Interaction of lattice dislocations with a grain boundary during nanoindentation simulation

Interaction of dislocations with a Σ = 5 (210) [001] grain boundary was investigated using molecular dynamics simulation with EAM potentials. The results showed that the dislocation transmitted across the grain boundary during nanoindentation and left a step in the boundary plane. Burgers vector analysis suggested that a partial dislocation in grain I merged into the grain boundary and it was dissociated into another partial dislocation in grain II and a grain boundary dislocation, introducing a step in the grain boundary. Simulation also indicated that, after the transmission, the leading partial dislocation in the grain across the boundary was not followed by the trailing partials, expanding the width of the stacking fault. The results suggested that the creation of the step that accompanied grain boundary motion and expansion of the stacking fault caused resistance to nanoindentation.     

Effect of hydrogen on deformation behavior of Alloy 725 revealed by in-situ bi-crystalline micropillar compression test

Nickel-based Alloy 725 bi-crystalline micropillars with different types of grain boundaries(GBs)were compressed in hydrogen-free and in-situ hydrogen-charged conditions to investigate the hydrogen effect on the deformation behavior of the selected GBs.In the presence of hydrogen,the compressive stresses on the micropillars increase regardless of the GB type.It was proposed that this hydrogen-induced hardening behavior is the synergistic effect of hydrogen-enhanced dislocation multiplication and interactions,the pinning effect of hydrogen on dislocation motion,and hydrogen-enhanced lattice friction.Transmission electron backscatter diffraction(t-EBSD)results demonstrate that both low-angle GBs and high-angle GBs can effectively suppress dislocation transmission through the GBs,resulting in dislocations pile up along the GBs in the hydrogen-charged condition.In contrast,this behavior was not observed in the micropillars with twin boundaries.     

A study of the influence of grain boundaries on short crack growth during varying load using a dislocation technique

The propagation of short cracks in the neighbourhood of grain boundaries have been investigated using a technique were the crack is modelled by distributed dislocation dipoles and the plastic deformation is represented by discrete dislocations. Discrete dislocations are emitted from the crack tip as the crack grows. Dislocations can also nucleate at the grain boundaries. The influence on crack growth characteristics of the distance between the initial crack tip and the grain boundary has been studied. It was found that crack growth rate is strongly correlated to the dislocation pile-ups at the grain boundaries.     

Density of grain boundaries and plasticity size effects: A discrete dislocation dynamics study

Discrete dislocation dynamics simulations are carried out to systematically investigate the microstructural and geometrical size dependence of films under tension that have a varying number of grains through their thickness. By varying film thickness, grain size and aspect ratio, more insight is gained into the competition between grain boundary hardening and film thickness effects. This provides a seamless link between previous dislocation plasticity studies and qualitative agreement with experimental data. In the simulations, plasticity arises from the collective motion of discrete dislocations of edge character. Their dynamics is incorporated through constitutive rules for nucleation, glide, pinning and annihilation. Grain boundaries are treated as impenetrable to dislocation motion. The numerical results show that the grain size dependence of yield in thin films as well as in bulk polycrystals is controlled by the density of grain boundaries.     

Sliding and Migration of Tilt Grain Boundaries in a Mg–Zn–Y Alloy

Sliding and migration of tilt grain boundaries in a Mg–Zn–Y alloy have been investigated on the atomic scale using aberration‐corrected scanning transmission electron microscopy. Grain boundary sliding is accommodated by non‐basal dislocations moving along the grain boundary; grain boundary migration is induced by the motion of grain boundary dislocations with synchronized grain boundary diffusion. Simultaneous sliding and migration of tilt boundaries take place in both Mg matrix and long period stacking ordered phases. These results provide evidence for occurrence of grain boundary motion, which may play a role in plasticity of this kind of Mg alloys.     

