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.     

Influence of hydrogen on the deformation morphology of vanadium (1 0 0) micropillars in the α-phase of the vanadium–hydrogen system

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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.     

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.     

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.     

Epitaxial Ni–Mn–Ga/MgO(1 0 0) thin films ranging in thickness from 10 to 100 nm

Thin films of Ni–Mn–Ga alloy ranging in thickness from 10 to 100 nm have been epitaxially grown on MgO(1 0 0) substrate. Temperature-dependent X-ray diffraction measurements combined with room-temperature atomic force microscopy and transmission electron microscopy highlight the structural features of the martensitic structure from the atomic level to the microscopic scale, in particular the relationship between crystallographic orientations and twin formation. Depending on the film thickness, different crystallographic and microstructural behaviours have been observed: for thinner Ni–Mn–Ga films (10 and 20 nm), the L2₁ austenitic cubic phase is present throughout the temperature range being constrained to the substrate. When the thickness of the film exceeds the critical value of 40 nm, the austenite-to-martensite phase transition is allowed. The martensitic phase is present with the unique axis of the pseudo-orthorhombic 7M modulated martensitic structure perpendicular to the film plane. A second critical thickness has been identified at 100 nm where the unique axis has been found both perpendicular and parallel to the film plane. Magnetic force microscopy reveals the out-of-plane magnetic domain structure for thick films. For the film thickness below 40 nm, no magnetic contrast is observed, indicating an in-plane orientation of the magnetization.     

Multiscale modeling of plastic deformation of molybdenum and tungsten: I. Atomistic studies of the core structure and glide of 1/2〈1 1 1〉 screw dislocations at 0 K

Owing to their non-planar cores, 1/2〈1 1 1〉 screw dislocations govern the plastic deformation of body-centered cubic (bcc) metals. Atomistic studies of the glide of these dislocations at 0 K have been performed using Bond Order Potentials for molybdenum and tungsten that account for the mixed metallic and covalent bonding in transition metals. When applying pure shear stress in the slip direction significant twinning–antitwinning asymmetry is displayed for molybdenum but not for tungsten. However, for tensile/compressive loading the Schmid law breaks down in both metals, principally due to the effect of shear stresses perpendicular to the slip direction that alter the dislocation core. Recognition of this phenomenon forms a basis for the development of physically based yield criteria that capture the breakdown of the Schmid law in bcc metals. Moreover, dislocation glide may be preferred on {1 1 0} planes other than the most highly stressed one, which is reminiscent of the anomalous slip observed in many bcc metals.     

Stress dependence of the Peierls barrier of 1/2〈1 1 1〉 screw dislocations in bcc metals

The recently formulated constrained nudged elastic band method with atomic relaxations (NEB + r) (Gröger R, Vitek V. Model Simul Mater Sci Eng 2012;20:035019) is used to investigate the dependence of the Peierls barrier of 1/2〈1 1 1〉 screw dislocations in body-centered cubic metals on non-glide stresses. These are the shear stresses parallel to the slip direction acting in the planes of the 〈1 1 1〉 zone different from the slip plane, and the shear stresses perpendicular to the slip direction. Both these shear stresses modify the structure of the dislocation core and thus alter both the Peierls barrier and the related Peierls stress. Understanding of this effect of loading is crucial for the development of mesoscopic models of thermally activated dislocation motion via formation and propagation of pairs of kinks. The Peierls stresses and related choices of the glide planes determined from the Peierls barriers agree with the results of molecular statics calculations (Gröger R, Bailey AG, Vitek V. Acta Mater 2008;56:5401), which demonstrates that the NEB + r method is a reliable tool for determining the variation in the Peierls barrier with the applied stress. However, such calculations are very time consuming, and it is shown here that an approximate approach of determining the stress dependence of the Peierls barrier (proposed in Gröger R, Vitek V. Acta Mater 2008;56:5426) can be used, combined with test calculations employing the NEB + r method.     

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.     

Preparation and characterization of sol–gel derived (1 0 0)-textured Pb(Zr,Ti)O3 thin films: PbO seeding role in the formation of preferential orientation

Lead zirconate titanate (PZT) thin films with a composition near the morphotropic phase boundary region were deposited onto Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates using a sol–gel method. A seeding layer was introduced between the most underlying surface of the PZT film and the platinum electrode surface to control the texture of the PZT thin film. The lead oxide seeding layer resulted in the formation of a single-phase perovskite and absolutely (1 0 0)-textured PZT film. SEM, XRD, XPS, and AES were used to characterize the evolution of the lead oxide layer and the PZT thin films. The growth kinetic mechanism of the (1 0 0)-textured PZT thin films was proposed phenomenologically. The ferroelectric and piezoelectric properties of the PZT films were also evaluated and discussed in association with different preferential orientations.     

Glass-like thermal conductivities in (La1-x1Yx1)2(Zr1-x2Yx2)2O7-x2 (x = x1 + x2, 0 ⩽ x ⩽ 1.0) solid solutions

The evolution of structure and thermal conductivity (k) has been studied for a range of Y–La₂Zr₂O₇ solid solutions. Within the pyrochlore range (x < 0.40) Y³⁺ solely substitutes for La³⁺ below a critical composition factor (x = 0.15), above which it substitutes for both La³⁺ and the Zr⁴⁺. A glass-like k, approaching the amorphous limit, is observed within a certain composition range (0.20 ? x < 0.40). The glass-like k behaviour is attributed to a phonon localization effect that arises from small and weakly bound Y³⁺ cations (rattlers) oscillating locally and independently in oversized anionic cages [(La/Y)O₈]. The ultralow and glassy k makes Y³⁺-doped La₂Zr₂O₇ pyrochlores promising candidate materials for high temperature thermal barrier coating topcoats.     

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).     

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.