直Volterra位错芯屈服区构型

根据各向同性连续介质中的弹性应力场和Mises屈服准则。计算出直Voterra位错芯屈服区的二维几何构型。结果表明：在无限弹性体中，直刃型Volterra位错屈服区呈哑铃状，且在滑移面上屈服半宽度最大reg=5.9b(b为Burgers矢量模），攀移面上屈服半宽度最小rec=1.4b;直螺型Volterra位错芯屈服区为圆形，屈服半径rs=4.1b，在半无限弹性体中，受自由表面镜像位错的作用，直螺型Volterra位错芯服区为圆形，屈服半径rs=4.1b。在半无限弹性体中，受自由表面镜像位错的作用，直螺型位错芯屈服中心向表面偏移，屈服区形态依赖 于直螺型位错线与自由表面的间距。     

小角度非对称倾侧晶界形貌和变形的晶体相场模拟

采用双模晶体相场模型,计算了二维正方相相图,并以正方相为研究对象,在原子尺度上,模拟了小角度非对称倾侧晶界结构及变形过程。结果表明,小角度非对称倾侧晶界由刃型位错和刃型位错组构成;在外加应力作用下,位错先于位错组滑移并进行短程的攀移,最后合并,位错组分离为滑移方向相反的2个刃型位错并最终与其它晶界位错组分离出的异号刃型位错合并,完成晶粒的合并。     

Ti_3Al基合金超塑形变的微观观察

Ｔｉ－２５Ａｌ－１０Ｎｂ－３Ｖ－１Ｍｏ（ａｔ．－％）合金在９００－９８０℃温度范围和１×１０~（－３）Ｓ~（－１）拉伸速率下显示了良好的超塑性能，δ＝３８２％．其超塑形变受控于晶界滑动（主要为α_２／α_２晶界滑动），并协调以扩散、位错、位错环和层错机制．扩散协调机制表现为螺旋位错和割阶的攀移，位错协调制主要为的位错的滑移．位错环主要为的切变型位错环，并伴有少量的扩散切变型位错环．     

双相TiAl基合金γ相中螺旋状普通位错的形成

利用自行开发的精确立体显微术研究了800℃压缩变形2％的双相TiAl基合金γ相中普通位错的空间形态。研究结果表明，同一位错往往并不处于同一晶面，其中一些位错段靠近（111）晶面，另一些则靠近（112）晶面，构成三维螺旋状曲线。普通位错形成这种螺旋状曲绠交滑移的结果，即原先在（111）密排面滑移的螺位错的部分位错段交滑移到（112）非密排面。随着处于不同晶面的位错段的进一步滑移，靠近位错段交界处的位错线方向将发生变化，具有较大刃型分量的部分可通过攀移而偏离（111）或（112）晶面。     

含Nb低碳钢的热变形行为和金属塑性形变中流变应力的预测

利用Gleeble 1500模拟试验机对含Nb低碳钢的热变形行为进行了实验研究,对热变形稳态流变应力和发生动态再结晶时的峰值流变应力与变形条件的关系进行了回归分析,阐述了由于形变诱发Nb(C,N)化合物沉淀对形变激活能的影响,在对金属塑性变形过程中位错增值,螺型位错交滑移回复和刃型位错攀移回复过程分析的基础上,建立了可预测动态回复和动态再结晶同时发生情况下的流变应力理论模型,并对不同变形条件下的流变应力进行了预测,预测结果与实验结果相当吻合。     

α-Ti中a型位错的运动及相互作用的分子动力学模拟

采用分子动力学模拟研究α-Ti中a型刃位错和螺位错的运动特点及其相互作用.对于独立的刃位错和螺位错,发现刃位错滑移时位错芯形态保持不变,而螺位错芯存在两种不同的分解方式:三维分解和基面分解,并导致不同的滑移特征;三维分解的螺位错沿基面滑移时需转变为基面分解,而基面分解的螺位错沿柱面滑移时需转变为三维分解状态,从而导致其开动困难.刃位错与螺位错之间存在多种不同形式的相互作用,其位错反应可形成其他形式复杂的缺陷.     

AA7005铝合金的热加工变形特性

研究了AA7005合金高温压缩变形时的流变应力、动态回复与再结晶以变形组织变化特征。合金稳态变形时,应变速度、温度和流变应力之间满足包含热激活材料常数的Arrhenius项的双曲正弦关系,变形过程为受位错增殖和相互销毁速率控制的热激活过程,螺型位错的交滑移和刃型位错的攀移为主要动态回复机制。动态回复时,形成典型的变形亚晶组织,亚晶尺寸随1nZ的减小而增大。高温低速变形条件下,合金发生局部几何动态再结晶,流变曲线呈现连续下降的特征,形成与原始纤维组织不同的细小等轴大角度再结晶晶粒。     

