Mechanical behaviour of an Al–matrix composite reinforced with nanocrystalline Al-coated B4C particulates

In metal–matrix composites (MMCs), interfacial bonding between the metal matrix and the ceramic reinforcement plays a crucial role in their mechanical performance. In the present study, B₄C particles were cryomilled with an Al alloy to produce a composite powder, in which the B₄C was uniformly distributed in nanocrystalline Al. The cryomilling developed a strong bond between the B₄C and the Al, allowing the nanocrystalline Al to act as a coating with a strong ceramic–metal interface. This cryomilled composite powder was then introduced, as a reinforcement, into a conventional Al alloy to strengthen the material. After consolidation, the result was a bulk Al–matrix composite reinforced with B₄C particles encapsulated in nanocrystalline Al. This composite exhibits greatly improved strength and stiffness.     

Twinning in cryomilled nanocrystalline Mg powder

Nanocrystalline (nc) Mg powder was synthesized via cryomilling. Extension twins were identified with high-resolution transmission electron microscopy in the cryomilled powders and the study presents the first evidence of twinning in unalloyed nc Mg. The formation of twins in the nc Mg is attributed to a high strain rate, the low (cryogenic) temperature and high local shear stresses present around the grain boundaries during deformation by cryomilling.     

Iron in solution with aluminum matrix after non-equilibrium processing: an atom probe tomography study

In this paper, we report on the influence of rapid solidification and severe plastic deformation on the solid solubility of Fe in Al. Atom probe tomography, for the first time, was performed on fine (3–4 μm diameter) and coarse (~100 μm) as-atomised Al-5 at.% Fe powder and cryomilled Al-5 at.% Fe powder. The atomised powders exhibited negligible Fe in solution with Al, whereas the cryomilled powder contained ~2 at.% Fe in solution. Moreover, our results suggest that severe plastic deformation is preferable to atomisation/rapid solidification for increasing the non-equilibrium solid solubility of Fe in Al.     

Stress-enhanced grain growth in a nanostructured aluminium alloy during spark plasma sintering

In this study, we report on the influence of high pressure on the microstructure evolution of cryomilled nanostructured Al alloy powders during spark plasma sintering (SPS). Our experimental results suggest that the particular mechanism that governs grain growth during SPS depends on the magnitude of the applied pressure. In the case of material consolidated at a high pressure (e.g. 500 MPa), grain coarsening occurs via a combination of thermally activated grain boundary (GB) migration, stress-coupled GB migration and grain rotation-induced grain coalescence. In contrast, in the case of the material consolidated at a low pressure (50 MPa), grain growth occurs primarily via thermally activated GB migration.     

‘Age-hardened alloy’ based on bulk polycrystalline oxide ceramic

We report here for the first time the development of ‘age-hardened/toughened’ ceramic alloy based on MgO in the bulk polycrystalline form. This route allows for the facile development of a ‘near-ideal’ microstructure characterized by the presence of nanosized and uniformly dispersed second-phase particles (MgFe₂O₄) within the matrix grains, as well as along the matrix grain boundaries, in a controlled manner. Furthermore, the intragranular second-phase particles are rendered coherent with the matrix (MgO). Development of such microstructural features for two-phase bulk polycrystalline ceramics is extremely challenging following the powder metallurgical route usually adopted for the development of bulk ceramic nanocomposites. Furthermore, unlike for the case of ceramic nanocomposites, the route adopted here does not necessitate the usage of nano-powder, pressure/electric field-assisted sintering techniques and inert/reducing atmosphere. The as-developed bulk polycrystalline MgO–MgFe₂O₄ alloys possess considerably improved hardness (by ~52%) and indentation toughness (by ~35%), as compared to phase pure MgO.     

Role of impurity elements in metal-metal bond strengths

Abstract

The influence of impurity elements on the strength of neighbouring metal–metal bonds is investigated using a simple tight-binding model. We show that the quantum-mechanical response function of the metal host plays a central role in determining whether the bond is weakened or strengthened. The Messmer–Briant model, which is based on classical concepts of charge flow due to differences in electronegativity, is not applicable to metals because of their perfect screening. Recent experiments on impurity core level and isomer shifts should not be taken as a direct measure of neighbouring metal–metal bond decohesion.     

Anagram software for cognitive research that enables specification of psycholinguistic variables

Anagram tasks are frequently used in cognitive research, and the generation of new scrambled letter combinations is a task well suited to a software solution. Most available programs, however, do not allow experimenters to generate new anagrams flexibly or to characterize existing anagrams using psycholinguistic criteria. They also do not provide detailed information on their source dictionaries. We present anagram software that interfaces with CELEX2, an internationally recognized psycholinguistic database. This software allows users to capitalize on lexical variables and thus enables direct control of psycholinguistic features that may influence the cognitive processes involved in anagram solution.     