Molecular dynamics simulation of crack growth under cyclic loading

The mechanical behaviors around a crack tip for a system including both a crack and two tilt grain boundaries under cyclic loading are examined using a molecular dynamics simulation. Not only a phase transition but also the emission of edge dislocations is observed in order to relax stress concentration around a crack tip during the first loading. Then, a dislocation pile-up is formed near the grain boundary after the edge dislocations reach the grain boundary, because they cannot move beyond the grain boundary. During the first unloading, the edge dislocations emitted from the crack tip return to the crack tip and disappear in the system. We observe several vacancies generated around the crack tip and crack growth corresponding to an atomic scale during cyclic loading. Conclusively, we propose the fatigue crack growth mechanism for the initial phase of the fatigue fracture. That is, a fatigue crack propagates due to coalescence of the crack and the vacancies caused by the emission and absorption of dislocations.     

Insights on slip transmission at grain boundaries from atomistic simulations

To fully understand the plastic deformation of metallic polycrystalline materials, the physical mechanisms by which a dislocation interacts with a grain boundary must be identified. Recent atomistic simulations have focused on the discrete atomic scale motions that lead to either dislocation obstruction, dislocation absorption into the grain boundary with subsequent emission at a different site along the grain boundary, or direct dislocation transmission through the grain boundary into the opposing lattice. These atomistic simulations, coupled with foundational experiments performed to study dislocation pile-ups and slip transfer through a grain boundary, have facilitated the development and refinement of a set of criteria for predicting if dislocation transmission will occur and which slip systems will be activated in the adjacent grain by the stress concentration resulting from the dislocation pile-up. This article provides a concise review of both experimental and atomistic simulation efforts focused on the details of slip transmission at grain boundaries in metallic materials and provides a discussion of outstanding challenges for atomistic simulations to advance this field.     

Cu—Cr—Zr合金强化机理电子理论研究

通过自编软件建立了Cu合金液体、位错、晶界等原子集团模型,采用递归法计算了Cu合金电子结构。研究表明：Y在晶粒、表面、液体的环境敏感镶嵌能依次降低,Y从晶粒内向晶粒表面、液体Cu中扩散。扩散过程中Y原子填补在Cu晶粒表面缺陷处,阻碍Cu原子结晶,同时进入液体中的Y在晶粒周围形成含有高浓度Y的薄层,使晶粒生长受阻,晶粒细化。Sn向位错扩散,抑制Cr的沉淀析出,并能钉扎位错的攀移运动,推迟回复和再结晶。S在晶界偏析,使晶界结合强度降低。偏聚在晶界的S可将合金中的Zr吸附到晶界,使晶界得到强化。Cu晶粒、晶界与位错处的费米能级不同,电子在这些区域之间发生偏移,使合金内产生微电场。微电场对电子产生散射作用,使合金电阻增大。     

Influence from geometrical features on the growth rate of a micro-structurally short fatigue crack

The influence on the crack growth rate on a micro-structurally short edge crack subjected to fatigue loading from changes in crack length, distance to grain boundaries and applied load has been investigated. The crack is assumed to grow in a single shear mechanism due to nucleation, glide and annihilation of dislocations along preferred slip planes in the material. The external geometry is modelled by distributed dislocation dipole elements in a boundary element approach under quasi-static and plane strain conditions. The evolving plasticity is described by individual discrete dislocations along a slip plane emanating from the crack in the crack direction. The crack growth rate is shown to be controlled by the plasticity, which in turn is controlled by geometrical parameters in combination with the external load.     

The study of grain boundary density effect on multi-grain thin film under tension

The grain-size effect on the yield strength and strain hardening of thin film at sub-micron and nanometer scale closely relates to the interactions between grain boundary and dislocation. Based on higher-order gradient plasticity theory, we have systematically investigated the size effect of multi-grain thin film arising from the grain boundary density under tensile stress. The developed formulations employing dislocation density and slip resistance have been implemented into the finite element program, in which grain boundary is treated as impenetrable interface for dislocations. The numerical simulation results reasonably show that plastic hardening rate and yield strength are linear to the grain boundary density of multi-grain thin film. The aspect ratio of grain size and orientation of slip system have distinct influence on the grain plastic properties. The research of slip system including homogeneous and nonhomogeneous distribution patterns reveals that the hardening effect of low-angle slip system is greater than that of high-angle slip system. The results agree well with the experimentally measured data and the solutions by discrete dislocation dynamics simulation.     

