Simulation of the interaction between an edge dislocation and a 〈1 0 0〉 interstitial dislocation loop in α-iron

Atomic-level simulations are used to investigate the interaction of an edge dislocation with 〈1 0 0〉 interstitial dislocation loops in α-iron at 300 K. Dislocation reactions are studied systematically for different loop positions and Burgers vector orientations, and results are compared for two different interatomic potentials. Reactions are wide-ranging and complex, but can be described in terms of conventional dislocation reactions in which Burgers vector is conserved. The fraction of interstitials left behind after dislocation breakaway varies from 25 to 100%. The nature of the reactions requiring high applied stress for breakaway is identified. The obstacle strengths of 〈1 0 0〉 loops, 1/2〈1 1 1〉 loops and voids containing the same number (169) of point defects are compared. 〈1 0 0〉 loops with Burgers vector parallel to the dislocation glide plane are slightly stronger than 〈1 0 0〉 and 1/2〈1 1 1〉 loops with inclined Burgers vector: voids are about 30% weaker than the stronger loops. However, small voids are stronger than small 1/2〈1 1 1〉 loops. The complexity of some reactions and the variety of obstacle strengths poses a challenge for the development of continuum models of dislocation behaviour in irradiated iron.     

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.     

Effect of climb on dislocation mechanisms and creep rates in γ′-strengthened Ni base superalloy single crystals: A discrete dislocation dynamics study

Creep of single-crystal superalloys is governed by dislocation glide, climb, reactions and annihilation. Discrete three-dimensional (3D) dislocation dynamics (DDD) simulations are used to study the evolution of the dislocation substructure in a γ/γ′ microstructure of a single-crystal superalloy for different climb rates and loading conditions. A hybrid mobility law for glide and climb is used to map the interactions of dislocations with γ′ cubes. The focus is on the early stages of creep, where dislocation plasticity is confined to narrow γ channels. With enhancing climb mobility, the creep strain increases, even if the applied resolved shear stress is below the critical stress required for squeezing dislocations into the γ channels. The simulated creep microstructure consists of long dislocations and a network near the corners of the γ′ precipitate in the low-stress regime. In the high-stress regime, dislocations squeeze into the γ channels, where they deposit dislocation segments at the γ/γ′ interfaces. These observations are in good agreement with experimentally observed dislocation structures that form during high-temperature and low-stress creep.     

Initiation and growth of the discontinuous precipitation reaction at [0 1 1] symmetric tilt boundaries in Cu–Be alloy bicrystals

The initiation and growth of discontinuous precipitation (DP) at [0 1 1] symmetric tilt boundaries in Cu–0.75 wt% Be alloy bicrystals have been studied. Transmission electron microscopy revealed that γ precipitates tended to nucleate at a boundary in such a manner that their habit plane, {1 1 2}_α or {4 4 5}_α, with one of the adjoining grains lay as close as possible to the boundary. The habit plane with a low energy formed on both sides of the precipitates behind an initially migrating boundary, indicating the importance of the existence of the low-energy habit plane for the initial boundary migration. The incubation period to initiate DP and cell growth rate for a boundary show a good correlation with the energy of the boundary. A kinetic analysis of DP using the models of Turnbull, and Petermann and Hornbogen has yielded boundary diffusion data. A higher-energy boundary has a higher diffusivity with a smaller activation energy.     

Microband evolution during large plastic strains of stable {1 1 0}〈1 1 2〉 Al and Al–Mn crystals

The deformation microstructures of Al and Al–Mn {1 1 0}〈1 1 2〉 single crystals have been characterized after room temperature channel-die compression up to true strains of 2.1. The evolution of local misorientations and microband structures were quantified by high-resolution electron backscatter diffraction in a field emission gun scanning electron microscope and their alignments compared with the traces of active slip planes and macroscopic shear stress planes. During plane-strain compression these “Brass” oriented crystals remain stable in terms of the final, average, orientation, with a small orientation spread. However, the microband alignment varies with strain and also with solute content. There is a general tendency for the microbands to be both crystallographic and non-crystallographic at low strains, then crystallographic, and finally mixed again at high strains (with some lamellar banding).     

