HREM examination of [101] screw dislocations in Ni3Al

Abstract

The dissociation of [101] screw dislocations in Ni³Al has been examined using high-resolution electron microscopy. [101] superdislocations are found to be dissociated into (a/2)[101] superpartial dislocations on the (010) cube cross-slip plane. These superpartials in turn dissociate into complex stacking faults on the (111) or (111) which are bounded by Shockley partials in agreement with theoretical predictions. The degree of antiphase boundary spreading on (010) was found to increase with deformation temperature while the superpartial core dissociations remain unchanged.     

On the ultra-high-strain rate shock deformation in copper single crystals: multiscale dislocation dynamics simulations

Multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper single crystals that have undergone shock loading at high strain rates ranging from 1?×?10⁶ to 1?×?10¹⁰?s^?1. Plasticity mechanisms associated with both the activation of pre-existing dislocation sources and homogeneous nucleation of glide loops are considered. Our results show that there is a threshold strain rate of 10⁸?s^?1 at which the deformation mechanism changes from source activation to homogeneous nucleation. It is also illustrated that the pressure dependence on strain rate follows a one-fourth power law up to 10⁸?s^?1 beyond which the relationship assumes a one-half power law. The MDDP computations are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models.     

The orientation and temperature dependences of dislocation velocity in Ni 3 Al single crystals

The average velocities of screw dislocations in Ni 3 Al single crystals have been directly measured as a function of resolved shear stress (RSS) and orientation in the temperature domain of the flow stress anomaly using the etch-pit technique. The velocity was found to be extremely sensitive to the RSS in all cases. In contrast with ordinary metals, the screw dislocation velocities in Ni 3 Al show anomalous behaviour; under a constant RSS, the velocities decrease dramatically with increasing temperature. Furthermore, the velocities and the tension-compression asymmetry of the velocities depend on the orientation of applied stress.     

Survey of plateau behaviour in the cyclic stress-strain curve of copper single crystals

Experimental results relating to the plateau behaviour in the cyclic stress-strain (CSS) curve of copper single crystals located on different sides of the stereographic triangle are summarized. Unlike the situation for single-sliporiented crystals, the crystallographic orientation has a strong effect on the plateau behaviour in the CSS curves of double- and multiple-slip-oriented crystals. The existence or non-existence of a plateau in the CSS curves, as well as the corresponding plateau stress amplitude, depend not only on the modes and intensities of dislocation interactions among slip systems operating in the crystals but also on the slip deformation characteristics associated with crystal orientations. The plateau region in CSS curve disappears only when multiple slip plays a determining role during cyclic deformation.     

Shape effect related to crystallographic orientation of deformation behaviour in copper single crystals

The deformation behaviour of pure copper single crystals has been investigated by scanning electron microscopy and synchrotron radiation using the in situ reflection Laue method. Two types of sample with the same orientation of tensile axes, but with different crystallographic orientations in the directions of the width and thickness of the samples, have been studied. They showed different characteristics of deformation behaviour, such as the activated slip systems, the movement of the tensile axis, and the mode of fracture.     

Creep strengthening by lath boundaries in 9Cr ferritic heat-resistant steel

The interactions between dislocations and lath boundaries in Grade 91 steel were observed by an in situ transmission electron microscopy tensile test at 973 K. Dislocations glided slowly and bowed out in a martensite lath interior. The ends of the dislocation were connected to the lath boundaries. In a tempered specimen, the pinning stress caused by the lath boundary was estimated to be >70 MPa with a lath width of 0.4 μm. In crept specimens, lath coarsening reduced the pinning effect.     

The effect of electropulsing on dislocation structures in [233] coplanar double-slip-oriented fatigued copper single crystals

The effect of high-current-density electropulsing on dislocation structures in a coplanar double-slip-oriented copper single crystal that had previously been fatigued is reported. The results show that, after electropulsing, vein structures are transformed to cell structures with some dark regions. It is proposed that the thermal compressive stress caused by electropulsing activates a coplanar slip system and leads to strong dislocation interactions between primary and coplanar slip systems, thereby forming cell structures. Partial recrystallization may occur by electropulsing, leading to the appearance of some dark regions.     

Size-dependent interaction between a screw dislocation and an inhomogeneity of arbitrary shape in an applied stress field

We present a general approximate solution for a screw dislocation interacting with an inhomogeneity of arbitrary shape in an applied stress field. The analysis is based on the Eshelby inhomogeneity theory. As special cases, explicit solutions for some common inhomogeneity shapes are obtained, from which size-dependent effects of dislocation stress field and the applied stress field on the interaction can be identified.     

