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.     

Formation of dislocation loops during He clustering in bcc Fe

The clustering of helium in bcc (body centered cubic) iron and the growth of a helium bubble are simulated at the atomistic level for the helium-rich vacancy-poor condition. It is shown that a ? 111 dislocation loop is formed as a sequential collection of 111 crowdions, the latter being the most stable self-interstitial atom configuration in the presence of a He cluster.     

Electronic selection rules controlling dislocation glide in bcc metals

The validity of the structure-property relationships governing the low-temperature deformation behavior of many bcc metals was brought into question with recent ab initio density functional studies of isolated screw dislocations in Mo and Ta. These relationships were semiclassical in nature, having grown from atomistic investigations of the deformation properties of the group V and VI transition metals. We find that the correct form for these structure-property relationships is fully quantum mechanical, involving the coupling of electronic states with the strain field at the core of long a/<2111> screw dislocations.     

Point-defect interaction in a dislocation core

By minimizing the thermodynamic potential we derive formulas for equlibrium length-distributions of dislocation segments and one-dimensional clusters formed from point defects in a dislocation core. The theoretical formulas are compared with results of computer modeling. Temporal variations of cluster-formation processes are discussed. Plots of the number of clusters and average and maximum lengths of clusters and dislocation segments as functions of time are presented. Moscow Technological University of Steel and Alloys. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 74–81, April 2000.     

Density functional theory studies of screw dislocation core structures in bcc metals

The core structures of d111 screw dislocations in bcc metals are studied using density functional theory in the local-density approximation. For Mo and Fe, direct calculations of the core structures show the cores to be symmetric with respect to 180 rotations around an axis perpendicular to the dislocation line. The magnetic moment in the Fe core is shown to be reduced relative to the bulk value. Calculations of nsurfaces and the elastic constants B , C ' and c 44 are reported for Fe and all group VB and VIB metals. Using a criterion suggested by Vitek and Duesbery the calculations point to symmetric core structures for all the studied metals.     

Effect of the alpha-gamma phase transition on the stability of dislocation loops in bcc iron

Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the alpha-gamma phase transition temperature T(c) = 912 degrees C that is associated with the dramatic reduction of the shear stiffness constant c' (c(11)-c(12))/2 near T(c). This reduction of c' has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a[100] and a/2[111] prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a[100], as opposed to the a/2[111], Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures.     

The effect of shear stress on the screw dislocation core in bcc metals

The screw dislocation core structure in bcc metals under an external shear stress is investigated using the model of generalized splitting as an approximation of the Peierls-Nabarro model of screw dislocation dissociated on three {110} slip planes. The shear stress for which the core structure in this model is unstable is found.     

Misfit dislocation loops in composite nanowires

A new mechanism for relaxation of misfit stresses in composite nanowires (quantum wires) is suggested and theoretically examined, namely the formation of misfit dislocation loops. The stress field of a prismatic dislocation loop in a cylinder (nanowire) is calculated. The parameters of two-phase composite nanowires at which the formation of misfit dislocation loops is energetically favourable are estimated. The effect of stress fields of dislocation loops on the formation of compositionally modulated nanowires is discussed.     

Non-singular dislocation loops in gradient elasticity

Using gradient elasticity, we give in this Letter the non-singular fields produced by arbitrary dislocation loops in isotropic media. We present the ‘modified’ Mura, Peach–Koehler and Burgers formulae in the framework of gradient elasticity theory.     

Mechanism of void nucleation and growth in bcc Fe: atomistic simulations at experimental time scales

Evolution of small-vacancy clusters in bcc Fe is simulated using a multiscale approach coupling an atomistic activation-relaxation method for sampling transition-state pathways with environment-dependent reaction coordinate calculations and a kinetic Monte Carlo simulation to reach time scales on the order of ~10? s. Under vacancy-supersaturated condition, di- and trivacancy clusters form and grow by coalescence (Ostwald ripening). For cluster size greater than four we find a transition temperature of 150 °C for accelerated cluster growth, as observed in positron annihilation spectroscopy experiments. Implications for the mechanism of stage-IV radiation-damage-recovery kinetics are discussed.     