Influence of annealing on the photoluminescence of nanocrystalline ZnO synthesized by microwave processing

Zinc oxide (ZnO) nanoparticles have been synthesized by a microwave heating method. The resulting powders were annealed at temperatures from 400°C to 800°C for 4?h. The annealed powders were characterized in terms of their structural, morphological, and optical properties by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and room-temperature photoluminescence (PL) spectroscopy, respectively. The XRD patterns of the samples revealed the formation of phase-pure ZnO, apparently without any impurities. However, FT-IR spectra revealed the presence of carboxylate impurities, the signals of which disappear as the annealing temperatures are increased from 400°C to 800°C. SEM pictures showed increasing size ZnO nanoparticles with increase in annealing temperature. The powder annealed at 800°C is identified as hexagonal wurzite ZnO. The improved PL spectral performance of the ZnO powder correlates well with the improved crystalline quality of the nanoparticles, as revealed by the XRD and PL. Sharp ultraviolet (UV) band edge luminescence has been observed at 375?nm, which is suitable for application in UV light emitting devices. The results also showed that the excitation wavelength dependence of the luminescence arises from quantum confinement effects.     

Engaging Environments Enhance Motor Skill Learning in a Computer Gaming Task

Engagement during practice can motivate a learner to practice more, hence having indirect effects on learning through increased practice. However, it is not known whether engagement can also have a direct effect on learning when the amount of practice is held constant. To address this question, 40 participants played a video game that contained an embedded repeated sequence component, under either highly engaging conditions (the game group) or mechanically identical but less engaging conditions (the sterile group). The game environment facilitated retention over a 1-week interval. Specifically, the game group improved in both speed and accuracy for random and repeated trials, suggesting a general motor-related improvement, rather than a specific influence of engagement on implicit sequence learning. These data provide initial evidence that increased engagement during practice has a direct effect on generalized learning, improving retention and transfer of a complex motor skill.     

White matter maturation supports the development of reasoning ability through its influence on processing speed

The structure of the human brain changes in several ways throughout childhood and adolescence. Perhaps the most salient of these changes is the strengthening of white matter tracts that enable distal brain regions to communicate with one another more quickly and efficiently. Here, we sought to understand whether and how white matter changes contribute to improved reasoning ability over development. In particular, we sought to understand whether previously reported relationships between white matter microstructure and reasoning are mediated by processing speed. To this end, we analyzed diffusion tensor imaging data as well as data from standard psychometric tests of cognitive abilities from 103 individuals between the ages of 6 and 18. We used structural equation modeling to investigate the network of relationships between brain and behavior variables. Our analyses provide support for the hypothesis that white matter maturation (as indexed either by microstructural organization or volume) supports improved processing speed, which, in turn, supports improved reasoning ability.     

Note taking, review, memory, and comprehension

In previous work assessing memory at various levels of representation, namely the surface form, textbase, and situation model levels, participants read texts but were otherwise not actively engaged with the texts. The current study tested the influence of active engagement with the material via note taking, along with the opportunity to review such notes, and the modality of presentation (text vs. spoken). The influence of these manipulations was assessed both immediately and 1 week later. In Experiment 1 participants read a text, whereas in Experiment 2 participants watched a video recording of the material being read as a lecture. For each experiment the opportunity to take notes was manipulated within participants, and the opportunity to review these notes before the test was manipulated between participants. Note taking improved performance at the situation model level in both experiments, although there was also some suggestion of benefit for the surface form. Thus, active engagement with material, such as note taking, appears to have the greatest benefit at the deeper levels of understanding.     

Age of acquisition predicts rate of lexical evolution

The processes taking place during language acquisition are proposed to influence language evolution. However, evidence demonstrating the link between language learning and language evolution is, at best, indirect, constituting studies of laboratory-based artificial language learning studies or computational simulations of diachronic change. In the current study, a direct link between acquisition and evolution is established, showing that for two hundred fundamental vocabulary items, the age at which words are acquired is a predictor of the rate at which they have changed in studies of language evolution. Early-acquired words are more salient and easier to process than late-acquired words, and these early-acquired words are also more stably represented within the community’s language. Analysing the properties of these early-acquired words potentially provides insight into the origins of communication, highlighting features of words that have been ultra-conserved in language.     

Deriving a Humanistic Theory of Child Development From the Works of Carl R. Rogers and Karen Horney

The aim of this theoretical investigation was to provide instructors at the college level with material that they might use to introduce humanistic developmental theory to students of child psychology. The study had several phases. First, the theories of child development embedded within the works of Carl R. Rogers and Karen Horney were articulated and outlined separately. Next, the highly significant structural features of the theories were synthesized to form a unified humanistic theory of child development. Finally, there was a brief discussion of the strengths and weakness of the humanistic developmental perspective derived from Rogers's and Horney's works.     

