Irreversible structural relaxation in a bulk Pd-Cu-Ni-P metallic glass

The evolution of the shear modulus and the damping decrement during irreversible structural relaxation in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in a temperature range below the glass transition temperature has been studied with an inverse torsion pendulum at a frequency of ～25 Hz. It is shown that the irreversible relaxation can be recovered via quenching from temperatures above the glass transition temperature. The spectrum shape, the characteristic activation energies, and the attempt frequencies of the irreversible structural relaxation are estimated.     

Isothermal kinetics and relaxation recovery of high-frequency shear modulus in the course of structural relaxation of Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> bulk glass

Isothermal kinetics of relaxation of the high-frequency (1.4 MHz) shear modulus during structural relaxation of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass below the glass transition temperature is studied by an in situ method of contactless electromagnetic acoustic transformation. The kinetic law of relaxation is established. It is shown that quenching of aged samples from the supercooled liquid state leads to a decrease in the absolute value of shear modulus to below the initial value; the degree of subsequent isothermal relaxation of the modulus may be several times higher than the initial value. Possible reasons for relaxation and recovery of the shear modulus are considered.     

Time and amplitude dependences of the damping decrement and shear modulus upon irreversible structural relaxation in a Zr-Cu-Ni-Al-Ti bulk metallic glass

The time dependences of the irreversible relaxation of the damping decrement and the shear modulus of a Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass are investigated using an inverse torsion pendulum in the range from room temperature to ～650 K. The spectrum of activation energies of irreversible structural relaxation is evaluated from the results obtained. Analysis of the amplitude dependences of the damping decrement and the shear modulus allows the conclusion that the relaxation centers responsible for the amplitude dependence differ from those associated with the irreversible structural relaxation at temperatures below and in the vicinity of the glass transition point.     

Relaxation of the high-frequency shear modulus in bulk metallic glass Zr46(Cu4/5Ag1/5)46Al8

Precision in situ measurements of the shear modulus of bulk metallic glass based on Zr have been performed. The contribution to the shear modulus due to the structural relaxation has been separated. Based on the interstitial theory, the relaxation model explaining the obtained experimental results has been constructed.     

Shear transformations and the relaxation of the magnetic permeability aftereffect in amorphous ferrogmagnets

The relaxation of the permeability aftereffect of amorphous ferromagnets is studied as a function of aging time at different temperatures, and interpreted in terms of a superposition of processes with a distribution of activation energies, whose boundaries are experimentally determined. The results are accounted for in terms of the progressive freezing of shear stress transformations, which are the source of the magnetic aftereffect in amorphous alloys. It is suggested that a correlation must exist between shear transformations and the processes of free volume annihilation responsible for the structural relaxation in the glassy state.     

Unified analysis of diffusion and relaxation processes in amorphous metallic alloys

Abstract

Self-diffusion data on amorphous metallic alloys determined in long-time radio-tracer experiments and activation-enthalpy spectra deduced from short-time structural relaxation studies on such materials are reviewed and analyzed in terms of current random transition rate models. It is shown that the seeming discrepancy between the self-diffusion enthalpies and the comparatively small enthalpies obtained from measurements of the magnetic after-effect arises from the existence of activation-enthalpy spectra. Whereas the short-time experiments reveal the small-activation-enthalpy parts of these spectra, the long-time self-diffusion experiments are controlled by the larger activation enthalpies. Assuming that the activation-enthalpy distributions are Gaussian, their characteristic parameters have been determined by comparing the two types of measurements. It is shown that the Arrhenius-type diffusion coefficients found by experiment are compatible with half-widths of the activation-enthalpy spectra of about 0.3 eV. Based on an analysis of the pre-exponential factors and other typical properties of the diffusion coefficients, potential mechanisms of the diffusion in amorphous alloys are proposed.     

Structural relaxation and viscous flow in amorphous ZrAlCu

The ternary metallic glass Zr₆₅Al_7.5Cu_27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.     

