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Abstract: Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu46Zr45Al7Y2 bulk metallic glass is 361kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson–Mehl–Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484kJ/mol in isothermal transformation for Cu46Zr45Al7Y2 bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu46Zr45Al7Y2 bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions. [Copyright &y& Elsevier]

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]

Abstract Dynamic mechanical behavior of Ti 40 Zr 10 Cu 36 Pd 14 bulk metallic glass was investigated by mechanical spectroscopy. The mechanical spectra of Ti 40 Zr 10 Cu 36 Pd 14 bulk metallic glass show the main α relaxation while the slow β relaxation is not evident. The isothermal relaxation process of the loss factor tanδ can be well described by the Kohlrausch-Williams-Watts (KWW) stretched exponential relaxation function. In the current study, the stretched exponential relaxation parameter β age remains constant below the glass transition temperature T g , while the value increases when the temperature approaches T g. In addition, atomic mobility of the Ti 40 Zr 10 Cu 36 Pd 14 bulk metallic glass decreases during the physical aging. Highlights • Dynamic mechanical behavior of Ti-based metallic glass was studied by DMA. • Ti-based metallic glass shows the main α relaxation while the slow β relaxation is not evident. • The isothermal relaxation process of the loss factor can be well described by the KWW function. • Atomic mobility of the Ti 40 Zr 10 Cu 36 Pd 14 bulk metallic glass decreases during the physical aging. [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]

In the current research, the dynamic mechanical spectrum and compressive deformation of Zr 65 Cu 18 Ni 7 Al 10 bulk metallic glass in the supercooled liquid region (SLR) are investigated. The experimental results prove the existence of transition from Newtonian flow to non-Newtonian flow in the metallic glasses. In addition, we found that the characteristic stress σ tc , which is obtained by a stretched exponential function based on the normalized viscosity, can be regarded as a transition point from Newtonian to non-Newtonian flow. The correlation between strain rate sensitivity exponent and corresponding strain rate was obtained at a certain temperature. It is noted that the variation of transition strain rate from Newtonian to non-Newtonian flow with different absolute temperatures follows the Arrhenius equation. The activation energy is in good accordance with that using the mechanical spectroscopy method. [ABSTRACT FROM AUTHOR]

In the current work, the size effect to plasticity of the Zr 50 Cu 40+x Al 10−x (x=0, and 2) bulk metallic glasses has been investigated experimentally. It is found that macroscopic plasticity are affected by different of leading factors. The mean stress drop of serrated flow is positively related to the number of weak regions. With greater number of weak regions, more pronounced stress drop occurs in the serrated flow process. The experimental results suggested that fewer number of weak regions lead to larger dimple patterns area. Besides, the average spacing of primary shear bands is associated with the weak regions and the position where shear bands terminate. [ABSTRACT FROM AUTHOR]

Zn 38 Mg 12 Ca 32 Yb 18 bulk metallic glass (BMG) presents higher corrosion resistance, lower glass transition temperature and lower density than most BMGs. Dynamic properties of Zn 38 Mg 12 Ca 32 Yb 18 BMG were investigated by mechanical spectroscopy. The structural (α) process in the isothermal spectra is well described by a Kohlrausch-Williams-Watts (KWW) function with β KWW = 0.505. At high frequency, an extra contribution to the α process (i.e. an excess wing) is observed with a much smaller frequency dependence of the loss shear modulus G″( f ) ~ f − 0.37 , indicating the presence of a submerged secondary peak. The temperature behaviour of the α relaxation time, τ α , is characterized by a very small steepness index (i.e. strong behaviour). We find that results for this BMG show a deviation of the correlation between fragility and β KWW generally found in glass formers . Results in the literature for other BMGs suggest that this deviation may not be an exception for BMGs, since very similar values of β KWW (~ 0.5) have been found for BMGs independently of their steepness index. [ABSTRACT FROM AUTHOR]

The dynamic mechanical relaxation behaviour of Zr 50 Cu 40 Al 10 bulk metallic glass was investigated by mechanical spectroscopy as a function of temperature. The mechanical spectra show the evidence of an “excess wing” process at the lower temperatures. The α-relaxation spectra of Zr 50 Cu 40 Al 10 bulk metallic glass was described with a Kohlrausch–Williams–Watts (KWW) function, with the exponent β KWW = 0.5 ± 0.02. The fragility parameter m of the Zr 50 Cu 40 Al 10 is m = 57 ± 5. The values of m and β KWW are found to agree with the correlation found for other glasses m = 250(± 30) − 320 β KWW [R. Böhmer, K.L. Ngai, C.A. Angell et al. J. Chem. Phys. 99 (1993) 4201–4209]. Aging experiments give strong evidence that the observed “excess wing” is a submerged secondary process and not part of the α process. [ABSTRACT FROM AUTHOR]

