Modeling of the Sub-Tg Relaxation Spectrum of Pd42.5Ni7.5Cu30P20 Metallic Glass

In this work we study the mechanical relaxation spectrum of Pd42.5Ni7.5Cu30P20 metallic glass. The effect of aging on the relaxation behavior is analyzed by measuring the internal friction during consecutive heating runs. The mechanical relaxation of the wellannealed glass state is modeled by fitting susceptibility functions to the primary and secondary relaxations of the system. The model is able to reproduce the mechanical relaxation spectrum below the glass transition temperature (sub-Tg) in the frequency- temperature ranges relevant for the high temperature physical properties and forming ability of metallic glasses. The model reveals a relaxation spectrum composed by the overlapping of primary and secondary processes covering a wide domain of times but with a relatively narrow range of activation energies.Postprint (author's final draft

Link between shear modulus and enthalpy changes of Ti16.7Zr16.7Hf16.7Cu16.7Ni16.7Be16.7 high entropy bulk metallic glass

Mechanical and thermal properties of materials are intricately linked. Particularly, this fully applies to metallic glasses. In this work, we study shear modulus behavior and heat effects occurring upon heating up of Ti16.7Zr16.7Hf16.7Cu16.7Ni16.7Be16.7 high entropy bulk metallic glass up to the full crystallization. In the framework of the Interstitialcy theory, we show that shear modulus relaxation data can be applied to quantitatively predict exo- and endothermal effects related to structural relaxation, glass transition and crystallization of this high entropy metallic glass. This fact suggests that the underlying physical mechanism responsible for this link can be conditioned by the relaxation of the system of structural defects, which by their properties are similar to dumbbell interstitials in metals.Peer ReviewedPostprint (author's final draft

Delayed elasticity of metallic glasses: Loading time and temperature dependences of the anelastic relaxation

One of the hallmarks of disordered matter is the large amplitude of the anelastic deformation, i.e., the fraction of reversible deformation that is not instantaneously recovered after the release of load but is delayed in time. In this paper, this delayed elasticity is studied for the glass-forming Zr46.25Ti8.25Cu7.5Ni10Be27.5 alloy by means of stress step and recovery experiments. Even at high temperatures, not far from the glass transition, the delayed elasticity can recover an important fraction of the deformation and endure for a long time. Analyzing the effects of loading time and waiting time on the strain evolution, we reveal the presence of an anelastic response with a timescale dependent on loading time and an invariant shape, which indicates the presence of a distribution of reversible relaxation modes following a t-n law with exponent n between 0.5 and 1. The underlying distribution of energy barriers activated at different temperatures is accordingly shape invariant. Moreover, we found that a distribution of reversible modes corresponding to the high-frequency side of the a-relaxation peak can reproduce the experimental results. The results establish a direct link between the dynamical spectrum and the distribution of activation energies, revealing the origin of the transient creep and anelastic recovery behaviors of metallic glasses.Peer ReviewedPostprint (published version

