Eloi Pineda

Atomic-Scale Relaxation Dynamics and Aging in a Metallic Glass Probed by X-Ray Photon Correlation Spectroscopy

We use x-ray photon correlation spectroscopy to investigate the structural relaxation process in a metallic glass on the atomic length scale. We report evidence for a dynamical crossover between the supercooled liquid phase and the metastable glassy state, suggesting different origins of the relaxation process across the transition. Furthermore, using different cooling rates, we observe a complex hierarchy of dynamic processes characterized by distinct aging regimes. Strong analogies with the aging dynamics of soft glassy materials, such as gels and concentrated colloidal suspensions, point at stress relaxation as a universal mechanism driving the relaxation dynamics of out-of-equilibrium systems.

On the validity of Avrami formalism in primary crystallization

Calorimetric data of primary crystallization is usually interpreted in the framework of the Kolmogorov Dokl. Akad. Nauk SSSR 1, 355 1937 , Johnson and Mehl Trans. AIME 135, 416 1939 , and Avrami J. Chem. Phys. 7, 1103 1939 ; 8, 212 1940 ; 9, 177 1941 KJMA theory. However, while the KJMA theory assumes random nucleation and exhaustion of space by direct impingement, primary crystallization is usually driven by diffusion-controlled growth with soft impingement between the growing crystallites. This results in a stop of the growth before the space is fully crystallized and induces nonrandom nucleation. In this work, phase-field simulations are used to check the validity of different kinetic models for describing primary crystallization kinetics. The results show that KJMA theory provides a good approximation to the soft-impingement and nonrandom nucleation effects. Moreover, these effects are not responsible of the slowing down of the kinetics found experimentally in the primary crystallization of glasses.Calorimetric data of primary crystallization is usually interpreted in the framework of the Kolmogorov [Dokl. Akad. Nauk SSSR 1, 355 (1937)], Johnson and Mehl [Trans. AIME 135, 416 (1939)], and Avrami [J. Chem. Phys. 7, 1103 (1939); 8, 212 (1940); 9, 177 (1941)] (KJMA) theory. However, while the KJMA theory assumes random nucleation and exhaustion of space by direct impingement, primary crystallization is usually driven by diffusion-controlled growth with soft impingement between the growing crystallites. This results in a stop of the growth before the space is fully crystallized and induces nonrandom nucleation. In this work, phase-field simulations are used to check the validity of different kinetic models for describing primary crystallization kinetics. The results show that KJMA theory provides a good approximation to the soft-impingement and nonrandom nucleation effects. Moreover, these effects are not responsible of the slowing down of the kinetics found experimentally in the primary crystallization...

X-Ray Photon Correlation Spectroscopy Reveals Intermittent Aging Dynamics in a Metallic Glass.

We use coherent x rays to probe the aging dynamics of a metallic glass directly on the atomic level. Contrary to the common assumption of a steady slowing down of the dynamics usually observed in macroscopic studies, we show that the structural relaxation processes underlying aging in this metallic glass are intermittent and highly heterogeneous at the atomic scale. Moreover, physical aging is triggered by cooperative atomic rearrangements, driven by the relaxation of internal stresses. The rich diversity of this behavior reflects a complex energy landscape, giving rise to a unique type of glassy-state dynamics.

Non-random nucleation and the Avrami kinetics

Abstract The theory available for obtaining the time evolution of the transformation for a nucleation and growth process, the well-known Kolmogorov-Johnson-Mehl and Avrami kinetic equation, is not accomplished for non-random nucleation processes. However, non-random nucleation protocols are very common; for example in first-order phase transitions where position-dependent nucleation protocols arise from the modification of the nucleation rates in the regions around the growing grains. The present paper attempts to study the effect of these position-dependent nucleation protocols in the overall kinetics of the transformation. Monte Carlo simulations of such processes are performed, and the deviations of the resulting kinetics of the transformation from the Avrami equation are analysed in detail. Consideration of the different effects resulting from the nucleation protocol allows us to model the transformation. A modified Avrami equation is obtained and compared with Monte Carlo simulations, giving excellent agreement. The modelling procedure enables us to extend the validity of the Avrami formulation beyond its limits of applicability.

Relaxation dynamics and aging in structural glasses

We present a study of the atomic dynamics in a Mg65Cu25Y10 metallic glass former both in the deep glassy state and in the supercooled liquid phase. Our results show that the glass transition is accompanied by a dynamical crossover between a faster than exponential shape of the intermediate scattering function in the glassy state and a slower than exponential shape in the supercooled liquid. While the crossover temperature is independent on the previous thermal history, both the relaxation rate and the shape of the relaxation process depend on the followed thermal path. Moreover, the temperature dependence of the the structural relaxation time displays a strong departure from the Arrhenius-like behavior of the corresponding supercooled liquid phase, and can be well described in the Narayanaswamy-Moynihan framework with a large non-linearity parameter.