• Novel model integrates Burgers & FV approaches. • Plastic viscosity decreases with temperature, increases with stress rates for the metallic glass. • Relaxation time increases with loading frequency. • Study sheds light on history-dependent behavior for metallic glasses. Establishing the intrinsic correlation between microstructural heterogeneity and mechanical properties is a challenging issue of metallic glasses. The ratchet behavior was examined in a Zr-based metallic glass under cyclic tensile loading well below the yield point, particularly near the glass transition temperature. It is found that strain evolution during cyclic loading shows heightened sensitivity to temperature and stress rate. Also, creep behavior mirrors the ratchet strain induced by cyclic loading. The proposed constitutive model, integrating the Burgers model with defect concentration based on free volume theory, effectively describes strain evolution during cyclic loading near glass transition temperature. Both macroscopic and microscopic perspectives are included in this model. The results verify that metallic glasses exhibit significant viscous characteristics, displaying noticeable creep deformation under low stress rates and amplitudes, which contributes to ratchet behavior. The fitted parameters show that plastic viscosity decreases with temperature and increases with stress rate, corroborating the decrease of tensile yield stress with temperature increasing; also, the fitted relaxation time increases with loading frequency, reflecting evolution of defect concentration. Structural relaxation competes favorably against stress-driven rejuvenation throughout the cyclic process, suggesting potential in tuning metallic glasses properties through innovation thermos-mechanical processing techniques. [ABSTRACT FROM AUTHOR]

• Dynamic mechanical behavior of La-based and CuZr-based MGs were investigated by DMA. • La-based MG shows an evident slow β relaxation process. • Evolution of the concentration of "defects" of MGs was analyzed by IT theory. Dynamic mechanical relaxation processes are very important to understand physical and mechanical properties of metallic glasses. In the current work, dynamic mechanical behaviors of La 43.6 Ce 18.4 Ni 24 Al and Cu 46 Zr 46 Al 8 metallic glasses were investigated by mechanical spectroscopy. Unlike Cu 46 Zr 46 Al 8 metallic glass, La 43.6 Ce 18.4 Ni 24 Al 14 glassy system shows an evident slow β relaxation process. In addition, physical aging below the glass transition temperature T g induces a reduction of intensity of the β relaxation. Evolution of the concentration of "defects" of metallic glasses was analyzed in the framework of Interstitialcy theory. Our research provides a new insight on understanding of the correlation between the structural heterogeneity and β relaxation of metallic glasses. [ABSTRACT FROM AUTHOR]

• High-temperature rheological behavior of a metallic glass was investigated. • Plastic deformation behavior and apparent viscosity strongly depend on temperature and strain rate. • Physical origin of the stress overshoot is analyzed based on the physical expressions of the parameters in the free volume model. • Strength of the stress overshoot is determined by the time of change from non-equilibrium to equilibrium defect concentrations. The high-temperature rheological behavior, around the glass transition temperature, of a Zr 48 Cu 34 Ag 8 Al 8 Pd 2 metallic glass was investigated by strain rate jump experiments. The results show that the plastic deformation behavior and apparent viscosity strongly depend on temperature and strain rate. The high temperature rheology can be interpreted within the framework of the free volume model. The relative defect concentration deduced from the free volume model depends on temperature and strain rate and it explains the evolution of apparent viscosity within testing conditions, which is inversely correlated with the defect concentration. The stress overshoot is found to be a characteristic feature of the high-temperature rheological behavior. The origin of the stress overshoot is analyzed based on the physical meaning of the parameters in the free volume model. The results demonstrate that the strength of the stress overshoot is determined by the time of change from non-equilibrium to equilibrium defect concentrations. This study highlights the link between the stress overshoot phenomenon and the initial structural state, examining the influence of physical aging and stress relaxation on the high-temperature deformation process of metallic glasses. [ABSTRACT FROM AUTHOR]

The viscosity and fragility of TiZrHfCuNiBe high entropy bulk metallic glass (HE-BMG) in the supercooled liquid region were investigated by the strain-rate jump tests. The Newtonian viscosities of HE-BMG are generally smaller than the conventional MGs due to higher mixing entropy. The fragility parameter was estimated to be 38 and implying that the supercooled liquid of HE-BMG is stronger. The interrelationship between fragility index and glass forming ability is discussed. The defects concentration of HE-BMG is much smaller than the traditional MGs, which enhances the difficulty of atomic rearrangement and further improves the stability of the supercooled liquid. • The Newtonian viscosities of HE-BMG are smaller than the conventional MGs. • The supercooled liquid of HE-BMG is stronger. • The defects concentration of HE-BMG is much smaller than the traditional MGs. [ABSTRACT FROM AUTHOR]

