【作者】 郑三龙；

【导师】 蒋炎尧； 张玮； 高增梁；

【作者基本信息】 浙江工业大学 ， 化工过程机械， 2013， 博士

【摘要】 镁及镁合金具有低密度,高比刚度和高比强度,易加工回收等优点,是电子电器、交通汽车、航空航天和兵工行业最具发展潜力的金属结构材料。镁及其合金塑性变形能力差、目前大部分镁合金构件为铸件,难满足工程实际在力学性能和产品形状尺寸上的需求。变形镁合金以优良机械性能和尺寸、规格多样优势受广泛关注,成为材料领域研究的重点方向之一。工程实际构件往往承受循环载荷,疲劳失效是在役设备最主要的失效方式之一。压力加工易引起镁合金晶粒择优取向形成织构,导致机械性能各向异性,对镁合金构件的疲劳行为和寿命评估有重要影响。研究织构镁合金的疲劳裂纹扩展行为、机理和寿命预测方法,具有重要的理论意义和应用价值。本文针对挤压纯镁和挤压AZ31B镁合金,研究不同方向疲劳裂纹扩展行为和断裂机理,利用光学显微镜(OM)、扫描电镜(SEM)和背散射电子衍射(EBSD)等手段对材料的组织、裂纹扩展行为及断口进行分析,讨论了不同方向塑性变形、断裂机理、裂纹扩展路径及特征,分析了织构对挤压圆棒不同方向裂纹扩展的影响；分析Walker和Wheeler模型以及Paris公式对常幅和变幅加载下疲劳扩展速率预测适用性。本文的主要工作和成果如下：研究表明挤压AZ31B镁合金圆棒具有纤维状分层不均匀组织,晶粒粗细不均匀,大晶粒平均约50μm,小晶粒平均约8μm,部分晶粒沿挤压方向严重拉长、长大。挤压纯镁主要由等轴晶粒组成,大部分晶粒平均约1501μm,部分晶间夹杂有粒径约20gm小晶粒,无纤维状拉长分层组织。挤压纯镁和AZ31B镁合金均为强基面织构,大部分晶粒基面平行于挤压方向,c轴以圆棒轴心为中心呈放射状分布,织构指数分别为15.85和12.65。试样的取向对挤压AZ31B镁合金疲劳裂纹扩展行为有重要影响,沿挤压(T-L)方向和垂直挤压(L-T)方向裂纹扩展应力强度因子幅门槛值在0.95～1.34MP(?)m之间,受应力比影响不大。径向(T-R)方向门槛值在1.04～2.43MP(?)m间,随应力比增加而增加。三个方向裂纹均以穿晶为主和部分沿晶模式扩展,T-L和L-T方向裂纹以第二系列<c+a>锥面滑移解理模型为主,而T-R方向有<c+a>锥面滑移也存在沿孪晶界扩展。T-L方向裂纹扩展路径沿Ⅰ型方向呈直线扩展,裂纹扩展速率最快；T-R方向裂纹呈曲折沿Ⅰ型扩展；L-T方向裂纹扩展严重偏离Ⅰ型方向,裂纹有朝{1120}晶面发生偏转或分枝趋向,裂纹扩展速率最慢,在近门槛附近出现稳定扩展平台,da/dN-△K双对数曲线呈三段线性关系。裂纹扩展速率均随应力比的增加和加载频率的降低而增加,Walker有效应力强度因子模型与Paris公式可以的描述挤压AZ31B三个方向常幅加载的裂纹扩展速率,可以用于预测AZ31B疲劳裂纹扩展寿命和安全评估。单幅过载对AZ31B镁合金疲劳裂纹扩展有延滞作用,但过载影响区均小于0.35mm,过载后裂纹扩展速率立刻降至一个最小值,但随着裂纹延展,扩展速率迅速上升达到过载前水平,过载中没有发生裂纹尖端撕裂或偏折现象。高低幅加载过程中,当前步加载最大载荷大于后步最大载荷时,裂纹扩展有过载效应。当前后两步最大载荷相同时,裂纹扩展受载荷幅控制。Wheeler模型可以合理模拟过载效应。挤压纯镁圆棒三个方向裂纹均以穿晶扩展为主,塑性变形第二系列<c+a>锥面(1122)<1123>和(1122)<1123>滑移机制为主,T-L和L-T方向趋于沿(1120)面扩展,T-R方向部分晶粒产生{1012)孪生协助滑移塑性变形,存在<a>和<c+a>两种滑移机制。三个方向均呈解理脆性断裂特征。L-T方向裂纹扩展严重偏离Ⅰ型方向,容易产生分叉,裂纹扩展速率最慢,T-L方向基本呈直线扩展,裂纹扩展速率最快,T-R方向裂纹扩展路径出现局部波动和分叉。裂纹偏离、分叉和微裂纹导致扩展速率降低。常幅加载下三个方向的裂纹扩展da/dN-△K双对数曲线型双线性关系,L-T和T-L方向在ΔK≈3.0MPa(?),T-R方向在ΔK≈3.4MPa(?)附近两边斜率不同。单幅拉伸过载对纯镁L-T和T-R方向裂纹扩展延滞的实际影响区域很小,分别不超过0.1mm和0.2mm,远小于Wheeler模型预测过载影响区域,T-L方向过载没有观察到延滞现象,有促进裂纹扩展迹象。压缩过载引起裂尖孪品和残余拉应力,促进裂纹扩展。更多还原

【Abstract】 Magnesium (Mg) alloys have excellent mechanical properties such as low density, superior specific strength and stiffness, good physical machinability and recyclability. They are important future engineering metallic structure materials in electrons, transportation, aerospace and weapon industry. Because of their poor ability of plastic deformation, most magnesium alloys components are made by casting process. Due to pores and inclusions left from the casting process, cast Mg alloys do not sastify the engineering requirement for strength. Wrought Mg alloys exhibit superior physical properties and they become a subject of research. Cyclic load is usually applied on a real structure, and fatigue failure is the main failure mode for a structure in service. The fatigue properties of wrought Mg alloys are anisotropic due to initial texture and microstructure caused by extrusion and rolling processes. It is significant to study the fatigue crack growth of magnesium alloys with strong textureIn this thesis, the study focuses on the a detailed investigation on the fatigue crack growth (FCG) behavior and fracture mechanism of extruded pure Mg and AZ31B Mg alloy bar with strong texture. The microstructure, texture, crack growth path and fracture face were investigated by optical