Relaxation processes and physical aging in metallic glasses

Since their discovery in the 1960s, metallic glasses have continuously attracted much interest across the physics and materials science communities. In the forefront are their unique properties, which hold the alluring promise of broad application in fields as diverse as medicine, environmental science and engineering. However, a major obstacle to their wide-spread commercial use is their inherent temporal instability arising from underlying relaxation processes that can dramatically alter their physical properties. The result is a physical aging process which can bring about degradation of mechanical properties, namely through embrittlement and catastrophic mechanical failure. Understanding and controlling the effects of aging will play a decisive role in our on-going endeavor to advance the use of metallic glasses as structural materials, as well as in the more general comprehension of out-of-equilibrium dynamics in complex systems. This review presents an overview of the current state of the art in the experimental advances probing physical aging and relaxation processes in metallic glasses. Similarities and differences between other hard and soft matter glasses are highlighted. The topic is discussed in a multiscale approach, first presenting the key features obtained in macroscopic studies, then connecting them to recent novel microscopic investigations. Particular emphasis is put on the occurrence of distinct relaxation processes beyond the main structural process in viscous metallic melts and their fate upon entering the glassy state, trying to disentangle results and formalisms employed by the different groups of the glass-science community. A microscopic viewpoint is presented, in which physical aging manifests itself in irreversible atomic-scale processes such as avalanches and intermittent dynamics, ascribed to the existence of a plethora of metastable glassy states across a complex energy landscape. Future experimental challenges and the comparison with recent theoretical and numerical simulations are discussed as well.

Temporal evolution of the domain structure in a Poisson–Voronoi transformation

The temporal evolution of the domain size and free boundary distributions is calculated for a Poisson?Voronoi transformation. In this kind of transformation a set of randomly distributed domain seeds start growing simultaneously, all with equal isotropic growth rate, occupying the original untransformed space. At the end of the transformation, all the space is occupied and the final configuration is the well-known Poisson?Voronoi tessellation. In this work, the temporal evolution of the domain structure in a two-dimensional transformation is obtained by means of a calculation method presented recently?(Pineda et al 2007?Phys.?Rev.?E?75?040107(R)). The method is based on the differentiation of the domains through their number of extended collisions. It is found that the probability distribution of geometrical configurations for domains with a certain number of extended collisions is time invariant throughout the transformation. The calculation of these time-invariant probability distributions allows us to obtain the probability density function of any geometric characteristic of the domains at any finite time during the transformation. In this work this is applied to obtain the size and the free boundary fraction distributions. As far as we are aware, this is the first time that an analytical solution has been obtained for this system.

Microstructural implications of non-random nucleation protocols in nanocrystallized metallic glasses

Macroscopic properties of nanocrystallized metallic glasses are dependent of their nanostructure. The knowledge of the crystallization kinetics and its effect on the nanostructure are then essential in designing production and annealing protocols. Deviations from the Avrami kinetics in many of such systems are often interpreted in terms of either non-random nucleation or soft impingement due to overlapping concentration gradients. In this work, simple simulations of non-random nucleation processes allow us to evaluate the main features of both kinetic parameters and the nanostructure. It is shown that although non-random nucleation highly affects the nanostructure, it has a small effect on the transformed fraction evolution. Consequently, the decreasing Avrami exponents often reported in primary crystallization of metallic glasses have to be associated to a time dependent growth rate. Moreover, as similar kinetic behaviors are observed for different nucleation and growth protocols the understanding of the kinetics-nanostructure relationship becomes fundamental in studying such systems.

Kinetic simulation of primary transformations in glassy alloys

Abstract The kinetic processes of primary transformations have recently been modelled using the Avrami formalism and a mean field hypothesis [D. Crespo, T. Pradell, M.T. Clavaguera-Mora, N. Clavaguera, Phys. Rev. B 55 (1997) 3435]. However, the importance of several of the factors influencing the transformation has not been studied. In particular, the lack of randomness in the nucleation rate induced by the change in the untransformed matrix concentration has never been evaluated. This factor is of interest because non-random nucleation has been described in the literature as one of the main factors responsible for deviations from the Avrami kinetics. In this work, a kinetic Monte Carlo simulation of a primary transformation is presented. The simulation attempts to account for both the reduction in the nucleation and growth rates and the non-randomness in the nucleation protocol, due to the changes in the untransformed matrix along the transformation. The effects on the predicted time-dependent transformed fraction as well as on the microstructure are shown.

Rapid degradation of azo-dye using Mn–Al powders produced by ball-milling

This study was conducted on the reduction reaction of the azo dye Reactive Black 5 by means of the Mn85Al15 particles prepared by melt-spinning and ball-milling processes. The morphology, the surface elementary composition and the phase structure of the powders were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The degradation efficiency of the ball milled powder was measured by using an ultraviolet-visible absorption spectrophotometer and the collected powder was analyzed by means of Fourier transform infrared spectroscopy technique to characterize the functional groups in the extract. The degradation of Reactive Black 5 and the analysis of the aromatic by-products were investigated by high performance liquid chromatography coupled with tandem mass spectrometry. The ball-milled powder shows higher degradation efficiency and the Reactive Black 5 solution was completely decolorized after 30 min. The degradation kinetics and the formation by-products depend on the pH and temperature of the solution. The analyses of the extracted product confirmed the cleavage of the (–NN–) bonds. Our findings are expected to pave the way for a new opportunity with regard to the functional applications of nanostructured metallic particles.