The constitutive behavior of Zr-based metallic glass ribbon around the glass transition temperature was investigated. The Zener-Hollomon equation was adopted to predict the flow stresses. The experimental results show that an exponent value of 0.75 and activation energy magnitude of 343 kJ/mol can be used in the hyperbolic sine model to predict the flow stress response of the steady-state stage. The activation energy is consistent with the free volume theory (335 kJ/mol). It is observed that the flow stress during steady-state is independent of the previously strain rate history and is only sensitivity to the current strain rate. A Z-parameter constitutive model to describe the hot deformation behavior of Zr-based metallic glass is developed. • The Zener-Hollomon equation was applied to predict the flow stresses. • The activation energy is consistent with the free volume theory (335 kJ/mol). • The formulation of the hot deformation behaviour of the Zr-based metallic glass MG is provided. [ABSTRACT FROM AUTHOR]

Abstract The dynamic mechanical relaxation behavior of Zr 64.13 Cu 15.75 Ni 10.12 Al 10 bulk metallic glass was investigated by mechanical spectroscopy. The activation energy of the main α relaxation was determined to be 4.24 eV. In order to well understand the α relaxation process, several physical and phenomenological models such as Debye model, Kohlrausch-Williams-Watt (KWW) model and quasi-point defect (QPD) model were employed to analyze the dynamic mechanical performance. It was found that Debye model was not so persuasive due to the asymmetry of the main relaxation of Zr 64.13 Cu 15.75 Ni 10.12 Al 10 bulk metallic glass. By contrast, KWW model agreed well with the experimental data if taking Kohlrausch parameter β kww as 0.571. The QPD model, which had a definite physical meaning, could not only match the experimental data perfectly but also correlated the relaxation behavior with the microstructure evolution versus temperature in this bulk metallic glass. [ABSTRACT FROM AUTHOR]

• A theoretical framework based on the hierarchically correlated atomic theory of the glassy materials was proposed. • The theoretical framework can be validated by the stress relaxation and creep experiments on typical La-based metallic glass. • It provides quantitative insights into the non-elastic deformation mechanisms in metallic glasses. Due to the structurally disordered arrangement of atoms and deviation from thermodynamic equilibrium, the physical and mechanical properties of metallic glasses can vary with time, temperature and magnitude of strain or stress. The current work provides a theoretical framework based on the hierarchically correlated atomic theory, which allows a quantitative description of the non-elastic deformation in metallic glasses. The defect concentration is adopted as an order parameter, which can evolve with temperature and non-elastic strain owing to correlated atomic movements. Through our hierarchically correlated atomic theory, we derive the characteristic times for local shear events in metallic glasses that entail activation, growth and/or annihilation of flow defects, which however are not accounted for in the previous mean field theories. Finally, we demonstrate that the current theoretical framework can be validated by the stress relaxation and creep experiments on typical La-based metallic glasses, which in turn provides quantitative insights into the non-elastic deformation mechanisms in metallic glasses. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: In this paper, Zr based bulk metallic glass (BMG) composite consisting of nanocrystals embedded in the amorphous matrix was synthesized by controlled annealing treatment of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 amorphous alloy. The homogeneous plastic deformation behavior of the BMG composite was characterized in the supercooled liquid region, exhibiting non-Newtonian steady state plastic flow over the investigated strain rate domain. Based upon the analysis of rheological data in terms of a modified mechanistic model, it is revealed that the mechanism of the non-Newtonian plastic flow varies with temperature, which is closely related to the presence of nanocrystals within the glassy matrix. [Copyright &y& Elsevier]

• The quasi-point defects theory is adopted to predict the atomic mobility and mechanical properties of the ZrCu-based metallic glasses. The correlated parameter χ is used to describe the concentration of the defects related to the spatial structural heterogeneity in metallic glasses. • The glass transition is facilitated, the thermal stability is decreased and intensity of the Johari-Goldstein (JG) relaxation is enhanced with the introduction of Dysprosium. • Micro-alloying of Dy increases the spatial structure heterogeneity and plays an important role in tailoring the mechanical behavior and relaxation behavior in ZrCu-based metallic glasses. The thermal stability and the dynamic mechanical relaxation behavior of (Zr 50 Cu 40 Al 10) 100-x Dy x (at.%) (x = 0 or 2) metallic glasses were investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the case of the (Zr 50 Cu 40 Al 10) 100-x Dy x (x = 0 or 2) metallic glasses, atomic mobility increases by introduction of the Dy element. The activation energy of the the main a relaxation process and fragility index were discussed according to the DMA measurements. With the help of the time-temperature superposition (TTS) principle, the master curves of the model alloys were established, the Kohlrausch-Williams-Watts (KWW) function and quasi-point defects (QPD) theory were used to descirbe the master curves of the metallic glasses. The characteristic parameters related to microstructural heterogeneity in the KWW equation and the QPD model, Kohlrausch exponent β KWW and the correlation factor χ were evaluated. In parallel, the elastic, viscoelastic and viscoplastic responses of the model alloys have been analyzed based on the DMA results. Our investigations demonstrated that introduction of the Dy increases the structure heterogeneity of the (Zr 50 Cu 40 Al 10) 100-x Dy x (x = 0 or 2) glassy system, which plays an important role in tailoring the dynamic relaxation behavior and mechanical properties of the metallic glasses. [Display omitted] [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]

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]