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Elastic properties in the medium can affect piezomagnetic fields. In the paper, the heterogeneity effect of elastic properties on piezomagnetic fields has been investigated by constructing different dislocation models. The results show that the burial depth of the source and elastic properties in near-surface layer affect the heterogeneity effect significantly. However, the heterogeneity effect can be neglected when the dislocation source is buried under 5 km depth. When the dislocation reaches the surface, the heterogeneity effect mainly depends on the elastic properties in the near-surface layer. The smaller the elastic properties in the near-surface layer, the larger heterogeneity effect. The non-uniform slip model can reduce the heterogeneity effect. [ABSTRACT FROM AUTHOR]

One of the key demands on tungsten (W) as designated plasma-facing material (PFM) is the capability to fulfill a structural function. Since W has refused ductilization strategies by alloying alone, the production of W materials with enhanced ductility has come into focus considering tailored microstructures. This work addresses the rolling-induced microstructural modifications of warm- and cold-deformed W sheets and is supplemented by a comprehensive fracture mechanical study as a fundament for correlations between the spatial distribution of boundaries and brittle-to-ductile transition (BDT) temperature. Here we show that an extended Hall–Petch-like relationship is well suited to describe the rolling-induced reduction in BDT temperature and moreover has the potential to reflect the anisotropic nature of the transition temperature in severely rolled W sheets. Using the data of warm- and cold-rolled W sheets and also of strongly recovered W, best description of the BDT temperature was achieved by using as microstructural variables (i) the mean spacing between boundaries which intersect with the crack front and (ii) the mean boundary spacing along the normal of the crack plane. Taking into account the similarity to recent simulative-derived relationships, our findings support the theory suggesting the stimulated dislocation nucleation at boundaries as the decisive factor for more effective shielding of the crack tip in UFG materials and, in consequence, significantly reduced BDT temperatures. Besides, this work gives strong indications that the reduction of the BDT temperature in UFG W is not related to coincidence site lattice (CSL) boundaries.

Identifying the character and the source of partial dislocations associated phase transformation from a hexagonal close-packed (HCP) structure to a face-centered cubic (FCC) structure is essential for understanding phase transformation mechanisms, but was rarely done using microscopy. Here, we report a stress-induced HCP to FCC phase transformation in pure hafnium during cold rolling. Detailed transmission electron microscopy investigations revealed that transformation-related partials stemmed from the dissociation of -type dislocations. Successive gliding of partials on every other basal plane resulted in the orientation relationship between the two phases of < 11 2 ¯ 0 > h c p // < 110 > f c c , < 10 1 ¯ 0 > h c p // < 1 ¯ 12 > f c c and [ 0001 ] h c p // < 111 > f c c .Besides, a new way to form a twin relationship in the FCC structure was discovered. [ABSTRACT FROM AUTHOR]

Nanomaterials with unique edge sites have received increasing attention due to their superior performance in various applications. Herein, we employed an effective ethylenediaminetetraacetic acid (EDTA)-assisted method to synthesize a series of exotic BiSe nanostructures with distinct edge sites. It was found that the products changed from smooth nanoplates to half-plate-containing and crown-like nanoplates upon increasing the molar ratio of EDTA to Bi. Mechanistic studies indicated that, when a dislocation source and relatively high supersaturation exist, the step edges in the initially formed seeds can serve as supporting sites for the growth of epilayers, leading to the formation of half-plate-containing nanoplates. In contrast, when the dislocation source and a suitably low supersaturation are simultaneously present in the system, the dislocation-driven growth mode dominates the process, in which the step edges form at the later stage of the growth responsible for the formation of crown-like nanoplates. [ABSTRACT FROM AUTHOR]

A mathematical model of generation and accumulation of interstitial atoms in plastically deformable pure FCC metals is suggested based on the concept of hardening and recovery that links the phenomena proceeding in the deformable crystal material with the behavior of crystal structure defects. The model comprises kinetics equations for point defects - mono- and bivacancies and interstitial atoms - written with allowance for mechanisms of their generation and precipitation on sinks. Special attention is given to investigation of the influence of the velocity and character of motion of helical segments of expanding dislocation loops on generation of interstitial atoms. Concentrations of interstitial atoms generated in the process of plastic deformation are calculated. [ABSTRACT FROM AUTHOR]

This paper presents synthesis of acoustic-emission (AE) wave propagation in multi-layer materials and simulation of AE wave responses at free surface. In particular, the AE source is modelled as an arbitrary-orientation dislocation over an inclined-to- surface fault within one layer or at the layer-to-layer interface, while the materials are assumed as multi-layer media, each of which is homogeneous, isotropic and linearly elastic. With the use of the integral transformation approach, the three-dimensional wave propagation in the materials is solved in transformed or frequency-wavenumber domain. Subsequently, a closed-form solution for wave responses at free surface is found, which can then be converted in time-space domain. Numerical examples are finally provided for illustration. [ABSTRACT FROM AUTHOR]