Mechanical properties of bulk metallic glasses : Influence of microstructure

Ce travail a porté sur l’étude des propriétés thermiques et mécaniques des verres métalliques massifs. Dans le premier chapitre nous avons rappelé l’historique des verres métalliques massifs, leurs propriétés intéressantes et quelques applications de ces matériaux et nous les avons situés par rapport aux autres matériaux amorphes, tels que les polymères ou les verres d’oxydes. Nous avons analysé par différentes méthodes expérimentales (calorimétrie, analyse mécanique dynamique, diffraction des rayons X, microscopies électroniques) les caractéristiques de cette évolution et leur incidence sur les propriétés mécaniques. Différents alliages base zirconium, cuivre, titane ou lanthane ont été étudiés. Nous avons notamment montré que : ● Un chauffage à une température inférieure à la température de transition vitreuse (Tg) conduit à une relaxation structurale, dont la cinétique, étudiée par calorimétrie, peut être modélisée par une fonction de type exponentielle étendue. Les paramètres caractéristiques ont été déterminés pour les différents alliages étudiés. Cette relaxation conduit à une augmentation du module élastique de stockage, mais à une diminution de la composante viscoélastique de ce module, autrement dit à une diminution de la mobilité atomique. Une déformation plastique conduit à un effet inverse. Ces évolutions ont été interprétées à l’aide d’un modèle reposant sur l’existence de défauts, dont la concentration diminue lors de la relaxation structurale, mais augmente lors de la déformation plastique. ● Lorsqu’une contrainte mécanique périodique de faible amplitude est appliquée, on observe des relaxations mécaniques. Quelle que soit la composition de l’alliage, une relaxation importante est toujours observée au voisinage de la transition vitreuse, comme dans tous les autres matériaux amorphes. En plus, dans certains verres métalliques massifs, (exemple les verres base Lanthane), une relaxation secondaire est détectée à basse température. Cette relaxation, de faible énergie d’activation, est attribuée à des mouvements locaux qui se produisent dans les zones faibles du matériau, zones résultant de l’existence d’hétérogénéités à une échelle nanoscopique. ● Lorsque qu’une contrainte de forte amplitude est appliquée (cas des essais de compression), on observe un comportement caractéristique de tous les matériaux amorphes : comportement essentiellement fragile à basse température et écoulement viscoplastique à haute température. Une courbe maitresse a pu être tracée pour la viscosité. La transition d’un régime newtonien à un régime non-newtonien apparait lorsque la vitesse de déformation augmente. Tous les résultats expérimentaux ont été discutés dans le cadre d’un modèle physique, basé sur l’existence de défauts activés par une augmentation de température ou par une contrainte.In the current work, we investigated the thermal and mechanical properties of bulk metallic glasses. The history of the bulk metallic glasses was described in the Chapter I. The clear interesting properties and applications of the bulk metallic glasses, compared with other amorphous materials, i.e. polymers and glassy oxides, were discussed. Different experimental methods [DSC, DMA, X-ray diffraction, electron microscopy] were used to investigate the features of evolution of the microstructure on mechanical properties for bulk metallic glasses. The different bulk metallic glasses, i.e. Zr-, Cu-, Ti- and La-based, have been studied in the current research. In particular, the main results as follows: ● A heat treatment performed below the glass transition temperature Tg induces the structural relaxation. The kinetics of the enthalpy relaxation was studied by differential scanning calorimetry and can be well fitted by a stretched exponential relaxation function. The characteristic parameters can be determined in different bulk metallic glasses. The structural relaxation leads to an increase of the storage modulus, on the contrary to a decrease of the visco-elastic component of the modulus. Namely, the structural relaxation induces a diminution of the atomic mobility. The plastic deformation leads to an inverse influence. Results are interpreted using a physical model, based on the existence of defects in the material, called quasi-point defects. Atomic mobility is reduced by structural relaxation due to decrease of the concentration of defects. In contrast, plastic deformation increases the concentration of defects and therefore enhances the atomic mobility. ● When a periodic mechanical stress with a low amplitude is applied, one can observe mechanical relaxation. The main (α) relaxation has been clearly observed near to the glass transition temperature in all the investigated bulk metallic glasses as well as other amorphous materials. In addition, in some cases of bulk metallic glasses (for example, Lanthanum-based metallic glass), a distant secondary relaxation has been detected at lower temperature. This relaxation presents lower activation energy, which is associated to dynamic heterogeneities and is related to local movements of “defect” on the nature of nanoscale order in glasses. ● When a large-amplitude stress is employed (case of the compression tests), one can acquire the characteristic behaviour of the amorphous materials: A typical brittle fracture phenomenon is observed at lower temperature and the flow stress can be detected at higher temperature. A master curve of the viscosity can be acquired based on the experimental results. The transition from a Newtonian to non-Newtonian regime appears when the strain rate increases. All the experimental results are discussed in the framework of physical models, based on existence of the defects, which can be activated by increasing temperature or stress

Propriétés mécaniques des verres métalliques massifs (Influence de la microstructure)

