Kinetic study of static recrystallization in an Fe–Al–O ultra-fine-grained nanocomposite

A nearly abrupt coarsening of grains is observed in a newly developed Fe–Al–O ultra-fine-grained nanocomposite with a significant volume fraction (4%) of alumina nano-precipitates. The microstructure of the alloy was analysed in different states (as-received and annealed) by means of scanning electron microscopy, transmission electron microscopy (TEM) and hardness. The initial grain size 150–200 nm increases up to 50 μm during annealing 1000 °C/8 h and thereafter demonstrates saturation. A linear correlation between volume fraction of coarse grains and hardness was found. It was identified by TEM that alumina nano-precipitates stabilize the dislocation microstructure against recovery very effectively and the grain coarsening is due to fast growth of very few dislocation free grains. Thus, the observed grain coarsening has the attributes of static recrystallization.     

Properties of three-dimensional bubbles of constant mean curvature

Foam bubbles of constant mean curvature with a number of faces between 3 and 32 have been computer generated. The surface area per unit volume is only weakly dependent upon the number of faces, which allows an approximate equation for the energy of a three-dimensional foam to be derived. A further relation between the rate of change in the volume of a bubble due to gas diffusion and its number of faces is compared with existing theory.     

Observation of Cu nanometre scale clusters formed in Fe85Si2B8P4Cu1 nanocrystalline soft magnetic alloy by a spherical aberration-corrected TEM/STEM

Microstructure of a nanocrystalline soft magnetic Fe₈₅Si₂B₈P₄Cu₁ alloy (NANOMET^®) was investigated by the state of the art spherical aberration-corrected TEM/STEM. Observation by TEM shows that the microstructure of NANOMET^® heat treated at 738 K for 600 s which exhibits the optimum soft magnetic properties has homogeneously distributed bcc-Fe nanocrystallites with the average grain size of 30 nm embedded in an amorphous matrix. Elemental mappings indicate that P is excluded from bcc-Fe grains and enriched outside the grains, which causes to retard the grain growth of bcc-Fe crystallites. The aberration-corrected STEM-EDS analysis with the ultrafine electron probe successfully proved that Cu atoms form nanometre scale clusters inside and/or outside the bcc-Fe nanocrystallites.     

Exaggerated grain growth and improved properties of Y1 Ba2 Cu3 O7-x by Pt addition

Abstract

Superconducting and mechanical properties of Y₁ Ba₂Cu₃ O₇ can be improved by the use of sintering aids. 0·2 wt% of finely divided laser-ablated platinum powder has been mixed with the parent material and produced a dense product with exaggerated grain growth and improved critical current density and Vickers hardness. At higher concentrations of platinum, while the mechanical properties are further improved, the increased density appeared to inhibit access of oxygen for the tetragonal to orthorhombic phase transition and the current density was reduced.     

A quantum-mechanical study of the stability of SnO 2 nanocrystalline grains

The purpose of this study is to gain insight into the instability which is observed, under operative conditions, in SnO 2 nanocrystalline materials. For this purpose, the binding and fragmentation energies of SnO 2 crystalline grains have been evaluated quantum-mechanically at a semiempirical level using the extended Debye-Hückel approximation. The inner structure of the grains is assumed to be an unreconstructed rutile lattice, as in the parent solid. The grain size and shape are variable and a parametric search has been carried out on both quantities. In broad terms the grains show a bulk-like behaviour, as their characteristic energies are approximately independent of their size and shape. However, the increases in the grain size and in the oxygen content may increase the grain stability. These features are discussed in the light of the properties of small clusters formed by tin and oxygen.     

Newly observed microscopic planar defects on {111} in natural diamond

Abstract

Circular disc-crack-like defects randomly distributed on {111} planes have been discovered by 300kV transmission electron microscopy in diamond that had crystallized on non-faceted growth surfaces of mean orientation {100}. In a sample of ～ 100 defects, disc diameters were narrowly distributed about a mean of 1·2 μm and one in seven discs were located on a coplanar stacking fault of average diameter 2 μm. Discs totally enclosed within the specimen exhibit concentric moiré fringes covering their image areas when viewed obliquely, showing them to be dilated into thin lenticles. Internal pressures, deduced from moiré-fringe counts on six lenticles, were in the range 1·5 to 1·8 GPa.     