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.     

Energetics of small hydrogen-vacancy clusters in bcc iron

Hydrogen may be trapped in voids in iron, leading to undesirable material properties. In this paper, the energetics of small hydrogen-vacancy clusters in body centered cubic iron are investigated. Results from two interatomic potentials are compared. We use molecular dynamics and Monte Carlo methods to find the minimum energy configurations of voids of up to ten vacancies containing up to 50 hydrogen atoms with ratios of hydrogen to vacancy of 10 or less. The formation energies and binding energies of defects to these clusters are calculated. Our results indicate that the hydrogen stabilizes bubbles by causing vacancies to be more tightly bound to clusters, while neighboring irons are less tightly bound. Hydrogen itself becomes less well bound to clusters as the inventory increases. The more physically relevant potential indicates a maximum supported ratio of hydrogen atoms to vacancies of about 4.     

Copper indium diselenide: crystallography and radiation-induced dislocation loops

Copper indium diselenide (CIS) is a prime candidate as the absorber layer in solar cells for use in extraterrestrial environments due to its good photovoltaic efficiency and ability to resist radiation damage. While CIS-based devices have been tested extensively in the laboratory using electron and proton irradiation, there is still little understanding of the underlying mechanisms which give rise to its radiation hardness. To gain better insight into the response of CIS to displacing radiation, transmission electron microscope samples have been irradiated in situ with 400?keV Xe ions at the Intermediate Voltage Electron Microscope facility at Argonne National Laboratory, USA. At room temperature, dislocation loops were observed to form and grow with increasing fluence. These loops have been investigated using g ?·? b techniques and inside/outside contrast analysis. They have been found to reside on {112} planes and to be interstitial in nature. The Burgers vector were calculated as b ?=?1/6 ?221?. The compositional content of these interstitial loops was found to be indistinguishable from the surrounding matrix within the sensitivity of the techniques used. To facilitate this work, experimental electron-diffraction zone-axis pattern maps were produced and these are also presented, along with analysis of the [100] zone-axis pattern.     

Dislocation structure and deformation hardening of bcc metals

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 23–34, March, 1991.     

Radiation damage in bcc metals studied by TDPAC

The time-differential perturbed angular correlation (TDPAC) method is an excellent tool for the study of radiation damage in metals and it has been applied successfully to many fcc metals. Recently, some investigations have been performed with bcc metals, where it is more difficult to obtain reliable results because the open lattice easily absorbs many impurities. After a short general introduction to the field, the power of the TDPAC method is demonstrated by showing in detail recent results obtained by our group in Bonn for the bcc metal tungsten. Finally, the present situation for all non-ferromagnetic bcc metals is summarized.     

注氢纯铝中间隙型位错环一维迁移现象的原位观察

核聚变堆材料在高能粒子辐照过程中会产生大量点缺陷,导致辐照脆性和辐照肿胀等现象.因而,研究点缺陷在辐照过程中的演变过程至关重要.点缺陷团簇的一维迁移现象是这种演变过程的主要研究内容之一.本文采用普通低压(200 kV)透射电镜,在室温条件下对注氢纯铝中的间隙型位错环在电子辐照下的一维迁移现象进行了观察和分析.在200 keV电子辐照下,注氢纯铝中的位错环可多个、同时发生一维迁移运动,也可单个、独立进行一维迁移运动.位错环沿柏氏矢量1/3<111>的方向可进行微米尺度的一维长程迁移,沿柏氏矢量1/2<110>的方向一维迁移也可达数百纳米.电子束辐照时产生的间隙原子浓度梯度是引起位错环一维迁移并决定其迁移方向的原因.位错环发生快速一维迁移时,其后会留下一条运动轨迹;位错环一维迁移的速率越快,运动的轨迹则越长,在完成迁移过后的几十秒内这些运动轨迹会逐渐消失.