Grain boundary influence during short fatigue crack growth using a discrete dislocation technique

We have studied the effect of a grain boundary in front of a short edge crack on its propagation under cyclic loading conditions in bcc iron. The used model is a combination of a discrete dislocation formulation and a boundary element approach where the boundary is described by dislocation dipole elements, while the local plasticity is modeled by discrete dislocations. The grain boundary is considered impenetrable, but dislocations positioned in the vicinity of a grain boundary give raise to high stresses in neighboring grains which, eventually, results in nucleation of dislocations and a spread of the plastic zone into the next grain. __________ Translated from Problemy Prochnosti, No. 1, pp. 163–166, January–February, 2008.     

高压扭转大塑性变形Al–Mg合金中的晶界结构*

利用透射电镜和高分辨透射电镜(HRTEM)研究了高压扭转大塑性变形纳米结构Al–Mg合金中的位错和晶界结构。结果表明: 对尺寸小于100 nm的晶粒, 晶内无位错, 其晶界清晰平直; 而尺寸大于200 nm的大晶粒通常由几个亚晶或位错胞结构组成, 局部位错密度可高达10¹⁷ m^-2, 这些位错往往以位错偶和位错环的形式出现。用HRTEM观察到了小角度及大角度非平衡晶界、小角度平衡晶界和大角度Σ9平衡晶界等不同的晶界结构。基于实验结果, 分析了局部高密度位错、位错胞和非平衡晶界等在晶粒细化过程中的作用, 提出了高压扭转Al–Mg合金的晶粒细化机制。     

High-temperature relaxations

In 1947, Kê observed a large relaxation peak around 0.46T_m in polycrystalline aluminum. This peak being absent in single crystal, Kê concluded that this relaxation effect was due to grain boundary sliding (GBS). In the 1970s, Woirgard, in the same temperature range, observed a peak in single crystal. Later on Rivière, Esnouf, and No systematically studied different relaxation effects in single and polycrystals and they concluded that the Kê peaks, as well as the other relaxations observed in this temperature range, are due to dislocation motion, the mechanism being probably the climb of jogs. More recently, in Ni–Cr, Cao clearly observed the presence of a large relaxation peak in polycrystals, which is absent in single crystal. He showed that the peak is due to grain boundary sliding, the mechanism of which is the climb of dislocations in the grain boundaries. Does a relaxation effect due to grain boundary sliding exist in metals? The question is still open and will be discussed in this paper considering recent results of the Chinese, French, Spanish and Swiss school.     

Atomistic simulations of metallic microstructures

This paper reviews recent results in the simulation of the mechanical response of metallic microstructures at the atomic level. The role of the grain boundary network in deformation process is the concentration of this paper as studied by virtual tensile and nanoindentation tests. The grain boundary network is found to contribute to plastic deformation through the process of dislocation nucleation, absorption and transmission, as well as grain boundary accommodation mechanisms such as grain boundary sliding and migration. The microstructural grain boundary network is also critical to the nucleation and propagation of cracks. The challenges and opportunities in this area are discussed.     

Quantized Grain Boundary States Promote Nanoparticle Alignment During Imperfect Oriented Attachment

Oriented attachment (OA) has become a well‐recognized mechanism for the growth of metal, ceramic, and biomineral crystals. While many computational and experimental studies of OA have shown that particles can attach with some misorientation then rotate to remove adjoining grain boundaries, the underlying atomistic pathways for this “imperfect OA” process remain the subject of debate. In this study, molecular dynamics and in situ transmission electron microscopy (TEM) are used to probe the crystallographic evolution of up to 30 gold nanoparticles during aggregation. It is found that Imperfect OA occurs because 1) grain boundaries become quantized when their size is comparable to the separation between constituent dislocations and 2) kinetic barriers associated with the glide of grain boundary dislocations are small. In support of these findings, TEM experiments show the formation of a single crystal aggregate after annealing nine initially misoriented, agglomerated particles with evidence of dislocation activity and twin formation during particle/grain alignment. These observations motivate future work on assembled nanocrystals with tailored defects and call for a revision of Read–Shockley models for grain boundary energies in nanocrystalline materials.     

