Hydrogen and vacancy clustering in zirconium

The effect of solute hydrogen on the stability of vacancy clusters in hexagonal closed packed zirconium is investigated with an ab initio approach, including contributions of H vibrations. Atomistic simulations within the density functional theory evidence a strong binding of H to small vacancy clusters. The hydrogen effect on large vacancy loops is modeled through its interaction with the stacking faults. A thermodynamic modeling of H segregation on the various faults, relying on ab initio binding energies, shows that these faults are enriched in H, leading to a decrease of the stacking fault energies. This is consistent with the trapping of H by vacancy loops observed experimentally. The stronger trapping, and thus the stronger stabilization, is obtained for vacancy loops lying in the basal planes, i.e. the loops responsible for the breakaway growth observed under high irradiation dose.Comment: submitte

Vacancy clustering in zirconium: an atomic scale study

International audienceThe stability properties of vacancy clusters in hexagonal close-packed Zr, cavities and dislocation loops, are investigated at the atomic scale, with a modeling approach based on density functional theory and empirical potentials. Considering the vacancy-vacancy interactions and the stability of small vacancy clusters, we establish how to build the larger clusters. The study of extended vacancy clusters is then performed using continuous laws for defect energetics. Once validated with an empirical potential, these laws are parameterized with ab initio data. Our work shows that the easy formation of loops can be explained by their thermodynamic properties

How does cognitive load influence speech perception? : An encoding hypothesis

Two experiments investigated the conditions under which cognitive load exerts an effect on speech perception. These experiments extend earlier research by using a different speech perception task (four-interval oddity task) and by implementing cognitive load through a task often thought to be modular, namely, face processing. In the cognitive-load conditions, participants were required to remember two faces presented before the speech stimuli. In Experiment 1, performance in the speech-perception task under cognitive load was not impaired in comparison to a no-load baseline condition. In Experiment 2, we modified the load condition minimally such that it required encoding of the two faces simultaneously with the speech stimuli. As a reference condition, we also used a visual search task that in earlier experiments had led to poorer speech perception. Both concurrent tasks led to decrements in the speech task. The results suggest that speech perception is affected even by loads thought to be processed modularly, and that, critically, encoding in working memory might be the locus of interference

Audio-visual speech perception in infants and toddlers with Down syndrome, fragile X syndrome, and Williams syndrome

Typically-developing (TD) infants can construct unified cross-modal percepts, such as a speaking face, by integrating auditory-visual (AV) information. This skill is a key building block upon which higher-level skills, such as word learning, are built. Because word learning is seriously delayed in most children with neurodevelopmental disorders, we assessed the hypothesis that this delay partly results from a deficit in integrating AV speech cues. AV speech integration has rarely been investigated in neurodevelopmental disorders, and never previously in infants. We probed for the McGurk effect, which occurs when the auditory component of one sound (/ba/) is paired with the visual component of another sound (/ga/), leading to the perception of an illusory third sound (/da/ or /tha/). We measured AV integration in 95 infants/toddlers with Down, fragile X, or Williams syndrome, whom we matched on Chronological and Mental Age to 25 TD infants. We also assessed a more basic AV perceptual ability: sensitivity to matching vs. mismatching AV speech stimuli. Infants with Williams syndrome failed to demonstrate a McGurk effect, indicating poor AV speech integration. Moreover, while the TD children discriminated between matching and mismatching AV stimuli, none of the other groups did, hinting at a basic deficit or delay in AV speech processing, which is likely to constrain subsequent language development

An order effect in English infants’ discrimination of an Urdu affricate contrast

An order effect was found in English infants’ discrimination of an Urdu contrast. In Experiment 1 7- and 11-month-old English infants were tested on the Urdu contrast between the affricates /tʃʰ/ and /tʃ/. The order of presentation was counterbalanced: At each age half the infants were habituated to the aspirated and tested on the unaspirated affricate, the other half habituated to the unaspirated and tested on the aspirated. As expected, younger infants discriminated the contrast whereas older infants did not, showing the expected decline in discrimination. Interestingly, order of presentation seemed to affect the older infants’ response. Experiment 2 tested the order effect directly. The results showed no asymmetry in the performance of 7-month olds but clear asymmetry in that of 11-month-olds, who discriminated the contrast only when the non-English-like aspirated affricate was presented first. Experiment 3 tested adult native-speakers of both Urdu and English. Although the English listeners showed a reduced sensitivity to the contrast, there was no effect due to order of presentation of the stimuli in either adult group. The finding of an asymmetry in the infants suggests that infants’ perceptual narrowing for speech sounds may be a more complex phenomenon than has generally been assumed