Novel fabrication of bulk Al with gradient grain size distributions via powder metallurgy

We describe a novel approach to synthesize gradient microstructures, defined hereafter as containing a broad but continuous distribution of grain sizes. These microstructures extend the concept of a bimodal grain size distribution and the ability to design with multiple length scales. We demonstrate the proposed approach via experiments involving cryogenic ball milling of Al–4.5Mg–0.4Mn–0.05Fe and Al–50Mg powder followed by subsequent consolidation. Our results reveal that the grains in the consolidated powder present a gradient size distribution ranging from <100 nm to >3 μm. Moreover, phase composition analysis revealed a unique “interfingered” structure where the two starting phases were intermixed in a complex three-dimensional mesh. Hardness studies of this gradient microstructure show average Vickers hardness values of 200 ± 2.6, 204 ± 4.3 and 266 ± 50 for macrohardness, microhardness and nanoindentation, respectively. The standard deviation values highlight that the gradient microstructure is disordered locally, but homogenous macroscopically.     

Direct observation of intergranular stress fields in polycrystalline materials

The ability to measure interparticle stress fields is crucial for a number of scientific fields. Detailed knowledge of such interaction stresses can shed light on a number of phenomena such as the fracture mechanics of polycrystalline materials, the mechanics of granular media, and crystallization process of microspheres and nanospheres. In this letter we report the use of a new micro-Raman-spectroscopy-based technique to measure directly the intergranular stress fields in polycrystalline systems. Using Raman active tracers (50Å graphite crystals) dispersed in the system, the technique was shown to be applicable for non-Raman-active polycrystalline systems (e.g. metals). For the first time, stresses within and around individual grains has been monitored in situ as the global system stress was increased to system failure. The effects of grain orientation and shape are monitored and discussed.     

Twin formation and structural modulations in orthorhombic and tetragonal YBa2(Cu1-xCox)3O7-δ

Abstract

The microstructure of YBa₂(Cu_1?xCo_x)₃O_7?δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x?0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x?0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20Å) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.     

Reversed Resolution in Reducing General Satisfiability Problem

In the following we show that general property S considered by Cowen [1], Cowen and Kolany in [3] and earlier by Cowen in [2] and Kolany in [4] as hypergraph satisfiability, can be constructively reduced to (3, 2) · SAT, that is to satisfiability of (at most) triples with two-element forbidden sets. This is an analogue of the“classical” result on the reduction of SAT to 3 · SAT.     

Phase field microelasticity theory of dislocation dynamics in a polycrystal: Model and three-dimensional simulations

A three-dimensional multidislocation system in a polycrystal under applied stress is treated as a particular case of the phase field microelasticity theory of multivariant stress-induced martensitic transformations in polycrystals. This approach reduces the problem of the evolution of a dislocation system to a solution of the nonlinear integrodifferential Ginzburg-Landau equation. In this formalism, the elastic interaction between dislocations and the elastic coupling between grains are taken into consideration through exact analytical solution of the elasticity problem. The dislocation reactions, such as multiplication and annihilation, are taken into account automatically. The dislocations are 'free' to choose the optimal evolution path. Examples of three-dimensional computer simulations are considered.     

Pressure-Induced transitions in amorphous TlxSe100−x alloys

Abstract

The electrical resistivity of bulk semiconducting amorphous Tl_xSe_100?x alloys with 0 ≤ x ≤ 25 has been investigated up to a pressure of 14 GPa and down to liquidnitrogen temperature by use of a Bridgman anvil device. All the glasses undergo a discontinuous pressure-induced semiconducting-to-metal transition. X-ray diffraction studies on the pressure-recovered samples show that the high-pressure phase is the crystalline phase. The pressure-induced crystalline products are identified to be a mixture of Se having a hexagonal structure with a = 4·37 Å and c = 4·95 Å and TlSe having a tetragonal structure with a = 8·0 Å and c = 7·0 Å     

Understanding associations between negatively biased attention and depression and social anxiety: positively biased attention is key

ABSTRACT

Background and objectives: Although research supports the premise that depressed and socially anxious individuals direct attention preferentially toward negative emotional cues, little is known about how attention to positive emotional cues might modulate this negative attention bias risk process. The purpose of this study was to determine if associations between attention biases to sad and angry faces and depression and social anxiety symptoms, respectively, would be strongest in individuals who also show biased attention away from happy faces.

