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Using high resolution inelastic x-ray scattering we studied the collective dynamics of the Pd77Si16.5Cu6.5 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. [ABSTRACT FROM AUTHOR]

The thermal stability and the quantification of the different transformation processes involved in the overall crystallization of the Fe 50 Cr 15 Mo 14 C 15 B 6 amorphous alloy were investigated by several characterization techniques. Formation of various metastable and stable phases during the devitrification process in the sequence α-Fe, χ-Cr 6 Fe 18 Mo 5 , M 23 (C,B) 6 , M 7 C 3 , η-Fe 3 Mo 3 C and FeMo 2 B 2 (with M = Fe, Cr, Mo), was observed by in-situ synchrotron high energy X-ray diffraction and in-situ transmission electron microscopy. By combining these techniques with differential scanning calorimetry data, the crystallization states and their temperature range of stability under continuous heating were related with the evolution of the crystallized fraction and the phase sequence as a function of temperature, revealing structural and chemical details of the different transformation mechanisms. [ABSTRACT FROM AUTHOR]

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 Ti 16.7 Zr 16.7 Hf 16.7 Cu 16.7 Ni 16.7 Be 16.7 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. • A relationship between the heat flow and shear modulus changes occurring upon structural relaxation and crystallization is analyzed. • All thermal effects are related to the alteration of shear modulus of metallic glass. • The anharmonicity of interatomic potential plays a critical role in the thermal flow and shear softening. [ABSTRACT FROM AUTHOR]

• Within the framework of the transition state theory, the isothermal creep behavior of metallic glasses is elucidated through a unique global approach, where the topological state is exclusively linked to measured strain. • Our methodology allows the computation of the average activation volume, activation energy, and relaxation decoupling of deformation units as a function of strain. • It provides comprehensive understanding of the creep deformation in metallic glasses and their mechanical response across different states. Within the framework of transition state theory, the isothermal creep behavior of metallic glasses is elucidated through a unique global approach, where the topological state is exclusively linked to measured strain. Our methodology allows the computation of the average activation volume and activation energy of deformation units as a function of strain. Experimental data from four representative metallic glasses (La 30 Ce 30 Ni 10 Al 10 Co 20 , La 65 Ni 15 Al 25 , La 56.16 Ce 14.04 Ni 19.8 Al 10 , and Cu 46 Zr 46 Al 8) reveal two distinct characteristics. Below the glass transition temperature, the mechanical response is primarily influenced by secondary relaxation processes and excess configuration entropy, with activation volume increasing with strain. Upon reaching the glass transition temperature, the activation volume becomes notably larger and strain-independent. Additionally, the activation energy exhibits an increase with strain, and deformation units of varying sizes are progressively activated, from smaller to larger units. The decoupling and competition among relaxation events are correlated with the increase in the activation volume of deformation units. These findings provide valuable insights into the dynamic behavior of metallic glasses and their mechanical response across different states. [Display omitted] [ABSTRACT FROM AUTHOR]

Although the atomic structure of amorphous alloys, which lacks long-range translational symmetry, may appear homogeneous at the macroscopic scale, their local dynamic and/or static properties however vary significantly according to the recent experimental and simulation results. In the literature of amorphous alloys, the nature of such local heterogeneities is currently an issue under debate. More importantly, since amorphous alloys are in a thermodynamically nonequilibrium state, their local structures constantly evolve during structural relaxation, physical aging and mechanical deformation. As such, local structural heterogeneities, which vary with the thermal and mechanical history of amorphous alloys, could provide a key to understand the structural origin of their mechanical behavior, such as anelasticity, viscoelasticity, plasticity and fracture. In this review article, we first review mechanical spectroscopy or dynamic mechanical analyses as an important tool to study the relaxation dynamics in amorphous alloys, with a focus on the possible correlation between the secondary (also called β) relaxation and the local structural heterogeneities of amorphous alloys. After that, we discuss the recent advances on the understanding of structural heterogeneities in metallic supercooled liquids and the influence of the structural heterogeneities on the overall mechanical properties of the corresponding amorphous alloys. Finally, we briefly discuss the further development of research on this subject. [ABSTRACT FROM AUTHOR]