Atomic scale modeling of twinning in zirconium and titanium

L'objectif de la thèse est d'identifier et de quantifier les paramètres régissant l'épaississement des macles dans trois métaux hexagonaux (Zr, Ti, Mg). Le mécanisme que nous étudions est le glissement de dislocations d'interfaces, i.e. les disconnections, le long des joints de macles parfaits. Nous nous intéressons alors à la germination des disconnections avant de nous concentrer sur leur migration.Une première étude en potentiel empirique permet de valider un couplage original avec la théorieélastique afin d'extraire des simulations atomiques l'énergie de cœur des disconnections. Cette méthode permet de partitionner l'énergie entre une contribution de cœur, intrinsèque à la disconnection et une partie élastique qui dépend de l'environnement de la disconnection. Cette partition faite, nous modélisons alors l'énergie de formation de dipôles de disconnections isolés, pour chacun des différents joints de macles. Cette modélisation nous permet alors de sélectionner les dipôles de disconnections les plus pertinents que nous étudions par la méthode abinitio.Nous modélisons ensuite la migration des disconnections le long des joints de macles parfaits.Pour cela, nous montrons via l'utilisation de la méthode Nudged Elastic Band, que l'énergie demigration est d'un ordre de grandeur inférieur à l'énergie de formation. Ainsi, l'énergie de formation des dipôles de disconnections apparait comme prédominant dans la croissance des maclesThe aim of this thesis is to identify and quantify the parameters of importance when dealing with twin thickening in three hcp metals (Zr, Ti, Mg). The mechanism we study is the glide of twinning dislocations, i. e. the disconnection, along the perfect twin boundaries. We first focus on the nucleation of disconnection before addressing their migration. A study using an EAM potential allows us to validate an original coupling between our atomistic simulations and linear elasticity in order to extract the core energy of disconnection.We then show how this coupling permits to divide the formation energy in two terms: the core contribution, intrinsec to the disconnections, and an elastic one, which depends on the disconnection's environment. Thanks to this partition, we model the formation of isolated disconnection that may appear along the different twin planes. We select the dipoles of lowest formation energies in order to perform ab initio calculations and compare the behavior observed the three different metals. We then model the migration of disconnections along the perfect twin planes. To do so, we use the Nudged Elastic Band method, and find out that the migration energy of disconnections is one order of magnitude lower than their formation energies. We therefor conclude that the disconnection nucleation is the rate limiting factor to explain twin thickening thanks to the creation and motion of disconnectio

Modélisation du maclage à l’échelle atomique dans les métaux hexagonaux : germination et migration de disconnections dans le zirconium, le titane et le magnésium

The aim of this thesis is to identify and quantify the parameters of importance when dealing with twin thickening in three hcp metals (Zr, Ti, Mg). The mechanism we study is the glide of twinning dislocations, i. e. the disconnection, along the perfect twin boundaries. We first focus on the nucleation of disconnection before addressing their migration. A study using an EAM potential allows us to validate an original coupling between our atomistic simulations and linear elasticity in order to extract the core energy of disconnection.We then show how this coupling permits to divide the formation energy in two terms: the core contribution, intrinsec to the disconnections, and an elastic one, which depends on the disconnection's environment. Thanks to this partition, we model the formation of isolated disconnection that may appear along the different twin planes. We select the dipoles of lowest formation energies in order to perform ab initio calculations and compare the behavior observed the three different metals. We then model the migration of disconnections along the perfect twin planes. To do so, we use the Nudged Elastic Band method, and find out that the migration energy of disconnections is one order of magnitude lower than their formation energies. We therefor conclude that the disconnection nucleation is the rate limiting factor to explain twin thickening thanks to the creation and motion of disconnectionL'objectif de la thèse est d'identifier et de quantifier les paramètres régissant l'épaississement des macles dans trois métaux hexagonaux (Zr, Ti, Mg). Le mécanisme que nous étudions est le glissement de dislocations d'interfaces, i.e. les disconnections, le long des joints de macles parfaits. Nous nous intéressons alors à la germination des disconnections avant de nous concentrer sur leur migration.Une première étude en potentiel empirique permet de valider un couplage original avec la théorieélastique afin d'extraire des simulations atomiques l'énergie de cœur des disconnections. Cette méthode permet de partitionner l'énergie entre une contribution de cœur, intrinsèque à la disconnection et une partie élastique qui dépend de l'environnement de la disconnection. Cette partition faite, nous modélisons alors l'énergie de formation de dipôles de disconnections isolés, pour chacun des différents joints de macles. Cette modélisation nous permet alors de sélectionner les dipôles de disconnections les plus pertinents que nous étudions par la méthode abinitio.Nous modélisons ensuite la migration des disconnections le long des joints de macles parfaits.Pour cela, nous montrons via l'utilisation de la méthode Nudged Elastic Band, que l'énergie demigration est d'un ordre de grandeur inférieur à l'énergie de formation. Ainsi, l'énergie de formation des dipôles de disconnections apparait comme prédominant dans la croissance des macle

Atomic-scale modeling of twinning disconnections in zirconium

International audienceTwin growth in hexagonal close-packed zirconium is investigated at the atomic scale by modeling the various disconnections that can exist on twin boundaries. Thanks to a coupling with elasticity theory, core energies are extracted from atomistic simulations and the formation energy of isolated disconnection dipoles is defined. For twin systems where several disconnections can exist, because of this core contribution, the most stable disconnection is not always the one with the smallest Burgers vector. Crystallographic parameters of the disconnection with the lowest formation energy correlate well with twin modes observed experimentally. On the other hand, disconnection migration, characterized here by computing their migration energy and Peierls stress, does not appear critical for twin mode selection