Yielding and strain hardening in aluminium single-crystal foils subjected to tension and bending

A series of tensile and microbend tests were conducted on aluminium single-crystal foil specimens with different thicknesses ranging from 20 to 90?µm. Two different crystal orientations relative to the tensile direction were considered in the tests: one is an orientation that was excepted to activate at least four slip systems on four different slip planes, and the other is an orientation that was expected to activate only a single-slip system. In the tensile tests, typical size dependence of initial yield strength was observed. The microbend test method was extended to include a reversal of bending direction. Using the curves of bending moment normalized by the square of foil thickness versus surface strain, which were obtained from the reversed bending tests, the total amounts of strain hardening were divided into isotropic and kinematic hardening components. In the microbend tests, a pronounced size-dependent kinematic hardening behaviour was clearly observed; meanwhile, the amounts of isotropic hardening were very small, particularly for the single-slip orientation.     

Influence of heat treatment on the microstructural evolution of Al–3 wt.% Cu during high-pressure torsion

Solution-treated, peak-aged and overaged samples of the model alloy Al–3?wt.% Cu, obtained by selective heat treatments of the pre-material, have been subjected to high-pressure torsion at room temperature and at 200?°C. The mechanical behaviour of the samples was investigated with torque measurements during deformation and microhardness measurements after deformation. Irrespective of the initial material condition, in the saturation regime a comparable equilibrium microstructure was found consisting of ultrafine aluminium grains stabilized by precipitates formed at grain boundaries.     

The aftereffects of ventriloquism: patterns of spatial generalization

We examined how visual recalibration of apparent sound location obtained at a particular location generalizes to untrained locations. Participants pointed toward the origin of tone bursts scattered along the azimuth, before and after repeated exposure to bursts in one particular location, synchronized with point flashes of light a constant distance to their left/right. Adapter tones were presented straight ahead in Experiment 1, and in the left or right periphery in Experiment 2. With both arrangements, different generalization patterns were obtained on the visual distractor's side of the auditory adapter and onthe opposite side. On the distractor side, recalibration generalized following a descending gradient; practically no generalization was observed on the other side. This dependence of generalization patterns on the direction of the discordance imposed during adaptation has not been reported before, perhaps because the experimental designs in use did not allow its observation.     

Very high temperature creep behavior of a single crystal Ni-based superalloy under complex thermal cycling conditions

Very high temperature thermal cycling has been performed on the single crystal superalloy MC2 to evaluate the effect of periodic overheating on creep behavior. The experiments consist of alternately performing 1 min dwell time at 1100°C and 1150°C for every 15 min during creep test at 1050°C/120 MPa. Both thermal cycling and prior γ′-rafting appear to be deleterious to the cyclic creep properties. The observed non-isothermal creep behavior is correlated with γ′-dissolution/coalescence processes, especially during overheatings where γ′ micro-rafts seem to play a significant role.     

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.     

Customer‐induced stress in call centre work: A comparison of audio‐ and videoconference

Call centre work was simulated in an experiment with 96 experienced call centre agents. The experimental design comprised two factors. First, agents communicated with customers either via phone, pc‐videoconference or pc‐videoconference with additional instructions increasing time pressure. The second experimental factor varied customer behaviour: half of the customers were friendly whereas the other half were rude. Several indicators of strain (e.g. emotional dissonance, tiredness) were assessed by self‐reports. Moreover, the levels of immunoglobulin A (IgA) in salvia were determined at three sampling points and specific behaviours of agents (e.g. smiling to customers) were assessed using video data. It was found that unfriendly customer behaviour led to more strain and lower call performance than friendly customer behaviour. Inducing time pressure increased IgA‐levels and reduced talking‐time with customers. However, contrary to the expectations, the availability of video data did not enhance strain of agents. Instead, it was found that videoconferencing increased activation of agents if customers were friendly. Since higher levels of activation can counteract boredom and because customers often prefer to see their service providers, adding videoconference facilities in call centres seem to be a fruitful way of enriching routine call centre work.     

