Effect of strain rate on the compressive behaviour of a Zr56Al10.9Ni4.6Cu27.8Nb0.7 bulk metallic glass

We report the study of the effect of strain rate on the compressive behaviour of a Zr₅₆Al_10.9Ni_4.6Cu_27.8Nb_0.7 bulk metallic glass. The results indicated that both the strength and plasticity of the glass increase with increasing the strain rate up to 10^?5 s^?1, above which the strength and plasticity start to decrease. The enhanced mechanical properties under a strain rate of 10^?5 s^?1 are due to the emission/propagation rate of the shear bands being consistent with the strain rate.     

Internal stress effect on grain-boundary diffusion in submicrocrystalline metals

Grain-boundary diffusion in the regime affected by internal stresses of triple-line disclinations is analysed. The concentration profiles in a stressed solid averaged over a distribution of hydrostatic stress gradients are calculated. It is shown that the stresses from triple-line disclinations in as-prepared submicrocrystalline metals can significantly increase the effective grain-boundary diffusion coefficient, which is obtained by fitting the concentration profiles to the solution of a diffusion equation for an unstressed polycrystal.     

BIS/BAS responses to emotional cues: Self report, autonomic measure and alpha band modulation

The study explored the effect of the subjective evaluation and of BIS/BAS (Behavioral Inhibition and Activation System) differences on psychophysiological and brain oscillation measures, in response to pleasant/unpleasant and high/low arousing stimuli. Skin conductance response, heart rate, and electromyography, and alpha frequency band, were registered, during viewing IAPS figures. Both BIS and BAS measures were significant in modulating behavioral, autonomic and brain responses, with a clear dichotomy BAS-appetitive/positive BIS-withdrawal/negative sensitivity. Withdrawal (BIS) and appetitive (BAS) behavior showed opposite patterns of responses by the subjects within the frontal cortical site more than other sites. In addition, a specific frontal lateralization effect was found as a function of BIS (more right-side activation for negative cues) and BAS (more left-side activation for positive cues). Moreover, autonomic variables and frequency band were found to be effected by arousal rating per se, with an increased response to high arousal in comparison with low arousal and neutral stimuli. The effects of subjective evaluation and individual differences were discussed in the light of the coping activity of emotion comprehension.     

Ab initio pseudopotential studies on an Al Σ = 9 grain boundary: Effects of Na and Ca impurities

The electronic properties of an Al = 9 tilt grain boundary with segregated Na and Ca impurity atoms have been calculated by a first-principles pseudopotential method. The results show that the boundary expands and the charge density decreases significantly over the whole boundary by the segregation of Na and Ca, and there are no stronger bonds than those associated with metallic bonding in the boundary even with the impurities. It is concluded that the embrittlement promotion mechanism by Na or Ca segregation in the Al boundaries is one kind of 'decohesion model'.     

Intrinsic factor controlling the deformation and ductile-to-brittle transition of metallic glasses

The energetic driving force and resistance for shearing and cracking in metallic glasses (MGs) are quantitatively evaluated. A universal thermodynamic criterion is proposed for better understanding the intrinsic correlations between fracture toughness and Poisson’s ratio, the competitions between various deformation modes and the ductile-to-brittle transition in MGs and other materials. A new cooperation parameter δ is also introduced to depict quantitatively the relative propensity of shearing versus cracking. This work could provide insights into the long-standing issues of deformation mechanisms of glassy materials, and be helpful in searching for ductile and tough MGs.     

Shear band evolution during large plastic deformation of brittle and ductile metallic glasses

Four monolithic metallic glasses (MMGs) with different plasticities varying from brittle to ductile behavior under unconstrained loading were subjected to small punch testing. All specimens undergo large plastic deformation with multiple cobweb-like shear bands under these conditions. The process of shear band evolution was carefully controlled and investigated. Plasticity of MMGs is characterized by equivalent plastic strain ε* (product of shear band density and critical shear offset). Thus, this article provides an experimental basis for a better understanding of the shear band evolution during plastic deformation of MMGs.     

Tensile deformation behaviour of Zr-based glassy alloys

This paper presents a study of the deformation behaviour of a glassy phase in two Zr-based alloys, Zr₆₅Ni₁₀Cu₅Al_7.5Pd_12.5 and Zr₆₅Al_7.5Ni₁₀Pd_17.5, performed in situ in a transmission electron microscope. In contrast to the case of shear localisation and formation of 10–20 nm thick shear bands in deformed bulk glassy samples studied earlier, it is found that in thin (electron-transparent) samples the glassy phase in front of a crack deforms more homogeneously and no nanocrystallisation takes place. The reasons for such behaviour are discussed. According to the observed results, one can conclude that the studied metallic glasses can be intrinsically ductile in submicrometre-sized volumes.     

Thurstone's discriminal processes fifty years later

Four issues are discussed concerning Thurstone's discriminal processes: the distributions governing the representation, the nature of the response decision rules, the relation of the mean representation to physical characteristics of the stimulus, and factors affecting the variance of the representation. A neural schema underlying the representation is proposed which involves samples in time of pulse trains on individual neural fibers, estimators of parameters of the several pulse trains, samples of neural fibers, and an aggregation of the estimates over the sample. The resulting aggregated estimate is the Thurstonian representation. Two estimators of pulse rate, which is monotonic with signal intensity, are timing and counting ratios and two methods of aggregation are averaging and maximizing. These lead to very different predictions in a speed-accuracy experiment; data indicate that both estimators are available and the aggregation is by averaging. Magnitude estimation data are then used both to illustrate an unusual response rule and to study the psychophysical law. In addition, the pattern of variability and correlation of magnitude estimates on successive trials is interpreted in terms of the sample size over which the aggregation takes place. Neural sample size is equated with selective attention, and is an important factor affecting the variability of the representation. It accounts for the magical number seven phenomenon in absolute identification and predicts the impact of nonuniform distributions of intensities on the absolute identification of two frequencies. 1977 Psychometric Society Presidential Address. This work was supported in part by a grant of the National Science Foundation to Harvard University. I wish to express my appreciation to S. Burbeck, D. M. Green, M. Shaw, and B. Wandell for their useful comments on an earlier draft of this paper.