The dynamic mechanical relaxation of metallic glasses is closely associated with the physical and mechanical properties. In the current work, the dynamic mechanical relaxation behaviors of Cu 46 Zr 45 Al 7 Y 2 and La 65 Al 14 (Cu 5/6 Ag 1/6 ) 11 (Ni 1/2 Co 1/2 ) 10 bulk metallic glasses are investigated by mechanical spectroscopy. In general, metallic glasses display two relaxation modes: main ( α ) relaxation and the slow secondary ( β ) relaxation. The α relaxation is linked to the dynamic glass transition phenomenon and viscous flow while the slow β relaxation is associated with many fundamental issues, such as diffusion and glass transition phenomenon. The experimental study shows La 65 Al 14 (Cu 5/6 Ag 1/6 ) 11 (Ni 1/2 Co 1/2 ) 10 bulk metallic glass displays a noticeable slow β relaxation. Contrarily, the Cu 46 Zr 45 Al 7 Y 2 bulk metallic glass relaxation process takes the form of an “excess wing”. In the framework of quasi-point defects (QPD) theory, the dynamic mechanical response of the metallic glasses is discussed. [ABSTRACT FROM AUTHOR]

Mechanical relaxation of glassy materials is strongly influenced by their mechanical and physical properties. In the current study, dynamic mechanical relaxation of Cu 46 Zr 46 Al 8 metallic glass has been investigated by mechanical spectroscopy and molecular dynamics simulations. Our results show that mechanical properties of metallic glasses are highly dependent on their thermal history. α relaxation in the frequency region of the Cu 46 Zr 46 Al 8 metallic glass was described by Havriliak-Negami (HN) model. In the framework of the quasi-point defects theory, the correlated factor χ could be used to characterize the concentration of “defects” of glassy materials. By molecular dynamics simulations of dynamic mechanical tests at different temperatures and frequencies, it is noted that adjusting of defects is inversely correlated to storage modulus at the atomic level. On the other hand, physical aging below the glass transition temperature T g reduces the concentration of defects and narrows down the width of shear bands, which corresponds to decreasing of the atomic mobility. [ABSTRACT FROM AUTHOR]

Abstract: Understanding phase changes, including their formation and evolution, is critical for the performance of functional as well as structural materials. We analyze in detail microstructural and chemical transformations of the amorphous steel Fe50Cr15Mo14C15B6 during isothermal treatments at temperatures ranging from 550 to 800°C. By combining high-resolution transmission electron microscopy and Rietveld analyses of X-ray diffraction patterns together with the local chemical data obtained by atom probe tomography, this research provides relevant information at the atomic scale about the mechanisms of crystallization and the subsequent phases evolution. During the initial stages of crystallization a stable (Fe,Cr)23(C,B)6 precipitates as well as two metastable intermediates of M3(C,B) and the intermetallic χ-phase. When full crystallization is reached, only a percolated nano-scale Cr-rich (Fe,Cr)23(C,B)6 and Mo-rich η-Fe3Mo3C structure is detected, with no evidence to suggest that other phases appear at any subsequent time. Finally, the corrosion behavior of the developed phases is discussed from considerations of the obtained atomic information. [Copyright &y& Elsevier]

We report here on a polyamorphic-phase transition upon application of pressure on a Ce-based metallic glass, Ce10Al10Ni10Cu10, investigated by x-ray diffraction and inelastic x-ray scattering. This alloy is found to display a strong hysteresis in volume per atom upon application and subsequent release of pressure. The observed structural changes are correlated with changes observed by inelastic x-ray scattering in the elastic constants, acoustic mode frequencies, and sound speed. The results reported here point toward three different amorphous phases for this alloy existing in the 0-25 GPa pressure region: low- and high-density states and an intermediate mixed state that displays a hysteresis behavior. Finally, we discuss the impact of Ce concentration on the polyamorphic transition for a series of Ce-based metallic glass alloys and link it to the phase transformation between γ-Ce and α-Ce under pressure. [ABSTRACT FROM AUTHOR]

Abstract: Metallic glasses with conventional glass-forming ability (Al–Fe–Nd, Fe–Zr–B, Fe–B–Nb compositions) and bulk metallic glasses (Ca–Mg–Cu compositions) were studied by synchrotron X-ray diffraction during annealing throughout glass transition and crystallization temperatures. The analysis of the first diffraction peak position during the annealing process allowed us to follow the free volume change during relaxation and glass transition. The structure factor and the radial distribution function of the glasses were obtained from the X-ray measurements. The structural changes occurred during annealing are analyzed and discussed. [Copyright &y& Elsevier]

The viscoelastic properties of Pd 20 Pt 20 Cu 20 Ni 20 P 20 high-entropy metallic glass were probed by dynamic mechanical spectroscopy and stress relaxation. The experimental evolution of stress can be characterized by the empirical Kohlrausch-Williams-Watts function during the tensile stress relaxation measurement. We develop a linear relaxation model able to describe the whole process of stress relaxation in a wide range of temperatures. The linear relaxation model takes into account the microstructural heterogeneity of the glass, and thus its dynamical heterogeneity, so that the thermal effect and stress-driven process are physically decoupled. The activation energy spectra at various temperatures reveal the changes of the deformation units during stress relaxation, which is the result of the interplay between stress and temperature. This study decomposes the widespread hierarchical dynamics due to structural heterogeneity which accommodates the viscoelastic deformation of the high-entropy metallic glasses. • Viscoelastic properties of HEMG were probed by DMA and stress relaxation. • A constitutive model was proposed to describe the whole process of stress relaxation. • Stress relaxation in the HEMG can be well described by KWW equation. [ABSTRACT FROM AUTHOR]