Nanoindentation creep behavior of grain boundary in pure magnesium

Nanoindentation creep tests were performed at the grain boundary and grain interior in pure magnesium. The grain boundary showed a high strain rate sensitivity exponent and was dominated by grain boundary sliding due to the high diffusion rate at the grain boundary. The grain boundary affected the deformation behavior of the area at a distance of 2 µm into the grain interior. On the other hand, the grain interior had a low strain rate sensitivity exponent, because its matrix was too large to be influenced by the grain boundary. The deformation mechanism in the grain interior was determined to be dislocation slip.     

Detrimental effect of cellular precipitation on the creep strength of Inconel740H

Cellular precipitation (also known as discontinuous precipitation) has been observed at the grain boundaries of a newly developed nickel-based Inconel740H alloy designed for use at 700?°C in advanced ultrasupercritical coal-fired power plants. By means of element mapping and selected area diffraction, the cellular precipitates were identified as Cr-rich M₂₃C₆ carbides. The onset of cellular precipitation was found to follow a pucker mechanism in Inconel740H. The cellular precipitates at the grain boundaries, even at low volume fractions, were severely detrimental to the creep strength at 750?°C. The creep rupture life of Inconel740H containing cellular precipitates at grain boundaries was only one-tenth of that for the alloy without cellular precipitates. The reason for the drastically decreased creep rupture life is attributed to the poor resistance of cellular precipitates to crack propagation during creep.     

Scaling relationships in indentation of power-law creep solids using self-similar indenters

We use dimensional analysis to derive scaling relationships for self-similar indenters indenting solids that exhibit power-law creep. We identify the parameter that represents the indentation strain rate. The scaling relationships are applied to several types of indentation creep experiment with constant displacement rate, constant loading rate or constant ratio of loading rate over load. The predictions compare favourably with experimental observations reported in the literature. Finally, a connection is found between creep and 'indentation-size effect' (i.e. changing hardness with indentation depth or load).     

Stress reduction experiments during constant-strain-rate tests in Cu and Ge

The technique of rapid reductions in applied stress (dip test) is occasionally performed in creep testing. In the work reported here, the technique was applied during a constant-strain-rate test. Both variants, the stress dip test and the strain dip test, give equivalent results. The specimen strain has to be measured in both cases; however, the strain dip technique is preferred since anelastic effects do not interfere. It is shown that the response of Cu and Ge single crystals to fast stress reductions, on the level of the internal stress and below it, is strongly material dependent, reflecting differences in microstructural processes involved in the plastic deformation. These processes are discussed.     

Biochemical costs of a three day long cohabitation in dominant and submissive male Betta splendens

A 3 day long cohabitation in pairs of previously isolated adult male Betta splendens was studied. The first two mornings started with intense fighting, the frequency of which decreased during the day. The bursts of aggression noticed in the mornings were associated with a significant carbohydrate metabolism activation, while daytime activity resulted in a reduction of muscle proteins in the evenings. These changes were more pronounced in submissives than in dominants. The third day marked a shift in the behavior of the pairs: bursts of attack activity were not noticed, while the threatening display frequency increased. During this day a similar reduction in lipids was observed in dominants and submissives. While during the first 2 days the dominant position offered some metabolic advantage, this advantage disappeared during the third day when both members of the dyad seemed to do equally less well than their isolated counterparts. Combined with previous findings obtained in this series of experiments, the present results support the assumption that in this species the presence of one opponent is less well tolerated than the presence of four conspecifics. © 1994 Wiley-Liss, Inc.     