Secondary relaxation and dynamic heterogeneity in metallic glasses:A brief review

Understanding mechanical relaxation, such as primary(α) and secondary(β) relaxation, is key to unravel the intertwined relation between the atomic dynamics and non-equilibrium thermodynamics in metallic glasses. At a fundamental level, relaxation, plastic deformation, glass transition, and crystallization of metallic glasses are intimately linked to each other, which can be related to atomic packing, inter-atomic diffusion, and cooperative atom movement. Conceptually, βrelaxation is usually associated with structural heterogeneities intrinsic to metallic glasses. However, the details of such structural heterogeneities, being masked by the meta-stable disordered long-range structure, are yet to be understood. In this paper, we briefly review the recent experimental and simulation results that were attempted to elucidate structural heterogeneities in metallic glasses within the framework of β relaxation. In particular, we will discuss the correlation amongβ relaxation, structural heterogeneity, and mechanical properties of metallic glasses.     

Structural relaxation and viscous flow in amorphous ZrAlCu

The ternary metallic glass Zr₆₅Al_7.5Cu_27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.     

Stress relaxation and viscosity of a bulk Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> metallic glass under isochronous heating conditions

Isochronous relaxation of tensile stresses is measured in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in the initial state and after certain thermal treatments. The results of measurements are used to find the energy spectrum of irreversible structural relaxation, from which the temperature dependence of shear viscosity is then calculated. This dependence is also found independently from measurements of creep in the same glass. The calculated viscosity is shown to agree well with the experimental data.     

Structural relaxation of Zr_(41)Ti_(14)Cu_(12.5)Ni_(10)Be_(22.5) bulk metallic glass under high pressure

Zr41Ti14Cu12.5Ni10Be22.5bulk metallic glass (BMG) is annealed at 573 K under 3 GPaand its structural relaxation is investigated by X-ray diffraction, ultrasonic study, compression as well as sliding wear measurements. It is found that after the ZrTiCuNiBe BMG sample was annealed under high pressure, the mechanical properties were improved. Moreover, the BMG with relaxed structure exhibits markedly different acoustic properties. These results are attributed to the fact that relaxation under high-pressure results in a microstructural transformation in the BMG.     

A comparison of the dynamic mechanical relaxation behavior of linear low- and high-density polyethylenes

The mechanical relaxation behaviors of oriented samples of a linear low-density polyethylene (LLDPE) and a high-density polyethylene (HDPE) were studied using tensile dynamic mechanical measurements. The anisotropy and activation energies of the relaxations were determined for several different samples to investigate why the α-relaxation in HDPE has characteristics similar in some respects to both the α- and β-relaxations in LLDPE. It is concluded that the anisotropy of the α-relaxation in LLDPE is determined by c-shear, whereas in HDPE it relates to interlamellar shear. The activation energy measurements, however, show that the thermally activated process for the α-relaxation in both the LLDPE and HDPE is c-shear. It is proposed that, in HDPE, c-shear has to occur before there is enough mobility at the fold surface for interlamellar shear. It is concluded that the β-relaxation is not observed in HDPE because the interlamellar regions are too constrained to allow interlamellar shear without c-shear.     

On the relaxation of volume and shear viscosities in vegetable oils

Longitudinal and shear ultrasonic measurements were performed on castor, tung, soyabean and cottonseed oils in the frequency ranges 50 kHz–30 MHz and 10–150 MHz using reverberation, pulse and impedance techniques respectively. In castor and tung oils relaxation spectra for both volume and shear viscosities are broad and identical in shape. In soyabean and cottonseed oils both volume and shear viscosities undergo single relaxation but the volume relaxation time is about 10 times larger than the shear. It is suggested that shear relaxation results from molecular orientation and volume relaxation from rearrangement of molecular packing.     

Stress relaxation in an Zr<Subscript>52.5</Subscript>Ti<Subscript>5</Subscript>Cu<Subscript>17.9</Subscript>Ni<Subscript>14.6</Subscript>Al<Subscript>10</Subscript> bulk metallic glass

The isothermal relaxation of stresses in a bulk metallic glass is measured at temperatures below the glass transition point. The kinetic law of relaxation is determined. It is argued that the stress relaxation in the temperature range covered is due to the irreversible structural relaxation oriented by an external stress and characterized by a distribution of activation energies.     

