拉-压及拉-扭载荷下6063合金的疲劳行为

对6063锻造铝合金进行了不同应力幅值下拉-压及拉-扭复合疲劳试验,并用透射电子显微镜观察了疲劳失效试样的位错结构。结果表明:在循环加载过程中,循环硬化占据主要地位,循环硬化的速率和程度对应力幅值和加载路径有依赖性;相同应力幅值下,试样在拉-扭复合加载下失效比拉-压失效形成更为复杂的位错结构,且位错密度更高;位错之间及位错与析出相的交互作用是材料发生循环硬化的主要原因,循环硬化程度越高,疲劳寿命越短。     

EBSD-Based Microscopy: Resolution of Dislocation Density

Consideration is given to the resolution of dislocation density afforded by EBSD-based scanning electron microscopy. Comparison between the conventional Hough- and the emerging high-resolution cross-correlation-based approaches is made. It is illustrated that considerable care must be exercised in selecting a step size (Burger's circuit size) for experimental measurements. Important variables affecting this selection include the dislocation density and the physical size and density of dislocation dipole and multi-pole components of the structure. It is also illustrated that simulations can be useful to the interpretation of experimental recoveries.     

Piezoelectric study for a coated hole of quasi-polygonal shape in an infinite plate

This paper studies the piezoelectric problems for a coated hole of quasi-polygonal shape embedded in an infinite matrix subjected to electromechanical loadings. The electromechanical loadings considered in this work include a screw dislocation, a point force, a point charge, a far-field anti-plane shear and an in-plane electric field. Each component is assumed to be transversely isotropic medium belonging to a hexagonal crystal class 6 mm and poled in the x₃ direction. Based on the complex variable and analytical continuation method, the general expressions for the complex potentials can be derived in each medium. Numerical results are provided to show the effect of hole shape, the material combinations and the loading condition on the electro-elastic fields due to the presence of the coated film. The image force exerted on a dislocation, which can be used to probe the mobility of the screw dislocation, will be calculated by means of the generalized Peach-Koehler formula.     

Quasicontinuum simulation of indentation on FCC metals

Quasicontinuttm simulations were performed to study the processes of incipient plastic deformation on three FCC metals （Ag, Ni and Pd） under the action of a rigid indenter. Four widths （9.3, 18.6, 27.9 and 37.2 A） of the indenters were modelled for each metal specimen. A series of load--displacement responses and the strain energy versus displacement responses of the indenter were presented. It is shown that the abrupt drop of the load in the load--displacement response is triggered by the nucleation of dislocations in the metals. The critical load of each metal specimen increases with the increase of the indenter width, while the hardness of the metal specimen decreases as the indenter width increases. Furthermore, the microscopic mechanism of deformation in the films was analyzed. Two dislocations are nucleated respectively beneath the right and left sides of the indenter. Each dislocation successively decomposes into two Shockley partial dislocations after the first nucleation process. The distances between the two partial dislocations are equal for both sides of the indenter, which are in agreement with the theoretical values.     

ATOMISTIC SIMULATION OF NANOSTRUCTURE FOR MACHINING-TENSION PROCESS

利用分子动力学在原子尺度模拟了单晶Cu (111)面纳构件的纳米加工过程和加工后纳构件的拉 伸过程, 分析了纳刻划过程的缺陷行为及加工缺陷对纳构件力学特性的影响. 结 果表明: 在纳刻划过程中, 在针尖的前方和下方形成加工变形区; 当刻划深度 较浅时, 位错仅在表面与亚表面繁殖; 随着刻划深度的增加, 加工后残留的缺 陷数量增加, 纳构件的有序度及首次屈服应力下降; 加工后的纳构件内部, 尤其在针尖退出处有较高的残余应力. 对加工后的纳构件施加拉伸载荷, 由于 存在残留加工缺陷和较高残余应力, 其应力--应变曲线在弹性上升阶段有局部下降; 在塑性阶段, 由于位错繁殖及位错塞积和中间部分原子的迁移重构使应力--应变曲 线呈锯齿状逐渐下降. 纳构件断裂失效前表现为单原子相连的纳 链. 纳构件的有序度随着刻划深度的增加而下降. 在应变为0.8处, 刻划较浅的 纳构件的有序度较首次屈服处的有序度略好.     