Correlation of the atomic and electronic structures and the optical properties of the Σ5(2 1 0)/[ 0 01] symmetric tilt grain boundary in yttrium aluminum garnet

A series of atomic models of the Σ5(2 1 0)/[0 0 1] symmetric tilt grain boundary in yttrium aluminum garnet (YAG) are constructed according to the coincident site lattice theory. Calculations performed using empirical potentials show that the O-termination configurations, namely G(1) and G(2), are the most energetically favorable boundary structures. First-principles density functional theory calculations have been further performed to understand the atomic and electronic structures, effective charge, potential distributions and optical properties of G(1) and G(2) grain boundaries. The simulated high-resolution transmission electron microscopy images of G(1) and G(2) are generally in good agreement with the experimental micrographs of the Σ5(2 1 0)/[0 0 1] grain boundaries. Results show that the effective charges of the atoms in the grain boundary region are related to their coordination numbers and bonding lengths. Moreover, the overall total density of states of G(1) and G(2) has been found to have similar features with the bulk YAG, with the exception of some defect states being introduced at the top of the valence band, resulting in the reduction of the band gap. The calculated optical properties show that the refractive indices of both grain boundary models are slightly larger than those of the bulk YAG. This is in agreement with the experimental observation that the refractive index of the polycrystalline YAG is higher than that of its single crystalline counterpart.     

Mechanism of depolarization with temperature for <0 0 1> (1 − x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals

The temperature dependence of polarization for Pb(Zn_1/3Nb_2/3)_1?xTi_xO₃ single crystals poled in the [0 0 1]-direction has been investigated. During the application of a temperature increase, the percentage of switched domains and the distortion of the crystalline lattice in (1 ? x)PZN–xPT single crystals were evaluated by X-ray diffraction (XRD) patterns. Using this method, intrinsic and extrinsic contributions to polarization variations were separated in the temperature range from 25 °C to the Curie temperature (T_c). Experimental polarization variations were simulated from microscopic data and details on micro–macro relationships were given. It was found that polarization variation with temperature is caused by the variation of the distortion of the crystalline lattice for temperatures below the Curie temperature and that only 90° domain switching occurs in the vicinity of the Curie temperature. Moreover, the hysteretic behavior of the polarization with temperature is due to motion of domain walls. The understanding of mechanisms of depolarization with temperature and the hysteresis associated with are of interest for the enhancement the pyroelectric properties of the material for detection and energy harvesting applications.     

Early stage oxidation of Ni–Cr binary alloy (1 1 1), (1 1 0) and (1 0 0) surfaces: A combined density functional and quantum chemical molecular dynamics study

First-principles and tight-binding quantum chemical molecular dynamics were used in this study. The chemisorption energies of O and OH on the Ni–Cr (1 1 0) surface are lower than those of other surfaces. The oxygen 2p orbitals hybridise with Ni 3d, 4s and small amounts of p orbitals for the (1 0 0) surface while Ni p orbitals have no contribution for the (1 1 0) surface, which might reduce the adsorption energy. Additionally, oxygen acquires the maximum depth into the Ni–Cr (1 1 0) surface. Applied strain increases the oxygen diffusivity. This study reveals that the Ni–Cr (1 1 0) surface is easier for oxygen diffusion accordingly oxidation accelerates.     

Atomistic structure of the Cu(1 1 1)/α-Al2O3(0 0 0 1) interface in terms of interatomic potentials fitted to ab initio results

We propose an atomistic model to describe the copper/sapphire interface by means of simple interatomic potentials involving only a few fitting parameters. Successful results are achieved when the copper atoms in the monatomic layer closest to the interface have properties different from the bulk. This layer is to accommodate the ionic/covalent bonding in the ceramics to the metallic bonding in copper. For an oxygen terminated interface, we fit the parameters of the potentials to the results of a rigid tensile test (explained in the text) simulated from first principles. The results of atomic relaxation near the interface are shown to be consistent with ab initio and experimental results available in the literature. Calculations reveal highly interesting relaxation dynamics near the interface. In the early stage of relaxation, a periodic network of partial misfit dislocations is formed, which later transforms into an irregular network due to the instability of the layer of copper atoms atop the oxygen atoms. This explains the interface incoherency observed in high-resolution electron microscopy. Calculations based on the FK model reproduce this effect.     