Interaction of a dislocation with an elliptical hole in icosahedral quasicrystals

The interaction between a dislocation and an elliptical hole in icosahedral quasicrystals is considered. An explicit expression for the complex potential is derived using the extended Stroh formalism. Based on the conformal mapping method and a perturbation technique, closed-form solutions are obtained. The field intensity factors at a crack tip and the image forces on the dislocation arising from the crack are calculated. The effects of phonon–phason elastic coupling on the mechanical behavior are also observed.     

Site preference and alloying effect of Re atoms in the edge dislocation cores in Ni3Al

Using Cambridge sequential total energy package method based on the density functional theory, we investigated the site preference and alloying effect of Re atoms in the [1 0 0] (0 1 0) edge dislocation cores in Ni₃Al. Due to the introduction of a Re atom, the energetic and electronic structure of dislocation core systems have been changed a lot. The binding energy results suggest Re atom prefers Al site, especially the centre site in the centre-Al system, which can be explained by the results of Mulliken orbital population and density of states. When Re atom occupies the centre site in the centre-Al system, the DC system has the highest stability and stronger bonds formed between the Re atom and its nearest neighbouring atom due to hybridization of the Re-5d and Ni-3d orbitals.     

Mechanical properties of nanocrystalline deformed layers induced by nanogrinding of CdZnTe single crystals

Nanocrystalline deformed layers were generated in cadmium zinc telluride (CZT) single crystals by nanogrinding using three different grit sizes. The mechanical properties of the deformed layers were measured using nanoindentation. The hardness of the deformed nanocrystalline layers in the soft-brittle CZT semiconductor was higher than that of the perfect single crystal. This result is different from those found for deformed layers of hard-brittle silicon semiconductors, where the hardness of the deformed layers is lower than those of the perfect single crystal.     

Formation of prismatic loop alignments in crystals deformed in single slip

In γ-TiAl deformed at room temperature in single slip, prismatic loops ar often organized in a staircase-like configuration, called strings whose generation by cross-slip is facilitated by the dislocation tendency to form screw locks as described recently by Grégori and Veyssière. The present letter is aimed at showing that strings may be encountered in virtually any crystal provided that the two impinging dislocations exhibit significantly different velocities. The crossslip annihilation mechanism of crossed dislocations considered by Tetelman is revisited and shown to evolve into a configuration significantly distinct from that originally predicted.     

Spatial reconstruction of human motion by means of a single camera and a biomechanical model

The value of the results of the inverse dynamic analysis procedures used in the study of human tasks is dependent on the quality of the kinematic and kinetic data supplied to the biomechanical model that supports it. The kinematic data, containing the position, velocity and acceleration of all anatomical segments of the biomechanical model, result from the reconstruction of human spatial motion by means of the evaluation of the anatomic points positions that enable to uniquely define the position of all anatomical segments. Furthermore, the motion data must be kinematically consistent with the structure of the biomechanical model used in the analysis. The traditional photogrammetric methodologies used for the spatial reconstruction of the human motion require images of two or more calibrated and synchronized cameras. This is due to the fact that the projection of each anatomical point is described by two linear equations relating its three spatial coordinates with the two coordinates of the projected point. The need for the image of another camera arises from the fact that a third equation is necessary to find the original spatial position of the anatomical point. The methodology proposed here substitutes the projection equations of the second camera with the kinematic constraint equations associated with a biomechanical model in the motion reconstruction process. In the formulation the system of equations arising from the point projections and biomechanical model kinematic constraints, representing the constant length of the anatomical segments, are solved simultaneously. Because the system of equations has multiple solutions for each image, a strategy based on the minimization of a cost function associated to the smoothness of the reconstructed motion is devised. It is shown how the process is implemented computationally avoiding any operator intervention during the motion reconstruction for a given time period. This leads to an automated computer procedure that ensures the uniqueness of the reconstructed motion. The result of the reconstruction process is a set of data that is kinematically consistent with the biomechanical model used. Through applications of the proposed methodology to several sports exercises its benefits and shortcomings are discussed.     

Influence of strain rate on the orientation dependence of microstructure in nickel single crystals

The deformation microstructures of nickel single crystals (99.945 wt.%) during dynamic plastic deformation and quasi-static compression to a true strain of 0.20 were comparatively investigated. The deformation microstructures are orientation dependent, forming cell structure, slip plane aligned or not slip plane aligned extended boundaries. It is found that the orientation spread decreases, remains unchanged and becomes enhanced when loading along 〈0?0?1〉, 〈0?1?1〉 and 〈1?1?1〉, respectively, as strain rate increases.     