microscope (OM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The study was conducted using compact tension specimens oriented in three different directions with respect to the extrusion direction. The effect of texture on FCG of different direction specimen is discussed. The experimental results are used to evaluate two existing fatigue models (Walkers model and Wheelers model) for their capabilities to account for the R-ratio effect, the overload effect, and the high-low loading sequence effect. The following conclusions were reached:The extruded AZ31B Mg alloy consists of an inhomogeneous microstructure exhibiting equiaxed large grains and equiaxed small grain clusters. The average sizes of the large and small grains are approximately50μm and8μm, respectively. On the plane parallel to the extrusion direction, large grains are elongated due to the extrusion process and a lamellar microstructure composed of alternatively elongated large grain and small grain cluster is developed. On the other hand, the extruded pure Mg is mainly composed of equiaxed large grains with an average size of approximate150μm and a few small grains with a size of20μm. No elongated grains and lamellar microstructure were abserved in extruded pure Mg. A strong basal texture was observed in both extruded AZ31B Mg alloy and pure Mg. The basal planes of most grains are parallel to the extrusion direction and the c-axes of most grains are orientated along the radial direction. The texture factor of the extruded AZ31B Mg alloy and pure Mg are15.85and12.65, respectively.The specimen orientation with respect to the extrusion direction plays a critical role in FCG of extruded AZ31B Mg alloy. The threshold stress intensity factor ranges are similar in the T-L and L-T specimens with a value of0.95～1.34MP(?) and they are not significantly influenced by the load ratio. The threshold stress intensity range of the T-R specimens is1.04～2.43MP(?) and increases with the load ratio. Transgranular cracking is the major fatigue propagation mode in all the three specimen orientations. In the T-L and L-T specimens, two sets of2nd order <c+a> pyramidal slip induced cleavage dominates the transgranular cracking whereas both twin boundary cracking and <c+a> pyramidal slip induced cleavage exist in the T-R specimens. The cracks of the T-L specimens grow in typical mode I with the fastest FCG rate. The overal crack path of the T-R specimens follows Model1cracking, and local branching and cracking deviated from the horizontal direction are developed. A large angle of deviation from the horizontal line was observed in the crack path of L-T specimens. The crack tends to deviate or branch following the {1120} crystal planes. The FCG rate in the L-T specimens is the lowest among the three orientations. A nearly constant FCG rate was observed in the L-T specimens right after the threshold zone, and the da/dN-ΔKcurve in log-log scale exhibits three linear relationship with different slopes. The FCG rate increases with increasing stress ratio in all the specimens with three orientations. Walker"s model and Paris law can correlate well the constant amplitude FCG