脉冲电流对2091Al-Li合金超塑变形机理的影响

分析了脉冲电流对２０９１Ａｌ－Ｌｉ合金超塑变形中晶内位错滑移、晶界位错滑移及原子扩散的影响。研究表明,脉冲电流促进位错滑移及增殖,降低原子扩散激活能,加速位错在晶界上的攀移,从而提高了超塑变形在高应变速率下的可能性。据此,建立了施加脉冲电流条件下的超塑变形速率方程。     

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．     

2Cr13钢高周疲劳微塑性损伤特性研究

研究了２Ｃｒ１３马氏体不锈钢高周疲劳载荷作用下微观塑性变形的演变过程,在三种选定循环载荷作用下,在疲劳裂纹萌生前试样一直处于弹性变形阶段,而且弹性模量基本不变,在１／２寿命时,普遍观察到局部微观考变形特征。有些板条内存在含扭折对的平行螺位错束或十字交叉网状位错组态,一些细长碳化物周围塞积了棱柱或滑移型位错环。达到疲劳寿命时,有些区域内形成位错胞状结构,经过高周疲劳实验的薄膜试样在透射电镜下进行动态     

Spacing defects and disconnections in grain boundaries

The process of cutting of a grain boundary by a gliding or climbing dislocation is considered. Some planar dislocation arrays with long-range stress fields are also treated. Defects formed on the grain boundaries by these mechanisms include edge and screw disconnections, grain boundary dislocations, spacing defects and line forces. The cutting defects can also acquire kinks and jogs. The results have implications for emission of lattice dislocations from grain boundaries, trapping of dislocations at grain boundaries and grain boundary topography.     

The effect of alloying elements on the dislocation climbing velocity in Ni: A first-principles study

By using density functional theory calculations in conjunction with the climbing images nudged elastic band method, the effects of alloying elements Re, W, Mo, Cr, Co and Ru on the velocity of dislocation climbing in gamma Ni were studied. The results shed a light on the mechanism of these elements suppressing the dislocation motion by connecting the stacking fault energy and the migration activation energy of vacancy with the dislocation climbing velocity. It is found that the elements can decrease the stacking fault energy of Ni and raise the migration activation energy of vacancy. The changes of these two energies result in the increase of the formation energy and the diffusion activation energy of the jog, thus the dislocation climbing is restricted. The results also reveal that the influences of alloying elements on dislocation climbing velocity depend on the characters of dislocations.     

Interstitial loop strengthening upon deformation in aluminum via molecular dynamics simulations

The formation of prismatic interstitial loops during plastic deformation, as well as their interaction with dislocations, is systematically investigated in aluminum, using molecular dynamics simulations. First, direct dislocation interaction is responsible for producing various types of defects, typically vacancies and their clusters and interstitial loops. Secondly, small interstitial loops act as obstacles to dislocation gliding. When close to each other, a loop either decorates a dislocation with jogs or drags it elastically. The mobility of a dislocation decorated by a loop is significantly reduced. Depending on the relative position of the dislocation and the loop, the Peierls stress can increase several hundred times. The present work shows a complete picture of dislocation–loop interaction with atomic-scale details, which provides reliable information for parameterizing dislocation–debris interactions in constitutive models.     

Interactions between non-screw lattice dislocations and coherent twin boundaries in face-centered cubic metals

In a first report [Jin ZH, Gumbsch P, Ma E, Albe K, Lu K, Hahn H, et al. Scripta Mater 2006;54:1163], interactions between screw dislocation and coherent twin boundary (CTB) were studied via molecular dynamics simulations for three face-centered cubic (fcc) metals, Cu, Ni and Al. To complement those preliminary results, purely stress-driven interactions between 60° non-screw lattice dislocation and CTB are considered in this paper. Depending on the material and the applied strain, slip has been observed to interact with the boundary in different ways. If a 60° dislocation is forced by an external stress into a CTB, it dissociates into different partial dislocations gliding into the twin as well as along the twin boundary. A sessile dislocation lock may be generated at the CTB if the transited slip is incomplete. The details of the interaction are controlled by the material-dependent energy barriers for the formation of Shockley partial dislocations from the site where the lattice dislocation impinges upon the boundary.     

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy during cold rolling

The evolution of dislocation microstructure in electron beam melted Ta-2.5W alloy was investigated by transmission electron microscope (TEM). Long straight dislocations and dislocation loops are formed in Ta-2.5W alloy cold-rolled by 5%. A set of long, continuous extending planar boundaries (EPBs) are formed when the reduction reaches 20%. In the early stage of development, EPBs are fragmented, diffused and curved, which are connected by non-crystallographic cells boundaries to maintain their continuity. The straight segments of EPBs are usually parallel with the trace of {110}, and incline at about 25–35° to the rolling direction (RD). Two groups of EPBs are formed in a grain when the reduction is larger than 30%. The dislocations within EPBs tend to rearrange themselves with increasing strain in a sequence, from tangled dislocations, followed by parallel long straight screw dislocations and finally into dislocation nets, which are composed by 1/2 < 111 > and [100] type dislocations. The relaxation process of dislocations and the interaction of dislocations with EPBs make EPBs appear wavy and deviate from the trace of slip planes.     