Ce travail a porté sur l étude des propriétés thermiques et mécaniques des verres métalliques massifs. Dans le premier chapitre nous avons rappelé l historique des verres métalliques massifs, leurs propriétés intéressantes et quelques applications de ces matériaux et nous les avons situés par rapport aux autres matériaux amorphes, tels que les polymères ou les verres d oxydes. Nous avons analysé par différentes méthodes expérimentales (calorimétrie, analyse mécanique dynamique, diffraction des rayons X, microscopies électroniques) les caractéristiques de cette évolution et leur incidence sur les propriétés mécaniques. Différents alliages base zirconium, cuivre, titane ou lanthane ont été étudiés. Nous avons notamment montré que : Un chauffage à une température inférieure à la température de transition vitreuse (Tg) conduit à une relaxation structurale, dont la cinétique, étudiée par calorimétrie, peut être modélisée par une fonction de type exponentielle étendue. Les paramètres caractéristiques ont été déterminés pour les différents alliages étudiés. Cette relaxation conduit à une augmentation du module élastique de stockage, mais à une diminution de la composante viscoélastique de ce module, autrement dit à une diminution de la mobilité atomique. Une déformation plastique conduit à un effet inverse. Ces évolutions ont été interprétées à l aide d un modèle reposant sur l existence de défauts, dont la concentration diminue lors de la relaxation structurale, mais augmente lors de la déformation plastique. Lorsqu une contrainte mécanique périodique de faible amplitude est appliquée, on observe des relaxations mécaniques. Quelle que soit la composition de l alliage, une relaxation importante est toujours observée au voisinage de la transition vitreuse, comme dans tous les autres matériaux amorphes. En plus, dans certains verres métalliques massifs, (exemple les verres base Lanthane), une relaxation secondaire est détectée à basse température. Cette relaxation, de faible énergie d activation, est attribuée à des mouvements locaux qui se produisent dans les zones faibles du matériau, zones résultant de l existence d hétérogénéités à une échelle nanoscopique. Lorsque qu une contrainte de forte amplitude est appliquée (cas des essais de compression), on observe un comportement caractéristique de tous les matériaux amorphes : comportement essentiellement fragile à basse température et écoulement viscoplastique à haute température. Une courbe maitresse a pu être tracée pour la viscosité. La transition d un régime newtonien à un régime non-newtonien apparait lorsque la vitesse de déformation augmente. Tous les résultats expérimentaux ont été discutés dans le cadre d un modèle physique, basé sur l existence de défauts activés par une augmentation de température ou par une contrainte.In the current work, we investigated the thermal and mechanical properties of bulk metallic glasses. The history of the bulk metallic glasses was described in the Chapter I. The clear interesting properties and applications of the bulk metallic glasses, compared with other amorphous materials, i.e. polymers and glassy oxides, were discussed. Different experimental methods [DSC, DMA, X-ray diffraction, electron microscopy] were used to investigate the features of evolution of the microstructure on mechanical properties for bulk metallic glasses. The different bulk metallic glasses, i.e. Zr-, Cu-, Ti- and La-based, have been studied in the current research. In particular, the main results as follows: A heat treatment performed below the glass transition temperature Tg induces the structural relaxation. The kinetics of the enthalpy relaxation was studied by differential scanning calorimetry and can be well fitted by a stretched exponential relaxation function. The characteristic parameters can be determined in different bulk metallic glasses. The structural relaxation leads to an increase of the storage modulus, on the contrary to a decrease of the visco-elastic component of the modulus. Namely, the structural relaxation induces a diminution of the atomic mobility. The plastic deformation leads to an inverse influence. Results are interpreted using a physical model, based on the existence of defects in the material, called quasi-point defects. Atomic mobility is reduced by structural relaxation due to decrease of the concentration of defects. In contrast, plastic deformation increases the concentration of defects and therefore enhances the atomic mobility. When a periodic mechanical stress with a low amplitude is applied, one can observe mechanical relaxation. The main (a) relaxation has been clearly observed near to the glass transition temperature in all the investigated bulk metallic glasses as well as other amorphous materials. In addition, in some cases of bulk metallic glasses (for example, Lanthanum-based metallic glass), a distant secondary relaxation has been detected at lower temperature. This relaxation presents lower activation energy, which is associated to dynamic heterogeneities and is related to local movements of defect on the nature of nanoscale order in glasses. When a large-amplitude stress is employed (case of the compression tests), one can acquire the characteristic behaviour of the amorphous materials: A typical brittle fracture phenomenon is observed at lower temperature and the flow stress can be detected at higher temperature. A master curve of the viscosity can be acquired based on the experimental results. The transition from a Newtonian to non-Newtonian regime appears when the strain rate increases. All the experimental results are discussed in the framework of physical models, based on existence of the defects, which can be activated by increasing temperature or stress.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Effect of the physical aging on the secondary

Dynamic mechanical relaxation processes, i.e., main (α) relaxation and secondary (β) relaxation, are important issues to understand mechanical deformation, atomic diffusion as well as glass transition phenomenon of metallic glasses. In current work, La68Ni15Al15Cu2 metallic glass was selected as a protocol glass system. Mechanical relaxation processes were probed by dynamic mechanical analysis. The effects of annealing at different temperatures were analyzed by Kohlrausch–Williams–Watts (KWW)-type equation. The Kohlrausch exponent βKWW reflects the deviation from a single Debye relaxation, indicating the fact that dynamics in metallic glass are actually heterogeneous originating from the structural heterogeneity. The effects of thermal treatments were also discussed, which provides a potential solution to tune the relaxation behaviors in metallic glasses

Aging and rejuvenation during high-temperature deformation in a metallic glass

High-temperature deformation has been demonstrated as an effective measure to rejuvenate and optimize the mechanical properties of metallic glasses (MGs). Clarifying the competition between aging and rejuvenation during high-temperature deformation is helpful in rejuvenating MGs accurately. Signatures of aging and rejuvenation in a La30Ce30Ni10Al20Co10 MG were investigated via high-temperature deformation and mechanical relaxation. The coupling of thermal history, aging, and mechanical disordering determines the transient deformation and the structural state of MGs. The stress overshoot and anelastic deformation induce structural rejuvenation, increasing the concentration of defects and erasing thermal history. Therefore, the eventually steady-state condition is dependent on ambient temperature and strain rate instead of the initial structure. Furthermore, the one-to-one relationship between defect concentration and strain rate clarifies the structural nature of rejuvenation in amorphous materials. Such a relationship also contributes toward a comprehensive understanding of the structural rejuvenation behavior in amorphous materials

The anelastic origin of mechanical cycling induced rejuvenation in the metallic glass

Mechanical cycling is one of the effective methods to rejuvenate metallic glasses (MGs) and improve their mechanical properties. The anelastic origin of the rejuvenation by mechanical cycling in a La30Ce30Ni10Al20Co10 MG was investigated via differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). We demonstrate that mechanical cycling promotes the activation of flow defects with short relaxation times, leading to anelastic strains and therefore considerable energy storage, which manifests itself as larger relaxation enthalpy on the DSC curves of MGs. However, the MGs release the excess relaxation enthalpy caused by anelastic strain with time, thus suppressing atomic mobility and elevating & beta; relaxation activation energies. The strategy of mechanical cycling at small strains, as demonstrated in the current work, can expand the energy states of MGs over a wide range of relaxation enthalpies