experiments with different R-ratios, and can be used to predict the FCG life and safety assessment for extruded AZ31B Mg alloy.For AZ31B Mg alloy, application of a single tensile overload during constant-amplitude loading results in an immediate decrease in the FCG rate to a minimum value in all the three specimen orientations. As the crack extends, the FCG rate recovers rapidly to the level expected during constant amplitude loading. The reduction in the FCG rate due to overloading is significant but the overload influencing zone size is small. No crack extension was detected during overloading in the experiment. In the high-low loading sequence experiment, the influence of the higher loading step on the crack growth of the subsequent lower loading step is dependent on the relative magnitude of the maximum loads in the consecutive loading steps. If the maximum loads in the two loading steps are identical, there is no loading sequence influence on the crack growth. With identical R-ratios or identical minimum loads in high-low two-step loading, the influence of the higher amplitude loading on the crack growth of the subsequent lower amplitude loading is similar to that of overloading in a constant amplitude loading. The modified Wheeler model can reasonably predict the influences of overloading and sequence loading on FCG.For extruded pure Mg, transgranular cracking is the major fatigue propagation mode in all the three specimen orientations. In the T-L and L-T specimens, two sets of2nd order<c+a> pyramidal slip (1122)<1123> and (1122)<1123> induced cleavage dominate the transgranular cracking whereas both{1012} twin boundary cracking and <a>,<c+a> pyramidal slip induced cleavage exist in the T-R specimens. Severe deviation from the pure Mode I cracking and many branchs were observed in the crack path of the L-T specimens. The FCG rate in the L-T specimens is the lowest in three orientations of specimens. The crack in the T-L specimen grows in typical Mode I direction with the fastest FCG rate among the three orienations for a given stress intensity factor range. The crack in the T-L specimen grows in Mode Ⅰ, and local deviation from Mode I cracking and a few branchs were observed in the crack growth path. The FCG rate discreases due to cracking dievation from Mode I, branching, and local microcracking. The da/dN-ΔKcurve under log-log scale exhibits two linear relationships with different slopes in all three orientations of specimens. The curves are divided to two parts at ΔK≈3.0MPa(?) for L-T and AK≡3.4MPa(?) for the T-L and T-R directions.For extruded pure Mg, application of a single tensile overload during constant-amplitude loading results in an immediate decrease in the FCG rate to a minimum value in L-T and T-R specimen orientations. The overload influencing zone size is less than0.1mm and0.2mm for L-T and T-R specimen orientations, respectively, which is much less than the predicted value by Wheeler’s model. No obvious overload influence was observed in the T-L specimen. Application of a compressure underload accelerates the FCG due to the residual tensile stress and twinning ahead of the crak tip.更多还原