Dynamic mechanical analysis was performed in two metallic glasses that show markedly distinct β relaxation features. The confined flow defects of free-volume zones and the inelastic deformation behavior result into a significant mechanical hysteresis loop in dynamic cyclic loadings, which is described with a viscoelastic model. Annealing below the glass transition temperature T g decreases the concentration of free-volume zones. The understanding of the inelastic deformation, which is related to free-volume zones, is critical and may provide a new avenue into the mechanical properties' improvement of high entropy metallic glasses. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Secondary β relaxation is a fundamental issue to understand the diffusion, plasticity and glass transition behavior of metallic glasses. Binary metallic glasses provide a simple glassy system to probe the dynamic mechanical processes. In the current research, dynamic mechanical behavior of the typical binary metallic glasses was investigated by dynamic mechanical analysis (DMA). The dynamic mechanical relaxation behavior of Cu 50-x Ti 50+x (x = 0, 7 and 9) and Cu x Zr 100-x (x = 30, 50, 56, 61.8 and 64) binary metallic glass system was studied. It is found that the β relaxation becomes pronounced with the increase of copper, which is analyzed in the framework of the principle of mixing enthalpy. However, the β relaxation of CuZr or CuTi binary metallic glasses is much more modest than that of Y 65 Co 35 , Dy 65 Co 35 and Y 67 Cu 33. Image 1 • Mechanical relaxation processes of binary MGs were probed by mechanical spectroscopy. • β relaxation of the Cu x Zr 100-x binary MG systems was suspended by increasing the content of zirconium. • The activation energy of the β relaxation obeys the empirical relation: E β ~26 RT g. [ABSTRACT FROM AUTHOR]

• Dynamic mechanical relaxation of metallic glass was studied by DMA. • Relaxation spectrum was analyzed by using the KWW equation, the QPD theory and the HN model. • The atomic mobility is affected by micro-alloying. The dynamic mechanical behavior of Zr 50-x Cu 34 Ag 8 Al 8 Pd x (x = 0 and 2) bulk metallic glasses was probed by mechanical spectroscopy. The results suggest that microalloying retards the relaxation dynamics. The physical aging was analyzed considering a model based on the variation of the concentration of flow units and the Kohlrausch-Williams-Watts (KWW) equation. In the current study, the characteristic parameters of the two models, β c and β aging , reflect the faster annihilation of flow units at higher temperature and the increase of dynamic heterogeneity with temperature, respectively. The mechanical relaxation spectrum was also analyzed. Based on the KWW equation and the quasi-point defect model, replacing Zr with a small amount of Pd can slightly increase the dynamic inhomogeneity and reduce the defect concentration. The analysis according to the HN function suggests that the addition of Pd increases the relaxation time. However, it does not significantly alter the shape of the relaxation time distribution. [ABSTRACT FROM AUTHOR]

Abstract: In this work we study the structural evolution of Al and Fe based metallic glass compositions. The samples were obtained as ribbons by melt-spinning, their glass stability and crystallization were analyzed by differential scanning calorimetry and dilatometry, structural changes were followed by in situ synchrotron X-ray diffraction during annealing throughout glass transition and crystallization. The synchrotron results are compared with the calorimetric and dilatometric measurements and the structural changes occurred during annealing are determined and described for each alloy. [Copyright &y& Elsevier]

Dynamic mechanical relaxation, stress relaxation and creep were performed in a La 56.16 Ce 14.04 Ni 19.8 Al 10 metallic glass with pronounced β relaxation features. The evolution of the microstructural heterogeneity with temperature is indirectly revealed via dynamic heterogeneity and analyzed in terms of several independent theoretical approaches. An apparent activation energy of β relaxation is obtained by assuming the Arrhenius relationship, which is calibrated by either the activation enthalpy of stress relaxation or the activation energy of creep. It is found that similar deformation mechanisms accommodate the dynamics of the LaCe-based metallic glass under different types of external stimuli applied in mechanical experiments. Image 1 • DMA, stress relaxation and creep experiments were launched to investigate the deformation mechanism of MG. • Activation enthalpy of stress relaxation is 0.46 eV, while the activation energy during creep process is 0.55 eV. • Evolution of microstructural heterogeneity with temperature is revealed via dynamic heterogeneity. [ABSTRACT FROM AUTHOR]