Spectral cues provided by the pinna for monaural localization in the horizontal plane

Six subjects located, monaurally, 1.0-kHz-wide noise bursts whose source originated on the side of the functioning ear and whose center frequency ranged from 4.0 through 9.0 kHz (Part 1). Irrespective of their actual locations, the stimuli appeared to migrate from the frontal sector of the arc toward the side as the center frequency was increased above 4.0 kHz. For some subjects, the sounds appeared again in front at the higher center frequencies. Comparable data were obtained with noise bursts 2.0 kHz in width. We referred to these constellations of location judgments, influenced by the frequency composition of the stimuli, as spatial referent maps. In Part 2, we measured, by means of a miniature microphone placed at the entrance of the external ear canal, the pinna amplification function for these same stimuli emanating from the same locations. The results showed a positive relation between the apparent location of noise bursts centered at 6.0 kHz and above and the relative amplification provided by the pinna. Localization performances by two subjects, chosen on the basis of their noncorresponding spatial referent maps, were examined for stimuli of wider bandwidths IPart 3). Their proficiency differed markedly from one another, which we accounted for in terms of different spatial referent maps that were associated with differences in the pinna amplification function.     

A comparison of thresholds for 1/3-octave filtered clicks and noise bursts in infants and adults

Behavioral thresholds of 6-month-old infants and adults were determined for 1/3-octave filtered clicks and 300-msec noise bursts with center frequencies ranging from .5 to 8 kHz. For noise bursts, differences between infant and adult thresholds were largest at low frequencies and smallest at 8 kHz. For clicks, infants’ thresholds were most like adults’ at 4 kHz, and age differences increased at both lower and higher frequencies. Differences between click and noise thresholds were significantly larger for infants than for adults at .5, 1, and 8 kHz, but not at 2 and 4 kHz. These results suggest that improvements in threshold for long-duration stimuli during infancy may not be accompanied by comparable changes in threshold at short durations. The delayed development of sensitivity to low- and high-frequency clicks appears consistent with maturational trends recently described for the auditory brainstem response.     

Acceleration and suppression of rats' responding to avoid foot shock and tail shock

Signalled response-independent shocks were superimposed on rats' wheel-turn responding to avoid shock administered to their feet through a grid floor or to their tails through fixed electrodes. In Experiment I, a tone paired with response-independent foot shock increased responding in three of four rats; a tone paired with tail shock increased responding in only one of four rats and suppressed responding in two rats. In Experiment II, a tone presented randomly with respect to response-independent shock had no reliable effect on responding to avoid foot shock or tail shock. In Experiment III, tail shock and foot shock were compared in a within-subject design while the temporal pattern of responding during conditioned stimuli was recorded. Responding during the conditioned stimulus preceding foot shock was characterized by initial suppression of responding at tone onset, followed by increased responding just before response-independent shock. Responding was suppressed throughout the conditioned stimulus preceding tail shock. Foot shock elicited bursts of responding, but tail shock did not.     

Children's aggressive behaviour and teacher–child conflict in kindergarten: Is teacher perceived control over child behaviour a mediating variable/

Background Research repeatedly showed young children's aggressive behaviour to predict relationship difficulties with the teacher. Aims To examine a possible mediating variable in this process and in the stability of relationship difficulties across the school year, namely teacher perceived control over child behaviour. Sample The sample consisted of 139 Belgian kindergartners and their teachers. Method Data were collected throughout kindergarten at three measurement occasions: children's aggressive behaviour was measured by means of a peer nomination procedure during the first trimester, teacher perceived control over child behaviour was assessed by means of a teacher questionnaire during the second trimester, and teacher–child conflict was measured by means of a teacher questionnaire during the first and third trimesters. Correlations among all study variables were calculated and different models were estimated and compared by means of structural equation modelling. Results Teacher perceived control completely mediated the relationship between aggressive behaviour and teacher–child conflict (after controlling for the concurrent association between aggressive behaviour and conflict, and the stability of conflict). In addition, teacher perceived control accounted for part of the stability in conflict across the school year. Conclusions Teacher perceived control over child behaviour has been found to act as a mediating mechanism between child aggressive behaviour and teacher–child conflict at the beginning of kindergarten and teacher–child conflict at the end of the year.     