Methods: Young adults (N?=?151; 79% female; M?=?19.63 years) completed self-report measures of depression and social anxiety symptoms and a dot probe task to assess attention biases to happy, sad, and angry facial expressions.

Results: Attention bias to happy faces moderated associations between attention to negatively valenced faces and psychopathology symptoms. However, attention bias toward sad faces was positively and significantly related to depression symptoms only for individuals who also selectively attended toward happy faces. Similarly, attention bias toward angry faces was positively and significantly associated with social anxiety symptoms only for individuals who also selectively attended toward happy faces.

Conclusions: These findings suggest that individuals with high levels of depression or social anxiety symptoms attend preferentially to emotional stimuli across valences.     

Grain orientation effects on delamination during fatigue of a sensitized Al–Mg alloy

Fatigue crack growth experiments were conducted in humid air (RH~45%) at 25 °C on 29-mm-thick plate samples of an aluminium–magnesium (Al–Mg) 5083-H131 alloy in the long transverse (LT) direction. Samples were tested in both the as-received condition and after sensitization at 175 °C for 100 h. Delamination along some grain boundaries was observed in the short transverse plane after fatigue testing of the sensitized material, depending upon the level of ΔK and K_max. Orientation microscopy using electron backscattering diffraction and chemical analyses using transmission electron microscopy and energy dispersive spectroscopy of grain boundaries revealed that Mg segregation and the orientation of grains had key roles in the observed grain boundary delamination of the sensitized material.     

Electron back-scatter diffraction study of inoculation of Al

The microstructure of cast commercial-purity aluminium inoculated by addition of an Al-Ti-C grain refiner has been studied by scanning electron microscopy and electron back-scatter diffraction (EBSD). It is found that a small fraction of the TiC particles present in the melt act as nucleation centres for grains. EBSD shows that the aluminium grains have a crystallographic cube-cube orientation relationship with the particles on which they nucleate. Nucleation occurs only on the largest particles, consistent with model predictions. Despite potential thermodynamic instability, TiC particles are effective nucleating agents.     

Microstructural characterization and high-temperature strength of hot-pressed silicon nitride ceramics with Lu2O3 additives

The microstructures of two hot-pressed Si₃N₄ ceramics, with 3.33 and 12.51 wt% Lu₂O₃ additive, have been characterized using transmission electron microscopy. The microstructures of both samples consisted of elongated β-Si₃N₄ grains and a secondary phase, contained in pockets surrounded by the grains, with a crystalline or amorphous form. In the 3.33 wt% Lu₂O₃-containing Si₃N₄ ceramic, all the multiple-grain junctions were completely crystalline while, in the 12.51 wt% Lu₂O₃-containing Si₃N₄ ceramic, approximately half the junctions were devitrified. A thin intergranular amorphous film present between the two-grain boundary was common; however, a film-free grain boundary was observed in the 12.51 wt% Lu₂O₃ sample. The film-free grain boundary was determined to be approximately 35%. Both ceramics fractured in four-point flexure between 1200 and 1600°C. Their high-temperature strength is closely associated with the nature of the grain-boundary phase formed during the sintering process.     

Strain-induced submicrocrystalline grains developed in austenitic stainless steel under severe warm deformation

Strain-induced grain evolution in a 304 type austenitic stainless steel has been studied in multiple compression with the loading direction being changed in each pass. The tests were carried out to total strains above 6 at 873 K (0.5 T _m) at a strain rate of about 10^-3 s^-1. Multiple deformation promotes the rapid formation of many mutually crossing subboundaries because various slip systems operate from pass to pass. The gradual rise in misorientations across dislocation subboundaries with increasing strain finally leads to the evolution of very fine grains with large-angle boundaries. It is concluded that a new grained structure can result from a kind of continuous reaction during deformation, namely continuous dynamic recrystallization. Such deformation-induced grains are characterized by relatively low densities of dislocations, and considerable lattice curvatures developed in their interiors. The latter observations suggest that high elastic distortions are developed in the grain interiors and so such strain-induced grain structures are in a non-equilibrium state.     