Shear Modulus Relaxation and Thermal Effects in a Zr65Cu15Ni10Al10 Metallic Glass after Inhomogeneous Plastic Deformation

Journal of Experimental and Theoretical Physics - The shear modulus and enthalpy relaxation in a deformed Zr65Cu15Ni10Al10 metallic glass has been investigated. It has been established that...     

Identification and characterization of potential shear transformation zones in metallic glasses

The stability of local atomic configurations in a Ni-Zr metallic glass is studied by molecular dynamics. It is shown that individual atom displacements induce irreversible atomic rearrangements under different glass relaxation, temperature, and strain conditions. The number of regions with an unstable topology depends on the glass relaxation degree. Their time evolution is governed by thermal activation, the activation energy decreasing with elastic strain. It is also shown that unstable regions are located in correspondence of shear transformation zones operating under plastic deformation.     

A fluctuation-dissipation theorem approach to mechanical relaxation in solid polymers

Abstract

Kubo theory formalism has been used to obtain expressions for shear and dilatational stress relaxation functions in terms of statistical mechanical time-dependent correlation functions. This is equivalent to obtaining expressions for the complex modulus or the complex viscosity for all frequencies. These results provide a basis for calculating the macroscopic consequences of molecular models presently used to provide qualitative understanding of relaxation peaks for solid polymers.

The shear and dilatational stress relaxation functions are quite different formally. For a particularly simple model it will be shown that the former is related to the frequency distribution of the kinetic energy and is also closely related to the dielectric relaxation function. The familiar results of the Rouse model are recovered in the results but no friction constant need be assumed in the present approach.     

金属玻璃流变的扩展弹性模型

玻璃-液体转变现象,简称玻璃转变,被诺贝尔物理学奖获得者安德森教授评为最深奥与重要的凝聚态物理问题之一.金属玻璃作为典型的非晶态物质,具有与液体相似的无序原子结构,因此又称为冻结了的液态金属,是研究玻璃转变问题的理想模型材料.当加热至玻璃转变温度,或者加载到力学屈服点附近时,金属玻璃将会发生流动.由于热或应力导致的流动现象对金属玻璃的应用具有重要意义.本文简要回顾了金属玻璃流变现象,综述了流变扩展弹性模型的研究进展和未来发展趋势.     

Spectral distribution of relaxation times in spin glasses

Recent neutron scattering measurements on spin glasses show that the dynamics of the spin systems can be best described in terms of wide spectral distribution of relaxation times evolving continuously with decreasing temperature but which is devoid of any critical behaviour, either speeding up or slowing down, at any finite temperature including the spin glass “freezing temperature T_sg”. It is argued that the latter temperature itself is dependent on the time constant of measurement for all spin glasses in general; the observed variation with frequency being less pronounced in some systems than others owing to some special characteristics of their spin dynamics such as, for example, the presence of parallel channels of rapid relaxation provided by the Korringa coupling in metallic spin glasses. The neutron scattering measurements presented here enable us to propose plausible forms for the density of relaxation times of the spin system and to show that the logarithmic frequency dependence of the freezing temperature observed in low frequency ac susceptibility measurements follows naturally from a uniform density of relaxation times at these frequencies.     

Ionic conduction and dielectric relaxation in polycrystalline Na2SO4

In the present paper, the ionic conductivity and the dielectric relaxation properties of Na₂SO₄ have been investigated by means of impedance spectroscopy measurements over wide ranges of frequencies and temperatures. The frequency dependent impedance data has been modeled by appropriate equivalent circuit representing the bulk and grain boundary properties of the sample. Phase transition temperatures and the activation energies of conduction in different phases have been determined from the temperature dependence of the dc conductivity. Dielectric relaxation has been studied using the complex electric modulus and permittivity formalisms. From the electric modulus formalism it is concluded that the relaxation mechanism is independent of temperature. The dielectric permittivity spectra were analyzed by the Cole–Cole formula where different parameters are determined.