Stress-driven migration of symmetrical 〈1 0 0〉 tilt grain boundaries in Al bicrystals

Stress-induced migration of planar grain boundaries in aluminum bicrystals was measured for both low- and high-angle symmetrical 〈1 0 0〉 tilt grain boundaries across the entire misorientation range (0–90°). Boundary migration under a shear stress was observed to be coupled to a lateral translation of the grains. Boundaries with misorientations smaller than 31° and larger than 36° moved in opposite directions under the same applied external stress. The measured ratios of the normal boundary motion to the lateral displacement of grains are in an excellent agreement with theoretical predictions. The coupled boundary motion was measured in the temperature range between 280 and 400 °C, and the corresponding activation parameters were determined. The results revealed that for mechanically induced grain-boundary motion there is a misorientation dependence of migration activation parameters. The obtained results are discussed with respect of the mechanism of grain-boundary motion.     

Interaction between martensitic structure and defects in β ↔ β′ + γ′ cycling in CuAlNi single crystals. Model for the inhibition of γ′ martensite

The authors have previously reported an estimate of the energy associated with the inhibition effect of γ′ martensite after β ↔ β′ + γ′ cycling in CuAlNi single crystals. In this paper, a microscopic model is proposed to explain the γ′ inhibition, related to the localized interaction between a dislocation array and the twinned γ′ structure. Dislocations with Burgers vector [1 0 0]_β and line direction [1 1 1]_β in an isotropic β matrix are considered. The model takes into account the interaction between the martensitic stress-free transformation strains and the stress field created by the dislocation arrays. It is shown that the interaction is different for each twin-related variant in the γ′ martensite. The energy necessary to maintain the right volume relationship of the twinned γ′ variants to produce an undistorted β/γ′ habit plane is defined as the inhibition energy. A value of around 12 J mol⁻¹ was obtained, which is in reasonable agreement with experimental results.     

Thermal stability of Al1−xInxN (0 0 0 1) throughout the compositional range as investigated during in situ thermal annealing in a scanning transmission electron microscope

The thermal stability of Al_1?xIn_xN (0 ? x ? 1) layers was investigated by scanning transmission electron microscopy (STEM) imaging, electron diffraction, and monochromated valence electron energy loss spectroscopy during in situ annealing from 750 to 950 °C. The results show two distinct decomposition paths for the layers richest in In (Al_0.28In_0.72N and Al_0.41In_0.59N) that independently lead to transformation of the layers into an In-deficient, nanocrystalline and a porous structure. The In-richest layer (Al_0.28In_0.72N) decomposes at 750 °C, where the decomposition process is initiated by In forming at grain boundaries and is characterized by an activation energy of 0.62 eV. The loss of In from the Al_0.41In_0.59N layer was initiated at 800 °C through continuous desorption. No In clusters were observed during this decomposition process, which is characterized by an activation energy of 1.95 eV. Finally, layers richest in Al (Al_0.82In_0.18N and Al_0.71In_0.29N) were found to resist thermal annealing, although the initial stages of decomposition were observed for the Al_0.71In_0.29N layer.     

Topological analysis of {1 1 0} slip in an α-iron crystal from in situ atomic force microscopy

In situ tensile tests were performed inside an atomic force microscope on commercially pure α-iron single crystals to examine the process of emerging slip at a free surface during plastic straining. The reported study corresponds to a crystal orientation which favors slip on a {1 1 0} plane. Statistical analysis of periodically stored images during the test allowed the collection of information on the planar and heterogeneous nature of slip, as well as on the evolution of this heterogeneity with strain. The average slip and the slip dispersion were step-wise estimated separately in each of the slip bands in a representative crystal observation zone. An estimate of the related heterogeneous evolution of the mobile dislocation density was also obtained. For the advanced microstructure-based modeling of intra-crystalline plastic behavior actually used in multiscale computational approaches of metal plasticity it is of interest to describe the relevant intra-crystalline slip process, which is not homogeneous even in single slip mode, although it is frequently simplified as such. These results also are of particular importance in validating the dislocation collective behaviour predicted by dislocation dynamics simulations.