Effect of isotropic stress on dislocation bias factor in bcc iron: an atomistic study

The effect of externally applied stress on the dislocation bias factor (BF) in bcc iron has been studied using a combination of atomistic static calculations and finite element integration. Three kinds of dislocations were considered, namely, a₀/2〈1 1 1〉{1 1 0} screw, a₀/2〈1 1 1〉{1 1 0} edge and a₀〈1 0 0〉{0 0 1} edge dislocations. The computations reveal that the isotropic crystal expansion leads to an increasing or constant dislocation bias, depending on the Burgers vector and type of dislocation. On the other hand, compressive stress reduces the dislocation bias for all the dislocations studied. Variation of the dislocation BF depending on dislocation type and Burgers vector is discussed by analysing the modification of the interaction energy landscape and the capture efficiency values for the vacancy and self-interstitial atom.     

Fundamentals in generalized elasticity and dislocation theory of quasicrystals: Green tensor,dislocation key-formulas and dislocation loops

The present work provides fundamental quantities in generalized elasticity and dislocation theory of quasicrystals. In a clear and straightforward manner, the three-dimensional Green tensor of generalized elasticity theory and the extended displacement vector for an arbitrary extended force are derived. Next, in the framework of dislocation theory of quasicrystals, the solutions of the field equations for the extended displacement vector and the extended elastic distortion tensor are given; that is, the generalized Burgers equation for arbitrary sources and the generalized Mura–Willis formula, respectively. Moreover, important quantities of the theory of dislocations as the Eshelby stress tensor, Peach–Koehler force, stress function tensor and the interaction energy are derived for general dislocations. The application to dislocation loops gives rise to the generalized Burgers equation, where the displacement vector can be written as a sum of a line integral plus a purely geometric part. Finally, using the Green tensor, all other dislocation key-formulas for loops, known from the theory of anisotropic elasticity, like the Peach–Koehler stress formula, Mura–Willis equation, Volterra equation, stress function tensor and the interaction energy are derived for quasicrystals.     

Hydrogen diffusion in the elastic fields of dislocations in iron

The effect of dislocation stress fields on the sink efficiency thereof is studied for hydrogen interstitial atoms at temperatures of 293 and 600 K and at a dislocation density of 3 × 10¹⁴ m^–2 in bcc iron crystal. Rectilinear full screw and edge dislocations in basic slip systems 〈111〉{110}, 〈111〉{112}, 〈100〉{100}, and 〈100〉{110} are considered. Diffusion of defects is simulated by means of the object kinetic Monte Carlo method. The energy of interaction between defects and dislocations is calculated using the anisotropic theory of elasticity. The elastic fields of dislocations result in a less than 25% change of the sink efficiency as compared to the noninteracting linear sink efficiency at a room temperature. The elastic fields of edge dislocations increase the dislocation sink efficiency, whereas the elastic fields of screw dislocations either decrease this parameter (in the case of dislocations with the Burgers vector being 1/2〈111〉) or do not affect it (in the case of dislocations with the Burgers vector being 〈100〉). At temperatures above 600 K, the dislocations affect the behavior of hydrogen in bcc iron mainly owing to a high binding energy between the hydrogen atom and dislocation cores.     

铝-镁合金的高温变形机构

用变温变速的拉伸试验,研究了Al-3％Mg和Al-6％Mg合金的高温变形行为,求得各温度下的激活能、激活体积和频率因子等。证明在250—400℃温度范围内,铝-镁合金变形机构可能是带割阶的螺位错作非保守性运动,变形方程为ε=Nι_jb²zAνexp{-(△H_s-ι_jb²τ)/(kT)}。然后计算得到两种合金在不同温度下的割阶间距ι_j,运动位错密度ρ=Nι_j,割阶密度N以及位错速度等数值。     

Interface dislocation structures at the onset of coherency loss in nanoscale Ni–Cu bilayer films

