【作者】 刘建忠；

【导师】 刘天模；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2010， 硕士

【摘要】 从20世纪晚期开始,人们就一直致力于降低能源消耗和环境污染。随着自然资源短缺的加重,大范围的生产和使用轻金属合金,从而减轻重量、提高效率、降低成本,已经成为一种必然的趋势。目前,镁合金作为最轻的金属结构材料在多种技术应用上都取得了引人注目的成就,尤其在汽车、航空和电子产品领域得到了广泛的应用。但是,由于镁合金是密排六方结构,可开动的滑移系比较少,使得镁合金的塑性较差,降低了成型能力,这大大限制了变形镁合金的推广应用。正因为如此,这些年镁合金变形和成型的研究已经成为该领域里的一个重要的课题。本论文的工作中,我们首先通过双向双通道变通径(DDE)挤压工艺改善镁合金的组织和性能。论文选取应用比较广泛的AZ31镁合金作为研究对象。采用金相显微分析(OM)、X射线衍射分析、扫描电镜(SEM)、透射电镜(TEM)等手段,对挤压态AZ31镁合金的性能及其随后的拉伸、压缩过程中的变形行为、失效行为、退火再结晶行为进行了系统的研究。结果表明:DDE挤压工艺可以细化镁合金的晶粒,改善镁合金的力学性能。随着挤压温度的升高,晶粒发生长大,组织变得不均匀,力学性能变差。400℃挤压过程中沿挤压方向形成了基面{0002}纤维织构。对400℃挤压态镁合金进行室温拉伸、压缩变形,变形过程中发生了孪生,随着变形量的增加,组织中孪晶的数量增加,孪晶形态和孪生模式也发生了变化,厚透镜片状的{10—12}<101—1>拉伸孪晶在变形初期形成,薄片状的{10—11}<10—12>压缩孪晶在变形后期形成。相同变形量下,压缩时产生的孪晶更多,这是挤压过程中形成的基面{0002}纤维织构引起的。变形后的试样在退火过程中发生了静态再结晶,变形过程中形成的孪晶为再结晶的发生和晶粒的细化起到了关键的作用。400℃挤压态镁合金在室温拉伸、压缩过程中出现了明显的拉伸-压缩屈服不对称现象。晶粒尺寸、Al元素含量和织构是影响拉-压不对称性的关键因素。晶粒尺寸越小、Al元素含量越高、织构越分散,镁合金的拉-压不对称性越弱。对挤压态镁合金进行热处理,经历固溶处理的试样的拉-压不对称性最弱,这是晶粒尺寸、织构、Al元素三方面综合作用的结果。均匀态AZ31镁合金拉伸破坏实验过程中发生穿晶断裂,微观断口分析显示其断裂机理为解理断裂。400℃挤压态AZ31镁合金拉伸断裂机理为准解理断裂。经热处理后的挤压态镁合金在拉伸过程中出现颈缩现象,断裂均属于延性断裂,断裂机理均为微孔型断裂。不同热处理状态试样断口中的韧窝数量、深浅、形貌有所差异,经历固溶处理的试样的塑性较好,固溶+时效态的最好,这是因为Al的固溶引起了“固溶软化”,降低了柱面滑移的临界分切应力(CRSS),使得柱面滑移容易开动,改变了变形的模式,第二相的析出也为改善镁合金的塑性起到了关键的作用。更多还原

【Abstract】 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.更多还原