Early extinction effects following intermittent reinforcement: Little evidence of extinction bursts

The occurrence of extinction bursts—transient increases in response rate in excess of those observed in baseline during the period immediately following discontinuation of reinforcement of a response—was examined. In Experiment 1, key pecking of pigeons was reinforced according to a multiple schedule in which a variable-ratio schedule alternated with an interval schedule in which the reinforcers were yoked to the preceding variable-ratio component. In Experiments 2 and 3, rats were screened such that the lever-press response rates of different rats maintained by variable-interval schedules were either relatively high or relatively low. Following these baseline conditions, in Experiments 1 and 2 responding was extinguished by eliminating the food reinforcer and in Experiment 3 by removing the response–reinforcer dependency. Responses immediately following extinction implementation were examined. Response increases relative to baseline during the first 20 min of a 324.75-min extinction session (Experiment 1) or during the first 30-min extinction session (Experiments 2 and 3) were rare and unsystematic. The results (a) reinforce earlier meta-analyses concluding that extinction bursts may be a less ubiquitous early effect of extinction than has been suggested and (b) invite further experimentation to establish their generality as a function of preceding reinforcement conditions.     

Interactions between interfacial dislocations and γ/γ′ interface during high temperature – low stress creep in nickel-based single crystal superalloy

ABSTRACT

The interaction between interfacial dislocations and γ/γ′ interface is critical to the high temperature creep properties of single crystal superalloys. However, only a few studies have paid attention to the detailed structure such as local interfacial morphologies and the elemental distribution around interfacial dislocations. In this paper, the interfacial protrusions and related dislocations in a single crystal superalloy after creep at high temperature – low stress have been investigated in detail. It is found that the morphology and size of the interfacial protrusions remain almost the same during the early and middle stages of high temperature creep, which indicates a local equilibrium at the interfacial protrusions. Steps at different height are formed at the γ/γ′ interface at the initial stage of high temperature creep since dislocations could move along the γ/γ′ interface, which indicates that dislocation motion at different creep stage may affect the morphology of γ/γ′ interface.     

BEHAVIORAL PERSISTENCE AND VARIABILITY DURING EXTINCTION OF SELF-INJURY MAINTAINED BY ESCAPE

The self-injurious escape behavior of a developmentally disabled adult was treated with extinction. Results of a reversal design showed substantial bursts of responding when extinction was introduced and reintroduced: self-injury remained at a variable and elevated rate for some time before stable, low rates were observed. Data on aggression, a nontarget behavior during both baseline and treatment, showed a pattern similar to that seen for self-injury during the extinction conditions.     

Descriptive characteristics of extinction bursts: A record review

Procedural extinction is sometimes associated with a temporary increase in responding known as an extinction burst. Extinction bursts present unique challenges in the context of treating behavior targeted for reduction. The present study updates the prevalence of extinction bursts using a clinical sample (N = 108) receiving treatment for targeted behavior. The prevalence of extinction bursts in our sample (24%) was consistent with that in prior literature. The extinction-burst magnitude decreased across sessions after extinction was contacted during treatment, but this sample did not demonstrate decreased persistence or magnitude of extinction bursts across successive transitions from baseline to treatment. We also examined the prevalence and magnitude of extinction bursts based on the function and topography of targeted behavior and treatment components and found no consistent relation among these variables. These findings should lead clinicians to prepare for transient extinction bursts when implementing extinction-based treatment for challenging behavior.     

Voids formed from solidifying tin particles in solid aluminium

In this study, voids commonly associated with tin particles in two aluminium alloys containing microalloying additions (0.01 at.%) of tin have been observed by transmission electron microscopy. The voids were generated by quenching the alloys at moderate rates (10²–10³ K s^?1) from a temperature (718 K) in excess of the melting temperature (501 K) of elemental tin in tin–microalloyed aluminium. Estimates of the void volume as a function of the volume of the associated tin particle reveal a linear relationship consistent with the excess volume resulting from the solidification of the tin particle. The formation and stabilisation of shrinkage voids in metallic alloys are suggested to arise from a combination of high vacancy supersaturation, the large volumetric misfit strain of the solidifying tin particle and a reduction in void surface energy associated with segregation of alloying elements.