We have performed a transmission electron microscopy study, using weak beam imaging, of the interface dislocation arrays that form initially at the (001) Ni–Cu interface during coherency loss. Interface dislocations were absent in the 2.5?nm Ni/100?nm Cu bilayers, but were present in the 3.0?nm Ni samples, indicating that the critical Ni film thickness for coherency loss is between 2.5 and 3?nm. The key features of the interface dislocation structure at the onset of coherency loss are: (i) the majority of interface dislocations are 60° dislocations, presumably formed by glide of threading dislocations in the coherently stressed Ni layer, and have Burgers vector in the {111} glide plane; (ii) the interface contained approximately 5% Lomer edge dislocations, with Burgers vector in the {001} interface plane, and an occasional Shockley partial dislocation and (iii) isolated segments of interface dislocations terminating at the surface are regularly observed. Possible mechanisms that lead to these dislocation configurations at the interface are discussed. This experimental study shows that near the critical thickness, accumulation of interface dislocations occurs in a somewhat stochastic fashion with favourable regions where coherency is first lost.     

Influence of normal stress on yielding behaviour in Fe -3 ·2% Si alloy single crystals

Specimens of Fe-3·2 wt% Si alloy single crystals of various orientations, both with 18 ppm C and decarburized, have been deformed in compression ( \(\dot \varepsilon \) ～ 10^?4 s^?1) at different temperatures between 125 K and 293 K. It has been found that the magnitude of CRSS, the choice of the slip planes and the shape of the stress-strain curves depend on the angle between the compression axis and the Burgers vector (angleξ). The stress normal to the maximum resolved shear stress plane is strongly altered on changing the angleξ. The discussion of the obtained experimental results seems to indicate that the normal stress influences the structure of screw dislocation core and subsequently the dislocation mobility.     

Which stresses affect the glide of screw dislocations in bcc metals?

By direct application of stress in molecular statics calculations we identify the stress components that affect the glide of 1/2?111? screw dislocations in bcc tungsten. These results prove that the hydrostatic stress and the normal stress parallel to the dislocation line do not play any role in the dislocation glide. Therefore, the Peierls stress of the dislocation cannot depend directly on the remaining two normal stresses that are perpendicular to the dislocation but, instead, on their combination that causes an equibiaxial tension-compression (and thus shear) in the plane perpendicular to the dislocation line. The Peierls stress of 1/2?111? screw dislocations then depends only on the orientation of the plane in which the shear stress parallel to the Burgers vector is applied and on the magnitude and orientation of the shear stress perpendicular to the slip direction.     

A model for the dynamics of loop drag by a gliding dislocation

Clusters of self-interstitial atoms are formed in metals by high-energy displacement cascades, often in the form of small dislocation loops with a perfect Burgers vector. In isolation, they are able to undergo fast, thermally activated glide in the direction of their Burgers vector, but do not move in response to a uniform stress field. The present work considers their ability to glide under the influence of the stress of a gliding dislocation. If loops can be dragged by a dislocation, it would have consequences for the effective cross-section for dislocation interaction with other defects near its glide plane. The lattice resistance to loop drag cannot be simulated accurately by the elasticity theory of dislocations, so here it is investigated in iron and copper by atomic-scale computer simulation. It is shown that a row of loops lying within a few nanometres of the dislocation slip plane can be dragged at very high speed. The drag coefficient associated with this process has been determined as a function of metal, temperature and loop size and spacing. A model for loop drag, based on the diffusivity of interstitial loops, is presented. It is tested against data obtained for the effects of drag on the stress to move a dislocation and the conditions under which a dislocation breaks away from a row of loops.     

Edge dislocations in crystal structures considered as traveling waves in discrete models

The static stress needed to depin a 2D edge dislocation, the lower dynamic stress needed to keep it moving, its velocity, and displacement vector profile are calculated from first principles. We use a simplified discrete model whose far field distortion tensor decays algebraically with distance as in the usual elasticity. Dislocation depinning in the strongly overdamped case (including the effect of fluctuations) is analytically described. N parallel edge dislocations whose average interdislocation distance divided by the Burgers vector of a single dislocation is L>1 can depin a given one if N=O(L). Then a limiting dislocation density can be defined and calculated in simple cases.     

