裂纹尖端局部形变带的TEM观察SCIEI

在透射电子显微镜(TEM)下对铁素体-奥氏体双相不锈钢薄膜拉伸试样进行了动态拉伸实验,发现在铁素体相和奥氏作相中在裂纹尖端有与裂纹扩展方向接近一致的局部形变带生成,其形成与裂尖Schmid因子、位借的屏蔽效应、潜在硬化和硬化系数的变化规律有关.当裂纹尖端优先在纯II型应力分量作用下开动与裂纹共面的滑移系、并且有合适的硬化系数下降速率时,裂尖易形成局部形变带。     

位错和位错偶沿单─滑移系从裂纹尖端的发射

采用计算机模拟了位错和位错偶沿单一滑移系从裂纹尖端的发射,考察了滑移面取向、外加载荷、晶格摩擦力以及位错发射的临界应力强度因子对所发射的位错数量、塑性区与无位错区大小以及裂关残余应力强度因子的影响研究表明,位错从裂纹尖端发射的临界应力强度因子对无位错区的存在和其大小起决定作用,而外加载荷与晶格摩擦力主要影响位错发射的数量以及塑性区大小．在Ｉ型载荷作用下,滑移面与裂纹面的夹角越大,从裂尖发射出的位错数量越多,位错对裂纹的屏蔽效应也越大当裂纹发射位错后的残余应力强度因子仍然较大时,位错偶就有可能在裂纹尖端附近产生井沿着几个滑移面发射,但发射出的位错偶对裂纹没有明显的屏蔽作用在滑移面不垂直于裂纹面时,发射出的位错或位错偶关于裂纹面呈不对称分布     

位和位错偶沿单一滑移系从裂纹尖端的发射

采用计算机模拟了位错和位借偶沿单一滑移从裂纹尖端的发射,考察了滑移面取向,外加载荷,晶格摩擦力以及位错发射的临界应力强度因子对所发射的位错数量,塑性区与无位错区大小以及裂尖残余应力强度因子的影响。     

不锈钢微裂纹形核扩展的TEM原位观察

３１０奥氏体不锈钢在透射电镜中的原位拉伸观察表明，如在裂尖发射位错后保持恒位移，则在裂尖能形成无位错区，这个弹性的ＤＦＺ不均匀减薄，其应力集中可等于原子键合力，从而导致纳米微裂纹在ＤＦＺ中形核，并很快钝化成空洞，如果裂尖发射位错后继续拉伸，则上述基本过程被掩盖，仅看到交滑移使裂尖区减薄，从而导致微米给空洞或裂纹的形核和连接。如稍稍卸载，则可观察到裂尖前反塞积位错群的逆返现象。     

Mg-Gd-Y-Zn稀土镁合金中的位错滑移转移现象

通过原位拉伸对Mg-Gd-Y-Zn稀土镁合金中位错在相邻晶粒内的滑移转移现象进行了研究。使用Luster-Morris因子m′评估相邻两个晶粒内滑移系之间的几何关系。研究发现,Mg-Gd-Y-Zn稀土镁合金中裂纹更容易在硬取向和软取向晶粒的交界处形成。对实验结果进行统计分析后发现,位错滑移转移发生时,相邻晶粒滑移系的m′大于0.77。当晶界取向差小于34.2°时,发生基面位错-基面位错滑移转移现象;当晶界取向差大于48.8°时,发生基面位错-锥面位错滑移转移现象。此外,研究发现,m′与基面滑移系的Schmid因子相乘,可以定量地判断基面位错-基面位错的滑移转移现象,而对于基面位错-锥面位错的滑移转移现象,m′与Schmid因子相乘的方法并不适用。     

Ti3Al金属间化全物氢致解理断裂及其机理

Ｔｉ－２４Ａｌ－１１Ｎｂ合金在室温下恒载荷动态充氢以及交变载荷动态充氢时，氢通过促进裂纹在滑移面上的解一，促进了裂纹扩散，不连续的氢致解理裂纹优先在α２相形核，α２／β相界面是裂纹扩展的有效障碍，氢通过促进裂尖位错的发射、解理微裂无位错区（ＤＦＺ）的形核以及解理裂纹，导致氢致解理断裂。     