The role of partial grain boundary dislocations in grain boundary sliding and coupled grain boundary motion

We study the process of grain boundary sliding through the motion of grain boundary dislocations, utilizing molecular dynamics and embedded atom method (EAM) interatomic potentials. For a Σ = 5 [001]{310} symmetrical tilt boundary in bcc Fe, the sliding process was found to occur through the nucleation and glide of partial grain boundary dislocations, with a secondary grain boundary structure playing an important role in the sliding process. While the homogeneous nucleation of these grain boundary dislocations requires shear strain levels higher than 7%, preexisting grain boundary dislocations are shown to glide at applied shear levels of 1.5%. The glide of the dislocations results in coupled motion of the boundary in the directions parallel and perpendicular to itself. Finally, interstitial impurities and vacancies were introduced in the grain boundary to study the effects on the sliding resistance of the boundary. While vacancies and H interstitials act as preferred nucleation sites, C interstitials do not. Both hydrogen and C interstitials stop dislocation glide whereas vacancies do not. A detailed study of the dynamic properties of these dislocations is also presented.     

The role of grain boundary energy in grain boundary complexion transitions

Recent findings about the role of the grain boundary energy in complexion transitions are reviewed. Grain boundary energy distributions are most commonly evaluated using measurements of grain boundary thermal grooves. The measurements demonstrate that when a stable high temperature complexion co-exists with a metastable low temperature complexion, the stable complexion has a lower energy. It has also been found that the changes in the grain boundary energy lead to changes in the grain boundary character distribution. Finally, recent experimental observations are consistent with the theoretical prediction that higher energy grain boundaries transform at lower temperatures than relatively lower energy grain boundaries. To better control microstructures developed through grain growth, it is necessary to learn more about the mechanism and kinetics of complexion transitions.     

Structure of Crystalline Interfaces

The structure of crystalline interfaces, as observed by transmission electron microscopy, is reviewed with emphasis on the similarity of grain and interphase boundaries of the dislocation type. Small-angle grain boundaries and low misfit interphase boundaries between similar crystal structures largely condense their mismatch into arrays of interfacial dislocations having Burgers vectors in common with dislocations located in the bulk crystals. Large-angle grain boundaries near certain misorientations corresponding to good fit between the abutting grains contain dislocations with Burgers vectors which are not found in the bulk crystal. Partially coherent interphase boundaries between quite dissimilar crystals, for example, f.c.c. and b.c.c., may also contain such dislocations. Principally, because of the difficulties involved in the acquisition of interfacial dislocations, dislocation interphase boundaries, in particular, usually do not have the minimum energy structure.     

Strengthening mechanism in micro-polycrystals with penetrable grain boundaries by discrete dislocation dynamics simulation and Hall–Petch effect

The grain-size effect on the yield stress and the flow strength in micro-polycrystals relates closely to the penetrability of grain boundary (GB) to dislocations. To simulate the dislocation transmission across grain boundary, a dislocation–grain boundary penetration model is proposed and then integrated into the two-dimensional discrete dislocation dynamics (DDD) framework by Giessen and Needleman (1995). By this extended DDD technology, the Hall–Petch effect in micro-polycrystals and the strengthening mechanism are computationally studied, with the main focus on the significant influence of the dislocation transmission across grain boundary that is not fully considered formerly. Results indicate that the Hall–Petch type relation is still applicable, but depends strongly on the GB-penetrability to dislocations, especially for the flow strength at large offset strains. The fitting values of Hall–Petch grain-size sensitive exponents n for initial yield stress and flow stress basically agree with experimentally measured data in published literatures.     

Grain boundary influence on short fatigue crack growth rate

The influence from different grain boundary configurations on the crack growth rate of a microstructurally short edge crack, located within one grain and subjected to remote fatigue loading, is studied. The study is performed using a dislocation formulation, were the geometry is described by dislocation dipole elements in a boundary element approach and the plasticity by discrete dislocations, located along specific slip planes in the material. Plane strain and quasi-static conditions are assumed. The crack is assumed to grow in a single shear mechanism due to nucleation, glide and annihilation of discrete dislocations. Different grain boundary configurations in front of the growing crack are considered, including both high angle and low angle grain boundaries. It is shown that both grain boundary configuration and distance between the crack and a grain boundary has a pronounced influence on the crack growth rate.