Interaction between Dislocation and Twinning Boundary under Incremental Loading in α-Titanium

The lattice dislocation interacting with grain boundary in the polycrystal exerts an evident influence on the materials' strength and toughness. A comprehensive study regarding the dislocation-twinning boundary(TB)interaction in a-titanium and TB migration is performed by employing molecular dynamic simulation. We analyze the interactions between dislocation and TB, under the conditions of plastic deformation and thermal stress, including the interaction between pure edge(a) dislocation and(1122) TB and the interaction between mixed type(a) dislocations and(1011) TB at 10 K/300 K. The(c + a) pyramidal transmitting slip mode is motivated in the case of edge dislocation-(1122) interaction at 300 K and then transforms into basal-dissociated dislocation after experiencing the complex dissociation and combination. The basal-dissociated pyramidal partial dislocation located in the second grain can be driven to penetrate through the second grain leaving the multiple stacking faults behind. Dissociation of incident basal dislocation on(1011) TB results in a nucleation of a(1011)twin embryo in twin crystals at room temperature. We determine the nature of the generated defects by means of the Burgers circuit analysis.     

Reprise: partial chemical strain dislocations and their role in pinning dislocations to their atmospheres

A correct solution for a dislocation atmosphere is provided using Hirth's Standard Model, confirming the errors in Hirth and Lothe. Contrary to what is given there, concentration changes in Cottrell atmospheres reduce an edge dislocation's stress and its elastic energy, thereby reducing the magnitude of the concentration changes. The chemical and elastic strain fields from Cottrell atmospheres are again shown to behave as partial dislocations with variable Burgers vectors that are not crystal translation vectors. The reality of partial dislocations provides a simpler explanation for pinning of dislocations by atmospheres. Much of the literature on dislocation properties in solid solutions should be re-examined.     

沿〈111〉晶向冲击加载下铜中纳米孔洞增长的塑性机制研究

 用分子动力学方法计算模拟了沿〈111〉晶向冲击加载过程中，单晶铜中纳米孔洞（直径约1.3 nm）的演化及其周围区域发生塑性变形的过程。模拟结果表明，在沿〈111〉晶向冲击加载后，在面心立方（fcc）结构中的4族{111}晶面中有3族发生了滑移。伴随孔洞的增长，在所激活的3族{111}晶面上，观察到位错在孔洞表面附近区域成核，然后向外滑移，其中在剪切应力最大的〈112〉方向上，其位错速度超过横波声速，其它〈112〉方向的位错速度低于横波声速。模拟得到的位错阻尼系数范围与实验值基本符合。由于孔洞周围产生的滑移在空间比较对称，孔洞增长形貌接近球形。在恒定的冲击强度下，孔洞半径增长速率近似保持恒定，其速率随着冲击强度的增加而增大。     

Nucleation of kink pairs on partial dislocations: A new model for solution hardening and softening

Nucleation and motion of kink pairs on partial dislocations are examined by elasticity theory for materials with a high Peierls stress. Two approaches are used: one considers the change in average stacking-fault energy (SFE) due to alloying elements and the other considers the change in local SFE due to a nearby solute atom. Both approaches highlight the role of SFE on kink nucleation, propagation and annihilation and both furnish strain rate as a function of stress, temperature and SFE. Model predictions are compared with yield stress data for two systems: firstly, an intermetallic, MoSi₂, which softens for alloying elements (V, Nb, Cr and Al) that decrease the SFE and hardens for Re additions that increase the SFE; secondly, a ceramic oxide, MgO-Al₂O₃ spinel, which softens with increasing addition of excess alumina and at the same time exhibits a decrease in SFE. The average SFE approach agrees qualitatively with the data while quantitative agreement is obtained with the local SFE approach. The possibility is considered that the model applies to other materials, such as TiAl, HfV₂ and Fe₃Al, which show softening with certain alloying additions. One requirement is that the dislocations are dissociated more than a few atomic distances; otherwise kink nucleation occurs on the perfect dislocation (or simultaneously on both partials). Hence the model does not apply to materials such as bcc metals which only have a core dissociation. Normal hardening effects of solutes from size and modulus misfits are additive with any softening effects from a decrease in SFE and so may mask the latter, as occurs for W additions to MoSi₂.     