Effects of temperature on structure and mobility of the 〈1 0 0〉 edge dislocation in body-centred cubic iron

Dislocation segments with Burgers vector b = 〈1 0 0〉 are formed during deformation of body-centred-cubic (bcc) metals by the interaction between dislocations with b = 1/2〈1 1 1〉. Such segments are also created by reactions between dislocations and dislocation loops in irradiated bcc metals. The obstacle resistance produced by these segments on gliding dislocations is controlled by their mobility, which is determined in turn by the atomic structure of their cores. The core structure of a straight 〈1 0 0〉 edge dislocation is investigated here by atomic-scale computer simulation for α-iron using three different interatomic potentials. At low temperature the dislocation has a non-planar core consisting of two 1/2〈1 1 1〉 fractional dislocations with atomic disregistry spread on planes inclined to the main glide plane. Increasing temperature modifies this core structure and so reduces the critical applied shear stress for glide of the 〈1 0 0〉 dislocation. It is concluded that the response of the 〈1 0 0〉 edge dislocation to temperature or applied stress determines specific reaction pathways occurring between a moving dislocation and 1/2〈1 1 1〉 dislocation loops. The implications of this for plastic flow in unirradiated and irradiated ferritic materials are discussed and demonstrated by examples.     

Dislocation interactions and low-angle grain boundary strengthening

The transmission of an incoming dislocation through a symmetrical low-angle tilt grain boundary (GB) is studied for {1 1 0}〈1 1 1〉 slip systems in body-centered cubic metals using discrete dislocation dynamics (DD) simulations. The transmission resistance is quantified in terms of the different types of interactions between the incoming and GB dislocations. Five different dislocation interaction types are considered: collinear, mixed-symmetrical junction, mixed-asymmetrical junction, edge junction, and coplanar. Mixed-symmetrical junction formation events are found not only to cause a strong resistance against the incident dislocation penetration, but also to transform the symmetrical low-angle tilt GB into a hexagonal network (a general low-angle GB). The interactions between the incident dislocation and the GB dislocations can form an array of 〈1 0 0〉 dislocations (binary junctions) in non-coplanar interactions, or a single 〈1 0 0〉 dislocation in coplanar interaction. We study how the transmission resistance depends on the mobility of 〈1 0 0〉 dislocations. 〈1 0 0〉 dislocations have usually been treated as immobile in DD simulations. In this work, we discuss and implement the mobility law for 〈1 0 0〉 dislocations. As an example, we report how the mobility of 〈1 0 0〉 dislocations affects the equilibrium configuration of a ternary dislocation interaction.     

Scaling of discrete dislocation predictions for near-threshold fatigue crack growth

Analyses of the growth of a plane strain crack subject to remote mode I cyclic loading under small scale yielding are carried out using discrete dislocation dynamics. Plastic deformation is modelled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules. An irreversible relation is specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading in an oxidizing environment. Calculations are carried out with different material parameters so that values of yield strength, cohesive strength and elastic moduli varying by factors of three to four are considered. The fatigue crack growth predictions are found to be insensitive to the yield strength of the material despite the number of dislocations and the plastic zone size varying by approximately an order of magnitude. The fatigue threshold scales with the fracture toughness of the purely elastic solid, with the experimentally observed linear scaling with Young’s modulus an outcome when the cohesive strength scales with Young’s modulus.     

Dislocation stability and deformation mechanisms of iron aluminides and silicide

Elastic stability of dislocations in FeAl alloys and Fe₃Si was examined with respect to the formation of jog pair(s) on a straight dislocation as well as the kink-pair formation. Dislocation configurations from annealed and deformed microstructures available in the literature are consistent with the predictions based on elastic instability. The difference in the formation enthalpy between a jog pair and a kink pair was obtained, and the activation enthalpy of a jog-pair pinning mechanism for edge dislocations was prescribed. Discussion is given on the driving forces for the pinning mechanisms, including the role of inhomogeneous internal stress in FeAl single crystals.     

Atomistic simulations of lattice defects in tungsten

The mechanical behavior of materials is ultimately determined by events occurring at the atomic scale. The onset of plastic yield corresponds to triggering of dislocation motion. Subsequent hardening is mainly controlled by interaction of gliding dislocations with other lattice defects such as forest dislocations, grain boundaries, interfaces and surfaces. Finally, material failure is influenced by processes at the tip of a crack propagating in a crystal lattice. In this work we review atomistic simulations of lattice defects in tungsten. We show that these studies are able to provide not only a detailed understanding of defect properties but also reveal how the fundamental processes at the atomic scale are linked to macroscopic material behavior.