mg-Zn-Gd alloy was studied in orderto obtain constitutive equation and the formation mechanism of nanoscale quasicrystalas well as the effect of nanoscale quasicrystal precipitation on the textureevolution.Thirdly, the three alloys were carried out hot extrusion under differentconditions, the effect of extrusion process parameters on microstructure andmechanical properties as well as texture were studied, especially focus on the effect ofextrusion process on the volume fraction of nanoscale quasicrystal precipitates, toestablish the relationship among the alloying content, processing parameter andvolume fraction of nanosacle quasicrystal. The orientation relationship and the atomicmatching of the interface between the nanoscale quasicrystal and the Mg matrix wasstudied to reveal the strengthening and toughening mechanisms of Mg-Zn-Gd-basedalloys reinforced with nanoscale quasicrystal. Then, the microstructure evolution ofthe extruded samples in the200℃high temperature tensile processing was studied soas to reveal the primary reasons why the extruded samples have excellent plasticity andto further discuss the crack behavior of quasicrystalline phase as well as the interaction between the dislocation and quasicrystal. Finally, a new double continuously extrusionprocessing was proposed and an extruded samples with high properties was fabricatedby this method. The main results for this study are summarised as follows:Microstructure and mechanical properties of the designed alloys at both as-cast andT4-treated conditions were studied. Three studied alloys were composed of primary-Mg solid solution, eutectic icosahedral quasicrystalline phase (I-phase), and smallamounts of Mg2Zn3phase and W-phase. With the increase of the alloying elementscontents, the dendritic microstructure is refined, the corresponding strength graduallyincreased, and the elongation gradually decreased because the increase of thesecondary phase sizes leads to stress concentration under as-cast and T4-treatedconditions. The large size I-phase has an orientation relationship with the Mg matrix asfollows:[1120]Mg∥[2-fold]I-phase，(0001)Mg∥(5-fold)I-phase. For as-cast alloys,micro-cracks originated in the-Mg matrix and quasicrystal interface. The fracturemode exhibits intergranular fracture characteristics. After T4treatment, the yieldstrength decreased, and the ultimate tensile strength and elongation increased clearlydue to the dissolution of Mg2Zn3phase and the decrease of second phase sizes.Correspondingly, the fracture mode changes to transgranular fracture.Deformation behavior of as-cast Mg-1.5Zn-0.25Gd and formation mechanism ofnano quasicrystal，as well as the effects of nano quasicrystal on texture randomizationwere investigated. The constitutive equation for the as-cast Mg-1.5Zn-0.25Gd alloydeformation range from200℃-350℃and0.001s-1-1s-1is as follows:=1.96×109exp（0.1410053255σ）exp（-22576.189229/T）. With the increasing of the deformationtemperature and/or strain, the texture was randomized. The needle-like Mg4Zn7phasesprecipitated in Mg matrix for as-cast sample are vanished during compression,consequently leading to formation of nanoscale quasicrystal, which is attributed to theatomic diffusion accelerated by defects. Orientation relationship among Mg matrix andMg4Zn7as well as nanoscale quasicrystal can be concluded as follows:[110]Mg∥[107]Mg4Zn7∥[2-fold]I-phase. Alloying element contents and temperature are veryimportant factors for the nanoscale I-phase precipitation and its formation meet theclassical theory of nucleation and growth mechanism. The quasicrystal could nucleateon the Mg4Zn7crystalline phase with the above orientation relationship.The optimum extrusion conditions for nano quasicrystal precipitation wereobtained.(1) Higher contents of alloying elements and larger extrusion ratio. After extrusion at250℃, the grains are refined and the ultimate tensile strength and elongation are improved as either the volume fraction of I-phase or the extrusion ratio is increased, which can be attributed to the secondary phase particle stimulate recrystallization nucleation and restrain grain boundary motion. Moreover, anisotropy is mitigated in all of the alloys as either the I-phase fraction or the extrusion ratio is increased due to the synergetic effect of texture weakening and grain refinement as well as the effect of I-phase on twinning. On the other hand, with the increase amount of the I-phase, the yield strength is decreased for the alloys extruded at low ratio owing to the texture weakening, yet increased for the alloys extruded at high ratio owing to the strengthening originating from the I-phase and refined grains.(2) Extrusion at as-cast condition directly with lower extrusion temperature. Compared with the samples without T4treatment, the samples extruded after T4treatment show larger grains, lower strength and elongation, which can be attributed to the secondary-phase particles precipitation in the matrix during T4treatment and to the fact that only a few volume fraction of nanoscale I-phase is precipitated in the matrix during extrusion. Moreover, the anisotropy is mitigated in the alloys extruded at as-cast condition because of the grain refinement and the I-phase precipitation.Micro structure evolution during high temperature tension, orientation relationship between nano quasicrystal and Mg matrix, and crack behavior of quasicrystal were investigated. The orientation relationship between the nanoscale I-phase and Mg matrix are as follows:[1120]Mg//[2-fold]I_phase,(2-fold)I_Phase//(0001)Mg,(5-fold)I_phase//(0111)Mg and interface atomic matching is coherent, which reveals that nanoscale I-phase is an excellent strengthening and toughening secondary phase. The crack behavior of the quasicrystal during deformation was studied by TEM, and the results showed that quasicrystal exhibited brittleness and2-fold plane was the main crack plane, while the other plane such as3-fold and5-fold plane also could be acted as crack propagation plane. Dislocation mainly bypassed microscale quasicrystal by climbing. With the increasing the strain, Mg-Zn phase precipitated in the matrix with the size range from5-20nm, many defects formed in the nanoscale quasicrystal and crack propagation along the edge of the icosahedral, and quasicrystal may be transformed to MgZn3Gd, W or W crystalline phase. The orientation relationship among the W, W and the Mg matrix are:[001]w’//[001]w//[7253]Mg.Large amount of non-basal dislocation were found in the sample during tensile at200℃because the samples had excellent plasticity. In addition, sub-grain boundary formed by dislocation accumulation may be is the omen for the broken.An ultrafine-grained quasicrystalline strengthened magnesium alloys withexcellent mechanical properties by conventional and double continuously extrudingas-cast Mg-1.5Zn-0.25Gd (at.%) ingot at100℃was developed. After extrusion,multimodal microstructure was formed and the grain sizes were smaller than1μm.The conventional extruded sample exhibited yield strength of about417MPa, with anelongation of8.3%. These values for double continuously extruded sample are423MPa and9%, respectively. Compared to the samples extruded at250℃, the notableimprovement in strength mainly attributed to the grain refinement, dense distributionof the fine precipitates inside the fine dynamical recrystallization grains and the highintensity of typical basal texture. |
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