Relaxation of internal friction of Zr 57 Nb 5 Al 10 Cu 15.4 Ni 12.6 metallic glass was investigated by mechanical spectroscopy. The stress relaxation of internal friction with different aging temperature was described by Kohlrausch-Williams-Watts (KWW) equation. The shear viscosity behavior during linear heating can be interpreted as a result of the local irreversible structural relaxation with distributed activation energies. The results show that the dynamic heterogeneity and the atomic mobility are stimulated by the increasing annealing temperature. Image 1 • Relaxation of internal friction of Zr-based metallic glass was studied by DMA. • Stress relaxation of internal friction can be described by KWW equation. • Dynamic heterogeneity and the atomic mobility are stimulated by the increasing annealing temperature. [ABSTRACT FROM AUTHOR]

The short-range ordered but long-range disordered structure of metallic glasses yields strong structural and dynamic heterogeneities. Stress relaxation is a technique to trace the evolution of stress in response to a fixed strain, which reflects the dynamic features phenomenologically described by the Kohlrausch-Williams-Watts (KWW) equation. The KWW equation describes a broad distribution of relaxation times with a small number of empirical parameters, but it does not arise from a particular physically motivated mechanistic picture. Here we report an anomalous two-stage stress relaxation behavior in a Cu46Zr46Al8 metallic glass over a wide temperature range and generalize the findings in other compositions. Thermodynamic analysis identifies two categories of processes: a fast stress-driven event with large activation volume and a slow thermally activated event with small activation volume, which synthetically dominates the stress relaxation dynamics. Discrete analyses rationalize the transition mechanism induced by stress and explain the anomalous variation of the KWW characteristic time with temperature. Atomistic simulations reveal that the stress-driven event involves virtually instantaneous short-range atomic rearrangement, while the thermally activated event is the percolation of the fast event accommodated by the long-range atomic diffusion. The insights may clarify the underlying physical mechanisms behind the phenomenological description and shed light on correlating the hierarchical dynamics and structural heterogeneity of amorphous solids. [ABSTRACT FROM AUTHOR]

Abstract: Amorphous steels are promising materials with potential structural applications. The glass-forming ability (GFA) and mechanical properties of metallic glasses are intimately related to the local structure. In Fe-based alloys, Cr and Mo content seem to play a key role in stabilizing the amorphous atomic-level structure. Here we present a study on the effects of changing Mo content in Fe72−x C7Si3.3B5.5P8.7Cr2.3Al2Mo x amorphous steels. We study the local structure of these alloys by Synchrotron X-ray diffraction and Mössbauer spectroscopy. The results show how the amorphous phase evolves from a ferromagnetic Fe-rich structure to a structure with predominance of paramagnetic environments with the increase of Mo content. The changes in the distribution of magnetic environments cannot be attributed only to the Fe–Mo substitution but to a change of local configuration in the amorphous phase. [Copyright &y& Elsevier]

Abstract: The mechanical response of bulk metallic glasses is ultimately controlled by interatomic forces between the randomly distributed atoms. The study of collective dynamics in amorphous systems, either liquid or glassy, opens a window to better understand this interaction. We present here results of recent IXS experiments performed at the ID16 beamline of the European Synchrotron Radiation Facility (ESRF) on a metallic glass. In particular, the high-frequency response of Pd77Si16.5Cu6.5 alloys, produced as a ribbon with an ultra-fast quenching rate (∼106 K/s) and as a bulk with a slow one (∼102 K/s), was determined. Previous experiments showed a discrepancy between the sound speed of metallic glasses measured by ultrasounds and by IXS. This was explained as a consequence of density fluctuations in the glass, although other explanations are possible. Here, the dispersion relation of the acoustic modes at room temperature and pressure are measured for both samples, allowing us to compute the sound speed and discuss the differences in the acoustic properties of both samples. [Copyright &y& Elsevier]

Abstract: The measurement of fragility of glass-forming liquids involves either the determination of viscosity at different temperatures above the glass transition or the measurement of the dynamic glass–liquid transition by calorimetry or dilatometry at different heating rates. Significant data dispersion is observed when comparing fragility parameters obtained by different techniques. Dynamic mechanical analysis (DMA) is widely used for characterizing glass transition in polymer science while it is still seldom used in metallic glasses. In this work we determine the relaxation time (τ) vs temperature of a Pd77.5Si16.5Cu6 glass by means of DMA measurements. The measured range of relaxation times cover the whole region from glass transition (τ >100s) to times of the order of 10−2 s. The fragility parameter obtained for this alloy is found significantly higher than the values obtained from previous viscosity and calorimetric techniques. The application range, limitations and correctness of the technique are discussed. [ABSTRACT FROM AUTHOR]