The lived experiences of single mothers in Swaziland of their son's aggressive behaviour

The study explored and described the lived experiences of single mothers and described guidelines for psychiatric nurses to facilitate the mental health of single mothers subjected to aggression by their adolescent sons. Participants were a purposive sample of 8 (single mothers, aged between 40 and 50 years, 7 mothers were employed and 3 were professionally trained, 4 mothers were domestic workers and 1 earned living by selling vegetables). The mothers attended the outpatient department at the National Psychiatric Hospital in Swaziland because of being exposed to aggression by their adolescent sons and had adolescent sons living with them 6 months prior to as well as during the study. Data were collected using individual in-depth phenomenological interviews, direct observation and field notes. Analyses used thematic coding. Results reflected that single mothers experienced their adolescent sons as disrespectful and violent, they had negative experiences about their adolescent sons which resulted in anxiety related symptoms, they had negative experiences of self and experienced poor relationships with their neighbours. Guidelines concerning intra- and interpersonal challenges were formulated for the psychiatric nurse to facilitate the mental health of these mothers.     

Behaviour of helium in UO2 single crystals: a transmission electron microscopy investigation

We have found that the structure of an Al-Ni-Co decagonal quasicrystal, which shows a diffraction pattern with many superlattice reflections (called a 'type I superlattice'), can be described as an ordered arrangement of two kinds of atomic columnar cluster with different orientations of their pentagonal symmetry. The fundamental lattice, which is constructed by connecting the clusters, is a rhombic quasiperiodic lattice with a bond length of about 2nm. The two kinds of cluster are placed at the lattice points in a definite manner such that the two connected clusters have different orientations.     

Reduced willingness to approach genuine smilers in social anxiety explained by potential for social evaluation,not misperception of smile authenticity

ABSTRACT

We investigate perception of, and responses to, facial expression authenticity for the first time in social anxiety, testing genuine and polite smiles. Experiment 1 (N?=?141) found perception of smile authenticity was unaffected, but that approach ratings, which are known to be reduced in social anxiety for happy faces, are more strongly reduced for genuine than polite smiles. Moreover, we found an independent contribution of social anxiety to approach ratings, over and above general negative affect (state/trait anxiety, depression), only for genuine smiles, and not for polite ones. We argue this pattern of results can be explained by genuine smilers signalling greater potential for interaction – and thus greater potential for the scrutiny that is feared in social anxiety – than polite smiles. Experiment 2 established that, relative to polite smilers, genuine smilers are indeed perceived as friendlier and likely to want to talk for longer if approached. Critically, the degree to which individual face items were perceived as wanting to interact correlated strongly with the amount that social anxiety reduced willingness to approach in Experiment 1. We conclude it is the potential for social evaluation and scrutiny signalled by happy expressions, rather than their positive valence, that is important in social anxiety.     

Synthesis of asymmetric movement trajectories in timed rhythmic behaviour by means of frequency modulation

Results from different empirical investigations on gestural aspects of timed rhythmic movements indicate that the production of asymmetric movement trajectories is a feature that seems to be a common characteristic of various performances of repetitive rhythmic patterns. The behavioural or neural origin of these asymmetrical trajectories is, however, not identified. In the present study we outline a theoretical model that is capable of producing syntheses of asymmetric movement trajectories documented in empirical investigations by Balasubramaniam et al. (2004). Characteristic qualities of the extension/flexion profiles in the observed asymmetric trajectories are reproduced, and we conduct an experiment similar to Balasubramaniam et al. (2004) to show that the empirically documented movement trajectories and our modelled approximations share the same spectral components. The model is based on an application of frequency modulated movements, and a theoretical interpretation offered by the model is to view paced rhythmic movements as a result of an unpaced movement being “stretched” and “compressed”, caused by the presence of a metronome. We discuss our model construction within the framework of event-based and emergent timing, and argue that a change between these timing modes might be reflected by the strength of the modulation in our model.     

Investigations of the optical spectra and local structure for nickel(II) ions in XF2 (X = Mg,Zn) single crystals

A complete diagonalization energy matrix method (CDM) is proposed for 3d ⁸ ions in orthorhombic site (D _{2 h} ) symmetry. As an application, the optical spectra of XF₂ : Ni²⁺ (X = Mg, Zn) is well explained on the basis of both the CDM and the semi-self-consistent field d-orbital (semi-SCF d-orbital) theory. In addition, by establishing the relationship between the optical spectra and local structure, it has been possible to calculate the crystal structure parameters for XF₂:Ni²⁺ from optical measurements on Ni²⁺ ions. The theoretical results are in good agreement with experimental findings.