序列学习中的内隐学习效应

该研究采用较为复杂的包含图形出现的位置规则和顺序规则的实验材料,通过对比被试直接测验和间接测验、内隐学习和外显学习成绩的异同,考察了序列学习中的内隐学习效应,并探讨了规则特点对内隐学习效应的影响。结果表明,在图形位置的序列学习中存在内隐学习效应,内隐学习效应的大小受规则特点的影响。     

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.     

Salient material properties and haptic volume perception: The influences of surface texture, thermal conductivity, and compliance

We investigated the influences of surface texture, thermal conductivity, and compliance on the haptic perception of the volume of small cubes. It was hypothesized that an object containing highly salient material properties would be perceived as larger in volume than the same object without these properties. Blindfolded subjects were asked to explore pairs of cubes differing in their material properties and to select the one with the larger volume. The results showed that, counterintuitively, a smooth cube was perceived as being significantly larger than a rough cube of the same physical volume, with average biases of about 19 %. Furthermore, cubes with a higher thermal conductivity were perceived as significantly larger than cubes with a lower thermal conductivity (average bias of about 7 %). In addition, the magnitude of the bias in this condition was not changed by increasing or decreasing the temperature of the test objects, suggesting that the effect of thermal conductivity could not be attributed directly to the heat flow. Finally, a hard cube was perceived as significantly larger than a soft cube of equal physical volume, with an average bias of about 25 %. These results reveal that the studied material properties have significant and consistent influences on the haptic perception of volume. The observed biases provide an indication of the levels at which the processing of haptic information on volume and material properties occurs.     

Childhood experiences and psychosocial influences on HIV risk among adolescent Latinas in southern California

This study determined how adverse childhood experiences influenced risky sexual behavior in a community sample of Latina adolescents in Los Angeles (N = 904) within a modified ecodevelopmental perspective. Psychosocial, sociocultural, and environmental mediators of the relations between childhood experiences and risky sexual behavior were tested. Many direct and mediated paths were revealed using structural equation modeling. Childhood maltreatment was associated with risk sexual practices, including more partners, less condom use, more frequent intercourse, and less HIV testing. Drug use and general self-efficacy mediated several relationships. Reducing childhood maltreatment reduced HIV risk and improved psychosocial functioning. Many factors tested did not influence HIV risk. Much of what is known about HIV risk among other populations may not apply to young Latina women.     

Tension and compression of bulk Al-7.5 wt% Mg alloy

The mechanical behaviour of bulk ultrafine-grained Al-7.5 wt% Mg alloy consolidated from cryomilled powders has been investigated. The experimental data show that the alloy exhibits high strength, low strain hardening, serrated flow and relatively high ductility. In addition, the data indicate that the yield strength in tension is essentially equal to that in compression. The yield and flow strengths of the alloy are discussed in terms of strengthening processes that are related to grain size, the Orowan mechanism and solid-solution hardening. The serrations in the stress-strain curve are discussed in terms of dynamic strain ageing and deformation twinning.     

HREM study of the chemical nature of twin planes in CuAlO2

Abstract

High-resolution electron microscopy has been used to study the atomic structure of twins in the binary oxide CuAlO₂. This compound is an occasional product of the bonding between pre-oxidized copper and alumina; its peculiarity is to exhibit a twinned microstructure, owing to the occurrence of two enantiomorphic right- and left-handed configurations. The microstructural examination at high resolution reveals that twin planes are located on copper basal planes rather than aluminium planes; this result is discussed on the basis of an analysis of the various possible stacking sequences which can reasonably be considered to occur at the twin interfaces.     

Impact of coherent and incoherent twin boundaries on the microhardness of annealed nanocrystalline Ni–W alloys

The microstructural evolution of nanocrystalline Ni–W alloys with annealing temperature and more specifically grain boundary (GB) character is investigated through several techniques and correlated with the hardening behaviour. It is shown that two distinct regions can be identified in relation to the annealing temperature and the microstructural evolution. At temperatures below 550 °C (Regime I), a small increase in grain size is observed and is accompanied by a significant hardening and an increase in the fraction of Σ3 incoherent twin boundaries. At temperatures above 550 °C (Regime II), the thermal stability is overcome and important grain growth occurs with a decrease in both the volumic fraction of GBs and the microhardness. It is suggested that the microhardness evolution during heat treatment is influenced by two opposing processes: an increase in the fraction of incoherent twin boundaries (hardening effect) and grain growth (softening effect). Both aspects are directly associated with the mean free path of mobile dislocations.