Abstract: By using microcompression methodology, deformation of nanolayered Cu/Zr micropillars was systematically investigated within wide ranges of intrinsic layer thickness (5–100nm) and extrinsic sample size (300–1200nm). The intrinsic size effect, extrinsic size effect and their interplay were respectively revealed. Competition between the intrinsic and extrinsic size effects leads to experimental observation of a critical layer thickness of ∼20nm, above which the deformation is predominantly intrinsic-size-related and insensitive to sample size, while below which the two size effects are comparable. The underlying deformation mechanisms were proposed to transform from bulk-like to small-volume materials behavior. Deformation mode is correspondingly transited from homogeneous extrusion/barreling to inhomogeneous shear banding, but the two competing modes coexist in the layer thickness range from ∼50 to 20nm. In the regime of shear deformation, the extrinsic size dependence is displayed in that the deformation was controlled by shear bands nucleation in larger pillars while controlled by shear bands propagation in smaller pillars. A deformation mode map is developed to clearly elucidate the coupling intrinsic and extrinsic size effects on the deformation mode of nanolayered pillars. [Copyright &y& Elsevier]

Abstract: Nanoscale twin boundaries with initial defects are found to be easy dislocation sources in experiments. Here, deformation of nano-twinned single crystals is performed by using atomistic simulations. It is revealed that perfect TBs potentially are not dislocation sources. Another observation is that plastic deformation under uniaxial tensile loading can be dominated by TB migration in samples with inclined TB orientation. Twin planes with disconnections induced by dislocation–TB interactions are obtained during nanoindentation. Under further plastic deformation, it is observed that nucleation of dislocations occur at the disconnections where coherency of TBs has been destroyed. [ABSTRACT FROM AUTHOR]

Abstract: Metallic multilayers exhibit a very pronounced size effect where the mechanical strength depends on the layer thickness. Traditionally the Hall–Petch relation is used to account for the size effect. However, rigorous application of dislocation pileup theory predicts significant deviation from the Hall–Petch relation due to elastic inhomogeneity, discreteness of dislocations and dislocation source operation. Elastic inhomogeneity leads to anomalous scaling where the scaling exponent deviates from 1/2 of the classical Hall–Petch relation. The discrete dislocation effect is properly accounted for by a piecewise approach that can be applied at all length scales. In this article, a key step in the formulation is taken: the dislocation source characteristics are taken into consideration. Thus, all the three effects are accounted for. Analytic formulas linking yield stress to microscopic interface strength, dislocation source activation stress and other easily obtainable parameters (the Burgers vector, the elastic constants of constituent materials, crystal structure and layer thickness) are provided for all length scales. The model is then applied to Cu/Ni multilayers and the predicted strength is compared with experimental data. [Copyright &y& Elsevier]

Using in situ atomic force microscopy (AFM) and post-mortem transmission electron microscopy (TEM), the activation of dislocation sources in γ-phases of nickel-base superalloys has been investigated. Sources, at the origin of slip bands containing numerous pile-ups of several tens of dislocations, are heterogeneously distributed. They are a few micrometers apart. The physical reason for such an heterogeneity is the presence of short-range order which induces a high friction stress acting against the dislocation movement. A description of the solid solution as a combination of soft and hard areas is proposed. Dislocation sources can be activated in areas where the friction is low while the presence of hard zones hinders their operation. This distribution of hard and soft zones results from the local variations of short-range order. The distribution of the sources indicates that soft regions are a few micrometers apart. [Copyright &y& Elsevier]

Interstitial-free (IF) steel with ultrafine-grained (UFG) size was used to clarify the underlying mechanisms of the 'extra-hardening' behavior from the Hall-Petch plot and the discontinuous yielding behavior from axial tensile tests. Using transmission electron microscopy (TEM) at different tensile strains, it was found that the TEM micrographs for the bulk sample of the UFG IF steel showed a decrease in dislocation density in the grain interior after experiencing a micro tensile strain of 1.0% before macro-yielding. Additionally, an increase in dislocation density was found in the grain interior, with a micro tensile strain of 15% in the necking region after macro-yielding. In-situ compression tests in a TEM for micropillars were performed to directly observe the motion of dislocations and corresponding interactions with grain boundaries in UFG IF steel. Before macro-yielding, pre-existing dislocations in the grain interior moved toward the grain boundary, wherein they were then absorbed. The dislocation density decreased significantly due to the annihilation, which was accompanied by an increase in load during pre-yielding. The decrease in dislocation density in the grain interior of the micropillar is consistent with the results obtained from the bulk sample. After macro-yielding, the burst-like dislocation emission emerged from the grain boundary, leading to a yield drop and a discontinuous yielding behavior. Furthermore, the higher yield stress and the yield drop in the UFG IF steel can be understood by the decrease in dislocation density according to a combination of the Orowan model as well as the Johnston-Gilman model. Image, graphical abstract [ABSTRACT FROM AUTHOR]