SiCP/ZL104泡沫复合材料的阻尼性能

对采用熔体直接发泡法制备的SiC_P /ZL104泡沫复合材料进行了阻尼性能和机理的分析。研究结果表明: 在孔径一定的条件下, SiC_P/ZL104泡沫复合材料室温下的阻尼性能表现为损耗因子随着孔隙率的增大呈非线性递增趋势, 且递增幅度越来越小; SiC_P/ZL104泡沫复合材料的阻尼性能好于相同孔隙率的泡沫铝、 SiC_P/ZL104复合材料和ZL104的阻尼性能; SiC_P/ZL104泡沫复合材料的损耗因子随着SiC_P含量的增加和SiC_P粒度的减小而增大。SiC_P/ZL104泡沫复合材料内部的气泡以及高密度位错对其阻尼性能的提高起主要作用。      

Relationship between Work Hardening Behaviour and Deformation Structure in Ni‐free High Nitrogen Austenitic Stainless Steels

The work hardening behaviour of high nitrogen austenitic steel (HNS) depends not only on the nitrogen content but also on the addition of substitutional alloying elements such as Mn and Ni, although the effect of nitrogen content has been considered to be a main factor controlling the work hardening rate in HNS. In this study, two kinds of high nitrogen austenitic steels containing nearly 1 mass‐% of nitrogen with and without Mn (Fe‐25%Cr‐1.1%N and Fe‐21%Cr‐0.9%N‐23%Mn alloys) were tensile‐tested and their work hardening behaviour was investigated for the purpose of clarifying the effect of Mn on the work hardening behaviour. Then the results were related to the change in deformation substructure. In the Fe‐25Cr‐1.1N alloy, the work hardening rate kept high until fracture occurred, while in the Fe‐21Cr‐0.9N‐23Mn alloy it tended to decrease gradually with tensile deformation in the high strain region. It was concluded that the difference in work hardening behaviour between both alloys is attributed to the change in dislocation substructure from planar dislocation array to dislocation cell by the addition of Mn.     

空间地震动场的相干性研究

摘 要 地震动的空间变化受震源破裂过程、传播途径和场地条件等因素影响,目前地震动场的相干性研究大多是用台阵实测数据,不具有普适性。本文采用震源位错模型,利用数值方法模拟均匀单一介质随机地震动场的理论地震图,通过对大量计算数据的分析,研究地表地震动场的空间相干性。分析结果表明：本文数值模拟结果所反应的地震动空间相干性规律与实测数据所反应的规律基本吻合,表明本文分析方法和模型的合理性。同时空间地震动场相干性随地震波的传播方向存在各向异性,断层走向是空间地震动场相干性的重要影响因素。本文方法是对地震动场相干性具有普适性的一个尝试。     

含富Cr沉淀颗粒Fe-Cr合金单晶体的疲劳变形特征

在恒塑性应变幅ε_pl控制条件下研究了含富Cr沉淀颗粒的单滑移取向 Fe-35%Cr合金(质量分数)单晶体的疲劳变形特征. 实验结果和分析表明,当ε_pl≧2.5×10^-3时,由于运动的位错易于切过细小富Cr沉淀颗粒,导致在循环刚开始第1周的拉伸阶段出现了明显的应力软化现象,并接着在疲劳早期阶段表现出轻微循环软化现象. 而且,晶体在循环过程中表现出一定程度的拉--压应力不对称性,这种增强的应力不对称性与细小富Cr沉淀颗粒的变形不稳定性有关.由大量细小滑移线聚集而成的粗滑移带以及高应变幅下形成的扭折带是该晶体主要的滑移变形特征.主裂纹大致沿主滑移面 发展并最终沿此开裂, 同时伴随有一些形态各异的二次裂纹在表面萌生.微观结构观察发现, 在低应变幅(ε_pl=5.0×10^-4ε_pl)下就有类驻留滑移带(PSB)楼梯结构形成, 其体积分数随ε_pl的增大而增加.当ε_pl增大到5.0×10^-3时,位错胞结构成为其主要微观结构特征.     

[011]取向镍基单晶合金蠕变特征

研究了一种[011]取向镍基单晶合金的拉伸蠕变特征及其变形期问的微观组织结构.结果表明:在750℃/680 MPa条件下,合金的初期蠕变和稳态蠕变速率相对较高,蠕变寿命较短.TEM观察显示,蠕变期间的变形特征是1/20<110>位错在基体中运动,发生反应形成1/3<112>超Shockley不全位错切入γ'相后产生层错;在870℃/500 MPa条件下,蠕变中期出现不均匀滑移带并有大量超不全位错剪切γ'相,使合金具有较高的应变速率;在980℃/200 MPa条件下,合金具有较长的蠕变寿命和较低的稳态蠕变速率.不同Burgers矢量的位错相遇发生反应形成界面位错网,位错网可以阻止位错切入γ'相,γ'相沿[010]方向扩散生长,逐渐转变成筏形组织.蠕变后期位错切入,γ'相,是合金变形的主要方式.