不同对称性下晶界结构演化及微观机理的晶体相场法研究

采用晶体相场法研究了单轴拉伸下三角相双晶变形过程及机理, 并重点分析了小角对称与非对称晶界和大角对称与非对称晶界在变形过程中的演化及微观机理, 变形过程中应力方向与初始晶界方向平行. 结果表明, 小角对称晶界由柏氏矢量夹角呈60°的两种刃型位错组成, 变形过程中不同类型的位错运动方向相反, 并各自与另一晶界上同一类型位错相互吸引以致部分位错发生湮没; 小角非对称晶界上的位错类型单一, 在应力作用下先沿水平方向攀移, 后各自分解成柏氏矢量约呈120°的两位错, 并通过位错运动和湮没最终形成理想单晶; 大角晶界在应力的作用下先保持水平状态而后锯齿化并发射位错, 伴随着位错运动和湮没, 最终大角非对称晶界发生分解, 而大角对称晶界则重新平直化, 表明大角对称晶界比大角非对称晶界更稳定, 这与实验和分子动力学模拟结果一致. 关键词： 晶体相场 双晶 晶界 对称性     

Formation of Ge self-assembled quantum dots on a SixGe1−x buffer layer

Ge self-assembled quantum dots (SAQDs) grown on a relaxed Si_0.75Ge_0.25 buffer layer were observed using an atomic force microscopy (AFM) and a transmission electron microscopy (TEM). The effect of buried misfit dislocations on the formation and the distribution of Ge SAQDs was extensively investigated. The Burgers vector determination of each buried dislocation using the g·b = 0 invisibility criterion with plane-view TEM micrographs shows that Ge SAQDs grow at specific positions related to the Burgers vectors of buried dislocations. The measurement of the lateral distance between a SAQD and the corresponding misfit dislocation with plane-view and cross-sectional TEM images reveals that SAQDs form at the intersections of the top surface with the slip planes of misfit dislocations. The stress field on the top surface due to misfit dislocations is computed, and it is found that the strain energy of the misfit dislocations provides the preferential formation sites for Ge SAQDs nucleation.     

On the role of weak interfaces in blocking slip in nanoscale layered composites

Layered composites of Cu/Nb achieve very high strength levels when the individual layer thicknesses are 1–10?nm, attributable to the interfaces acting as barriers to slip. Atomistic models of Cu/Nb bilayers were used to explore the origins of this resistance. The models clearly show that dislocations placed near an interface experience an attraction toward the interface, regardless of the sign of the Burgers vector or the material in which it is placed. This attraction is caused by shear of the interface induced by the stress field of the dislocation. Furthermore, the dislocation, upon reaching the interface, is absorbed by it in the sense that the core spreads within the interface. We develop a model, using a fractional dislocation approach, which provides an estimate of the strength of the attraction as a function of distance from the interface and also the dependence of the interaction on the type of dislocation. A screw dislocation is much more effective in shearing the interface, and the resulting attractive forces on screws are larger than for edge dislocations.     