316L奥氏体不锈钢应力腐蚀裂纹的形成机制

利用EBSD分析了316L奥氏体不锈钢应力腐蚀穿晶裂纹的形成机制。结果表明,裂纹扩展受晶体滑移性能影响,Schmid因子小的晶粒抵制应力腐蚀裂纹扩展,Schmid因子大的晶粒有利于裂纹扩展。裂纹形核和扩展涉及晶体滑移、钝化膜破裂和阳极溶解,这些过程均在{111}滑移面上进行。在高温低应力和含氯离子的腐蚀环境的交互作用下,应力腐蚀的裂纹扩展过程符合滑移溶解机制。     

疲劳裂纹尖端的位错结构

在双相钢物理短裂纹门槛区，观察到稳定的位错胞和墙结构；长裂纹门槛区，在铁素体／马氏体相界堆垛位错密度大，有形成位错胞的趋势．长裂纹扩展第二阶段，铁素体晶粒内具有单向滑移线（Ｒ＝０，－１）和正交网状（Ｒ＝－１）的位错结构，长裂纹扩展第三阶段，位错稀少，但单滑移、双交滑移位错线明显拉长，说明裂纹尖端位错组态是应变历史的产物．疲劳裂纹扩展门槛区形成的位错胞和墙是一亚稳态结构，与门槛循环应力应变处于动态平衡，也是一微观结构参数．     

310奥氏体不锈钢应力腐蚀的透射电镜原位观察

用自制的恒位移加载台，在透射电镜中原位观察310奥氏体不锈钢在纯水中局部溶解前后裂尖位错组态的变化以及微裂纹的形核和扩展。结果表明，310奥氏体不锈钢在室温纯水中局部阳极溶解能促进位错发射，增殖和运动，在低应力下，纳米级微裂纹在无位错区中连续或不连续形核，由于介质的作用，纳米级微裂纹并不钝化成空洞或缺口，而是解理扩展。     

310奥氏体不锈钢应力腐蚀的透射电镜原位观察

用自制的恒位移加载台，在透射电镜中原位观察３１０奥氏体不锈钢在纯水中局部溶解前后裂尖位错组态的变化以及微裂纹的形核和扩展。结果表明，３１０奥氏体在室温纯水中局部阳极溶解促进位错发射，增殖和运动，在低应力下，纳米级微裂纹在无位错区中连续或不连续形核，由于介质的作用，纳米级微裂纹并不钝化成空洞或缺口，而是理解扩展。     

IN-SITU OBSERVATION OF CRACK TIP DEFORMATION IN A DUPLEX STAINLESS STEEL

The development of dislocation structures in the plastic zone ahead of a crack tip has been in-vestigated in a duplex stainless steel during in-situ deformation experiments in a scanningtransmission electron microscope.It was found that the dislocation distribution wassignificantly different in the ferrite and in the austenite.In the ferrite grains,the dislocationsemitted by the crack tip may cross-slip out of the original slip planes and form a broad plasticzone.However,in the austenite,the dislocation free zone is small and the dislocations emittedby the crack pile up in its slip plane.The selection of slip systems at the crack tip depends onthe crack tip Schmid factors in both phases.But after large deformation,the selection of thesecond slip systems at the craek tip in austenite does not depend on the Schmid factors.     

TEM OBSERVATION OF LOCAL DEFORMATION BAND AT CRACK TIP

The thin foil specimen of a ferrite-austenite duplex stainless steel was tensiled under transmis-sion electron microscope(TEM).It was found that both in ferrite and austenite the localdeformation band at crack tip was formed near to the crack propagating direction.Its forma-tion was related with the crack tip Schmid factor,dislocation shielding,latent hardening andhardening coefficient.When the crack tip emitted dislocations to a slip system by the action ofpure mode Ⅱ stress resolute,and the decreasing rate of hardening coefficient was suitable,lo-cal deformation band was easy to form.     

含有{111}织构的纳米孪晶多晶铜在拉伸变形过程中塑性各向异性机制的分子动力学模拟

基于分子动力学模拟,从主导滑移系的斯密特因子和位错机制两个方面系统研究含有{111}织构的纳米孪晶多晶铜在拉伸变形过程中的塑性各向异性机制.结果表明:主导滑移系的斯密特因子随着孪晶界倾角的改变而变化,但屈服应力或流动应力并不严格遵守斯密特定律.在拉伸变形过程中存在涉及不同位错机制的硬取向和软取向.硬取向的硬化机制在于塑...     

