P. Bruna

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...

Structural and magnetic characterization of FeNbBCu alloys as a function of Nb content

In this work we describe the changes in the crystallization behaviour and the magnetic properties with variation of the Nb content in Fe79−xNb5+xB15Cu1 (x = 0,2,4) alloys. The microstructure of the samples, as-quenched and after several heat treatments, is analysed by transmission Mossbauer spectroscopy (TMS), x-ray diffraction, differential scanning calorimetry and vibrating sample magnetometry. The saturation polarization, Curie temperature, magnetic entropy change and the maximum value of the hyperfine field distribution of the amorphous phase are composition dependent and are enhanced with a reduction in Nb content. Devitrification is produced by the nanocrystallization of bcc-Fe followed by the precipitation of iron borides. The calorimetric analysis indicates that Nb stabilizes the alloy against nanocrystallization. Both the amount of bcc-Fe precipitates and their mean grain size decrease significantly with increasing Nb content, suggesting a higher level of disorder at the interface between the amorphous matrix and the nanocrystal that reduces grain growth. The variation in the Fe environments after crystallization events deduced from TMS measurements are analysed in terms of the amount of Nb in the alloy. Both amorphous and nanocrystallized alloys show a soft magnetic behaviour with coercivity values in the range 4–10 A m−1.

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.

Communication: Are metallic glasses different from other glasses? A closer look at their high frequency dynamics

Using high resolution inelastic x-ray scattering we studied the collective dynamics of the Pd(77)Si(16.5)Cu(6.5) metallic glass, focusing on the energy-momentum region where the boson peak appears. The dispersion relation and the width of the acoustic excitations are determined showing how the longitudinal acoustic modes maintain their dispersive character for frequencies well above the boson peak frequencies. Moreover, we prove that close to these frequencies there is a softening of the apparent sound speed indicating a failure of the Debye continuum approximation at the boson peak frequencies and challenging previous results on other metallic glasses.

Microstructural characterisation and kinetics modelling of vermicular cast irons

Abstract Several experimental techniques are used for phase identification and microstructure characterisation of austempered vermicular cast irons (XRD, SEM, TEM and Mössbauer spectroscopy). Acicular structures were found to be composed by ferrite and austenite with average sizes compatible with those reported for bainitic ferrite in steels and austempered ductile iron. An assessment of the free energy change involved in the austenite→bainite transformation assuming a plate-like nucleation shape for bainite gave an average characteristic length close to that observed from statistical length distributions. The kinetics of the transformation was modelled in the Avrami framework; both the diffusion controlled and the diffusionless growth hypotheses were considered to elucidate the mechanism underlying the austempering phase transformation. Results indicated that diffusion of C is responsible for the nucleation process of the bainite sheaves, which appear as a consequence of a localised displacive transformation when the C concentration is adequate, but further growth of the bainite plates is almost suppressed.

Size distribution evolution equations in space-competing domain growth systems

A model of microstructure development for phase transformations driven by nucleation and growth is presented. The model is based on a set of Fokker-Planck-like equations which allows us to compute the particle grain density distribution at any time during the transformation, provided that the nucleation and growth dependences on time and/or grain radius are known. The model is applicable to any kind of nucleation and growth protocols fulfilling the Johnson–Mehl–Avrami–Kolmogorov conditions, namely spatially random nucleation and isotropic growth. Comparison with stochastic (Monte Carlo) simulations is presented, giving quantitative agreement in all cases. This work shows the relationship between kinetic parameters and microstructure evolution as well as the accuracy of the developed model.

Mössbauer Study of the Ball Milling Disordering Process of FeAl Intermetallic Compounds

Structural changes during ball milling of ordered Fe50Al50 intermetallic compounds were studied. X‐Ray diffraction allowed the computation of a Long Range Order parameter (LRO) which dropped to zero after a short milling time. The initial B2 ordered structure gradually transforms into a disordered BCC structure, with a final crystallite size of about 25 nm. Mossbauer spectroscopy was used for obtaining a Chemical Short Range Order parameter (CSRO). Using a semiempirical n‐body noncentral potential a model of the partially disordered B2 structure was built allowing computing the distribution of Quadrupole Splitting during the disordering process. Comparison between experimental and simulated Mossbauer spectra shows a maximum of disorder in the system for 5h milling, related to the highest value of the lattice spacing and the broader quadrupole hyperfine distribution. However, after milling for times longer than 5h, there is a change on the behavior of the experimental data that cannot be explained by the s...

A Mössbauer Study of Nanocrystallisation Under Continuous Heating Conditions

The kinetics of nanocrystallisation of a Finemet metallic glass of composition Fe73.5Si17.5B5Cu1Nb3 under isothermal and continuous heating conditions has already been widely studied. However, the kinetics of the nanocrystalline precipitation in Finemet alloys under a constant heating rate has never been studied by Mossbauer spectroscopy. Mossbauer analysis has the capability of giving information on either the short range order of the amorphous phase and of the nanocrystalline precipitates. The present study examines the differences and similarities in the kinetics of the nanocrystallisation and on the final nanostructured obtained for both isothermally heated and continuously heated materials.

Rare Earth Additions in a Ferritic Steel: A Mössbauer Study

Rare earth metal additions in carbon steels promote better control of the inclusions morphology, resulting in better mechanical properties at room temperature. However, their hot flow behaviour has received scarce attention in the literature. In the present study, uniaxial hot compression tests are carried on two similar carbon containing steels with and without Ce and La (<0.1 wt%). As a general trend, the rare earth containing steel (RE) evidenced larger stress values than the rare earth-free steel (REF), particularly at testing temperatures below 1000°C. The rare earth metals may appear either as precipitates or dissolved into the austenitic phase. A Room Temperature Mossbauer study was performed to clarify the role played either as precipitate or solute by the rare earth elements. For this purpose samples of both steels were austenized at 900°C and 1000°C and quenched at room temperature. Hyperfine parameters of the ferritic and austenitic phases were obtained and compared. Mossbauer results indicate that rare earth metals are mainly affecting the flow stresses through a drag solute mechanism, and consequently are basically dissolved in the austenitic phase.