Ultrafine grain development in copper during multidirectional forging at 195 K

Strain-induced evolution of ultrafine grains in pure copper was studied in multidirectional forging (MDF) at 195?K. The stress–strain behaviour was characterized by rapid strain hardening during early processing and the rate of strain hardening gradually decreased with straining, leading to an apparent steady-state flow at large cumulative strains of more than 5. The structural changes were associated with the development of high-density microshear bands crossed by MDF. The new fine grains 0.16?µm in size, which was smaller than the subgrain size evolved during early deformation, were evolved primarily at microshear band intersections, and then the new fine grains filled out the whole sample as the number of microshear band intersections increased at large strains. This is essentially similar to continuous dynamic recrystallization. The size of new grains can be expressed by a power law function of flow stress with a grain size exponent of about –0.3. The kinetics of the strain-induced grain evolution is analyzed and the mechanisms are discussed.     

The feasibility of an automated eye-tracking-modified Fagan test of memory for human faces in younger Ugandan HIV-exposed children

Objective: The Fagan Test of Infant Intelligence (FTII) uses longer gaze length for unfamiliar versus familiar human faces to gauge visual-spatial encoding, attention, and working memory in infants. Our objective was to establish the feasibility of automated eye tracking with the FTII in HIV-exposed Ugandan infants.

Method: The FTII was administered to 31 perinatally HIV-exposed noninfected (HEU) Ugandan children 6–12 months of age (11 boys; M = 0.69 years, SD = 0.14; 19 girls; M = 0.79, SD = 0.15). A series of 10 different faces were presented (familiar face exposure for 25 s followed by a gaze preference trial of 15 s with both the familiar and unfamiliar faces). Tobii X2-30 infrared camera for pupil detection provided automated eye-tracking measures of gaze location and length during presentation of Ugandan faces selected to correspond to the gender, age (adult, child), face expression, and orientation of the original FTII. Eye-tracking gaze length for unfamiliar faces was correlated with performance on the Mullen Scales of Early Learning (MSEL).

Results: Infants gazed longer at the novel picture compared to familiar across 10 novelty preference trials. Better MSEL cognitive development was correlated with proportionately longer time spent looking at the novel faces (r(30) = 0.52, p = .004); especially for the Fine Motor Cognitive Sub-scale (r(30) = 0.54, p = .002).

Conclusion: Automated eye tracking in a human face recognition test proved feasible and corresponded to the MSEL composite cognitive development in HEU infants in a resource-constrained clinical setting. Eye tracking may be a viable means of enhancing the validity and accuracy of other neurodevelopmental measures in at-risk children in sub-Saharan Africa.     

Texture and grain boundary character distribution (GBCD) in rapidly solidified and annealed Fe–6·5 mass% Si ribbons

Abstract

The type and frequency of grain boundaries, the so-called grain boundary character distribution (GBCD), has been determined in rapidly solidified and subsequently annealed Fe-6·5 mass% Si alloy ribbon by the scanning electron microscopy-electron channelling pattern (SEM-ECP) technique. High frequencies of low-angle boundaries and coincidence boundaries with Σ3, Σ9, Σ11, Σ17 and Σ19 were observed in a fully annealed ribbon with well defined {110} texture. The total frequency of low-angle boundaries and coincidence boundaries is almost one-half of all grain boundaries. The coincidence boundaries which occurred more frequently are exactly those predicted theoretically from the coincidence orientations for 〈110〉 rotation in cubic crystals, similar to those observed previously in {100} textured ribbons of the same alloy produced by the same processing method. The presence of a close relationship between the type of texture and GBCD has been confirmed by experiment on differently textured ribbons of the same material.