双相不锈钢中奥氏体相内裂纹扩展路径的透射电镜原位观察

在透射电镜下原位观察了裂纹在双相不锈钢中的扩展过程。结果表明,裂纹在双相不锈钢中奥氏体晶粒内扩展时常呈较大的Z字形扩展路径。并且,在主裂纹前端奥氏体晶粒内形核的小裂纹与主裂纹有较强的交互作用,小裂纹尖端二次滑移系的选择和扩展路径为小裂纹尖端应力场和主裂纹尖端应力场共同支配。     

单晶硅裂纹尖端的位错发射行为

利用透射电镜原位观察了单晶硅压痕裂纹尖端位错及位错偶沿滑移面的发射行为,考察了滑移面取向,外荷对发射位错及塑性区的影响,结果表明：在I型载荷作用下,滑移面与裂纹面夹角要影响从裂纹尖端发射的位错数量及塑性区,发射出的位错可沿最大切应力方向改变运动方向或交换滑移面运动,实验观察的位错宽度平均值为22．0nm,与Peierls位移框架模型计算的23．6nm相近。     

裂纹尖端塑性区和无位错区的微观模拟

用细观断裂力学研究了刃型位错从Ⅰ型，Ⅱ型及Ⅰ Ⅱ复合型裂纹尖端沿多个滑移面的发射以及裂纹周围无位错区和塑性区的形状和大小．结果表明，Ⅱ型裂纹塑性区形状和宏观断裂力学算出的塑性区形状有所不同，而I型裂纹塑性区形状则类似；当外载荷相同时，Ⅱ型裂纹的塑性区比Ⅰ型裂纹的塑性区要大得多；复合型裂纹的塑性区形状和大小更接近Ⅱ型裂纹．各种裂纹尖端周围均存在无位错区，其形状与相应的塑性区相似。     

Crystallographic and macroscopic orientation of planar dislocation boundaries—correlation with grain orientation

A novel geometric analysis method for determination of the three-dimensional orientation of extended planar dislocation boundaries in polycrystals based on TEM measurements is presented. The analysis is applied to data for tensile deformed aluminium, revealing that the boundaries have a strong preference for certain crystallographic planes, depending on the crystallographic orientation of the grain. The crystallographic boundary planes are distributed around, but do not coincide with, the most stressed macroscopic planes inclined 45° to the tensile axis. The strong correlation between the crystallographic boundary planes and the grain orientation shows that the boundary orientation is closely linked to the active slip systems. The observed correlation can be explained by a Schmid factor analysis assuming activity on the five most stressed slip systems.     

Numerical simulations of hydrogen–dislocation interactions in fcc stainless steels.: part II: hydrogen effects on crack tip plasticity at a stress corrosion crack

The discrete simulation method for hydrogen–dislocation interactions is applied to the study of Stress Corrosion Cracking (SCC). We recall the main results of the experimental study of the fracture micro-crystallography in austenitic stainless steels, along with the successive stages of the Corrosion Enhanced Plasticity Model. Numerical simulations allow the assessment of the critical parameters affecting the model stages. Solute hydrogen promotes the formation of dense dislocation pile-ups, and a ‘zigzag’ type of fracture along alternating slip planes at the SC crack tip. We provide an analytical expression for the stress field of a dilatation line in the vicinity of a crack, from which we derive all the hydrogen–crack–dislocation elastic interactions terms. Diffusing hydrogen also has a marked pinning effect on a dislocation source at a crack tip. This effect exhibits a strong dependence on the crystal orientation. These results are discussed from the viewpoint of SCC fracture mechanisms.     

Study of Dislocation Boundary Structure in Al–Li Alloy During Bending

The dislocation boundary structures of 2060-T8 alloy during bending were investigated by backscattered electron imaging, electron backscattered diffraction, and misorientation axes maps. Experimental result shows that typical dislocation boundary structures, which depend on grains' orientation, are formed in grains during bending. The microstructure of type A is mainly observed in grains near brass, copper, and Goss orientations; microstructure of type B is mainly found in grains near S orientation; microstructure of type C is mainly seen in grains near Cube orientation. The angle between geometrically necessary boundaries(GNBs) and force axis is in the range of-45° to-30° and 30° to 45°.Most of the GNBs are approximately parallel to the trace of {111} slip planes which are identified by Schmid factor analysis.