【作者】 李林；

【导师】 孙奇；

【作者基本信息】 西南交通大学 ， 材料科学与工程， 2022， 硕士

【摘要】 镁合金作为最轻的工程用金属结构材料,具有质轻、导电导热性能好和阻尼减震性能佳等优点,在交通、通讯、航空航天和电子工业等领域具有广阔的应用前景,被誉为21世纪资源与环境可持续发展的绿色材料。然而,相较于钢铁和铝合金等材料而言,镁合金因其独特的六方密排晶格结构,在室温条件下存在严重的拉压不对称性,这极大地影响了镁合金在复杂交变载荷下的服役寿命。从本质上讲,镁合金材料出现拉压不对称现象是由于镁合金晶粒在受到不同方向的应力时会激活不同的塑性变形机制(位错或孪晶)。因此,认识和理解镁合金材料在不同方向的循环加载下微结构的演变规律及微裂纹的萌生和扩展特点并明晰其背后的微观机制,对设计、开发和改善具有优异性能的新一代镁合金材料具有重要的理论和工程意义。本文以热轧AZ31商用镁合金为研究对象,综合运用包括光学显微镜、扫描电子显微镜以及电子背散射衍射等表征手段,同时结合对位错和孪晶的斯密特因子(Schmid factor,SF)以及几何应变协调因子的计算分析,阐明AZ31镁合金板材沿不同方向施加循环载荷时的微结构演变规律、孪晶变体的选择和界面微裂纹的形成特点以及其背后的微观机理。研究结果表明:(1)AZ31镁合金板材沿轧制方向(Rolling direction,RD)施加循环载荷时,会产生{10(?)2}孪晶-退孪晶现象。在此过程中,多数晶粒内部激活单一的{10(?)2}孪晶变体或者出现{10(?)2}孪晶变体对。{10(?)2}孪晶内部存在大量的基面位错滑移迹线,导致明显的“挤出”现象。疲劳微裂纹沿着{10(?)2}孪晶界面形核和扩展。孪晶内部的位错与孪晶界面交互作用时诱发的局部应力集中被认为是微裂纹在孪晶界面处形成和传播的主要原因。(2)AZ31镁合金板材沿轧板法向方向(Normal direction,ND)施加循环载荷时,同一晶粒内部会形成多种{10(?)2}孪晶变体。在此加载条件下,晶粒中出现了不满足斯密特规律的{10(?)2}-{10(?)2}二次孪晶。通过对二次孪晶的斯密特因子和几何应变协调因子进行分析,结果表明,该种{10(?)2}-{10(?)2}二次孪晶的形成与位错-孪晶界面交互作用导致的局部高应变状态有关。此外,相较于沿着RD施加循环应力的情况,不同孪晶变体之间的交互作用会阻碍{10(?)2}孪晶界面的逆向迁移,从而导致沿ND施加循环应力时退孪晶过程更难发生。(3)当对AZ31镁合金板材沿着RD方向进行应变控制的拉-拉疲劳加载时,塑性变形主要通过位错滑移承载,滞回曲线呈对称特征。在此条件下,疲劳裂纹大多为垂直于加载方向的长裂纹或短裂纹。裂纹主要萌生于硬取向晶粒和软取向晶粒之间的界面处,随后沿着晶界或者晶粒内部的滑移迹线扩展。当沿着AZ31镁合金板材的ND方向进行应变控制的拉-拉疲劳加载时,变形孪晶为其主要的塑性变形方式,滞回曲线亦存在明显的对称性。此时,疲劳裂纹大多为被限制在晶粒内部的短平直裂纹,主要沿着孪晶界萌生和扩展。更多还原

【Abstract】 As the lightest engineering metal structure materials,magnesium alloys have the advantages of low density,outstanding electrical and thermal conductivity and good damping properties,as a result of have a broad application prospect in the fields of transportation,aerospace and electronics industry.Magnesium alloys are regarded as the green material for sustainable development of resources and environment in the 21 st century.However,compared with steels,aluminum alloys and other materials,magnesium alloys exhibit serious tension-compression asymmetry at room temperature due to the hexagonal close-packed structure,which seriously affects the service life of magnesium alloy under complex alternating loads.In essence,the phenomenon of tension-compression asymmetry in magnesium alloy materials is due to the different plastic deformation mechanisms(sliping or twinning)that are activated when the grains are subjected to stresses in different directions.Therefore,a comprehensive and profound understanding of the evolution of the microstructure and the potential microscopic mechanism of the formation and propagation of microcracks for the magnesium alloys under cyclic loading in different directions,is of great theoretical and engineering significance for the design,development and improvement of a new generation of magnesium alloy materials with excellent properties.In this dissertation,the microstructural evolution,the selection of twin variant,and the formation characteristics of interfacial microcracks of AZ31 magnesium alloy are analyzed and discussed when cyclic loading is applied in different directions by using the characterization methods including optical microscopy,scanning electron microscopy and electron backscatter diffraction as well as the Schmid factor(SF)and the geometrical compatibility parameter.The main conclusions are shown as following:(1)The {10 (?) 2} twinning-detwinning phenomenon occurs when a cyclic loading is applied along the Rolling direction(RD)of the rolled AZ31 magnesium alloy sheet.During the deformation process,most of the grains activate a single {10(?)2} twin variant or a {10(?)2}twin variant pair inside the grain.A large number of basal dislocation slip traces generate inside the {10(?) 2} twins,leading to a significant extrusion characteriation.Fatigue microcracks nucleate and propagate along the {10(?)2} twin interface.The local stress concentration induced by the interaction between the dislocations inside the twin and the twin interface is considered to be the main reason for the formation and propagation of microcracks at the twin interface.(2)When the cyclic loading is applied along the Normal direction(ND)of the rolled AZ31 magnesium alloy sheet,multiple {10(?)2} twin variants are activated within the same grain.Compared with the case of cyclic loading applying along RD,the interaction between different twin variants prevents the reverse migration of {10(?)2} twin interfaces,which leads to more difficulty in the occurrence of the detwinning process when cyclic stress is applied along ND.In addition,under this loading condition,a type of {10(?)2}-{10(?)2} secondary twin that does not satisfy Smit’s law generated in the grains.The results of the analysis of the Schmid factor and the geometrical compatibility parameter indicate that the formation of this{10 (?) 2}-{10 (?) 2} secondary twin is related to the local high stress state caused by the dislocation-twin interface interaction.(3)When the strain-controlled tensile-tensile fatigue loading is applied to the AZ31 magnesium alloy sheet along the RD direction,the plastic deformation is mainly carried by dislocation slip and the hysteresis curve is symmetrical in character.In this case,the fatigue cracks are mostly exhibited long or short cracks which perpendicular to the loading direction.The cracks mainly generate at the interface between the hard and soft oriented grains,and then propagate along the grains or the slip traces inside the grains.When the strain-controlled tensile-tensile fatigue loading is applied along the ND direction,the deformation twin is the main plastic deformation mode,and the hysteresis curve also has obvious symmetry.In this case,most of the fatigue cracks are short flat cracks which are confined inside the grains,mainly forming and propagating along the twinning boundary.更多还原