磷酸铝(AlPO4)晶体中缺陷的X射线形貌研究

用X射线衍射形貌法研究了AlPO₄晶体中的微观缺陷。在所研究的晶体中,主要晶体缺陷是生长层,沉淀物和位错。位错密度在晶体表面附近最大,晶体中部较低。位错主要起源于热应力和由沉淀物或生长层所造成的晶格畸变。多数位错的柏氏矢量是b=(a+c)<1123>型,部分的是b=a[2110]。分析了晶体缺陷与生长条件之间的关系。控制生长过程中的温度波动,特别是晶体出炉时的冷却速度,对提高晶体完美性是重要的。 关键词：     

Atomistic simulation analysis of the effects of void interaction on void growth and coalescence in a metallic system

Material deformation caused by the interaction between defects is a significant factor of material fracture failure. The present study employs molecular dynamics simulations of single-void and double-void crystalline Ni atomic systems to investigate inter-void interactions. Furthermore, simulations showing the evolution of dislocations for three different crystallographic orientations are conducted to study the void growth and coalescence. The simulations also consider the effect of the radius of the secondary void on dislocation evolution. The results show that double-void systems are more prone to yield than single-void systems. Further microstructural analysis indicates that the interaction between voids is realized by dislocation reactions. The simulation results of the dislocation evolution of the three orientations reveal that a relationship exists between the evolution of the dislocation density and the stress-strain curve. At the initial stage of dislocation, the dislocation grows slowly, and consists of Shockley partial dislocation. The dislocation growth rate then increases significantly in the sharply declining stage of the stress-strain curve, where most of dislocations are Shockley partial dislocation. Analysis of the dislocation length during the overall simulation indicates that the dislocation length of the [110] orientation is the longest, followed by that of the [111] orientation and the [100] orientation, which has the shortest dislocation length.     

Emission of Dislocation Loops from Nanovoids in an FCC Crystal Subjected to Shear Deformation under Post-Cascade Shock Waves

The method of molecular dynamics is applied to the study of the effect of post-cascade shock waves generated in a solid irradiated by high-energy particles on the heterogeneous formation of dislocation loops in a simulated gold crystal containing a spherical nanovoid, which is subjected to shear deformation. The interaction between atoms is described with the use of a potential calculated by the embedded atom method. Shock waves are created by assigning a velocity exceeding the speed of sound in the simulated material to the boundary atoms of the computational cell. It is shown that two regions of increased mechanical stress are formed under shear deformation near the surface of a nanovoid, which are the sources of emerging partial dislocations. The main mechanism for the formation of dislocations is the displacement of a group of atoms towards the inner surface of the void, which does not contradict modern ideas about the heterogeneous formation of dislocations. It is shown that, when the values of shear stress are insufficient for the formation of dislocations, loop emission can be initiated by a post-cascade shock wave generated in the computational cell. As temperature increases, the number of nucleated dislocation loops increases, and, in addition, the formation of Lomer–Cottrell dislocations is observed, which is attributed to the additional tangential stresses created by the unloading wave. In this case, the formation of a stable dislocation loop in which the linear tension is balanced by the Peach–Koehler force due to external stress requires that the shock wave front affect the regions of increased stress near the void surface while propagating through the simulated crystal.     

Study of yield stress anomaly of Fe2MnAl single crystal by in situ TEM straining

L2₁-structured Fe₅₉Mn₁₇Al₂₄ shows a yield stress anomaly with a peak yield stress at 700?K. The aim of the work reported here was to determine the dislocation mechanisms involved in this anomalous behaviour by performing in situ straining on Fe₅₉Mn₁₇Al₂₄ single crystals in a transmission electron microscope at 300–900?K. Cross-slip of ?111? dislocations was frequently found to occur between {110} and {211} planes at all temperatures tested. At 300?K, dislocations were four-fold dissociated and the partials moved together under stress. At 700–800?K, partial dislocations with a Burgers vector of a/2?111? dominated the deformation. These partial dislocations moved independently in the ordered matrix in a jerky manner, with rapid motion between long periods of locking. X-ray diffraction measurements showed that the degree of L2₁-order slightly decreased with increasing temperature in the range 300–700?K, and dropped rapidly above 700?K. At 900?K, the material was B2-ordered. The increased yield stress at elevated temperatures is primarily attributed to the uncoupling of a/2?111? partial dislocations trailing shear-induced anti-phase boundaries.