Structural Relaxation In Metallic Glasses Research Articles

The kinetics of internal structural relaxation of metallic glasses probed with ion beams and resistivity measurements

The kinetics of structural relaxations in glassy metal Zr65Al7.5Cu27.5 films are monitored using resistivity measurements. As-prepared samples irreversibly relax towards their metastable equilibrium or steady-state in close relation to co-occurring surface smoothing, when exposed to annealing and ion treatment. Vice versa, ion beams are used as a tool to probe the response to small structural disturbances which are found to relax thermally following a stretched exponential time dependence. A strongly decreasing Kohlrausch exponent with temperature indicates increasing dynamic heterogeneities during successive freezing out of relaxational modes.

The kinetics of irreversible structural relaxation and rheological behavior of metallic glasses under quasi-static loading

It is shown that the directional structural relaxation (DSR) model supplemented by the Maxwell rheological model allows the derivation of a set of simple equations that describe within a common framework a number of mechanical relaxation phenomena induced by irreversible structural relaxation of metallic glasses under quasi-static loading. A comparison of the model predictions with literature experimental data is made. It is concluded that the equations derived are supported by available experimental findings.

Non-isothermal strain recovery as a result of irreversible structural relaxation of metallic glasses

Abstract A quantitative model is proposed for the non-isothermal strain recovery induced by stress pre-annealing of metallic glasses. It is assumed that the non-isothermal strain recovery is determined by irreversible atomic rearrangements occurring in directional internal stress fields created by unloading of samples preannealed under stress. Special experiments are performed to make a thorough test of the model. It is shown that the model correctly reproduces the temperature position and height of the recovery strain rate peak observed in the experiment. It is concluded that the macroscopically reversible mechanical behaviour (strain recovery) can be determined by irreversible structural relaxation of a metallic glass.

Non-isothermal strain recovery as a result of irreversible structural relaxation of metallic glasses

Abstract A quantitative model is proposed for the non-isothermal strain recovery induced by stress pre-annealing of metallic glasses. It is assumed that the non-isothermal strain recovery is determined by irreversible atomic rearrangements occurring in directional internal stress fields created by unloading of samples preannealed under stress. Special experiments are performed to make a thorough test of the model. It is shown that the model correctly reproduces the temperature position and height of the recovery strain rate peak observed in the experiment. It is concluded that the macroscopically reversible mechanical behaviour (strain recovery) can be determined by irreversible structural relaxation of a metallic glass.

Structural relaxation in metallic glasses

Structural relaxation in several transition metal based metallic glasses has been studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), electrical resistivity, acoustic emission and Mossbauer spectroscopy techniques. Annealing of metallic glass below the crystallization temperature gives rise to discontinuous changes in electrical resistance. These changes are accompanied by acoustic emissions. The latter process provides an indication for the viscous flow in the matrix. In diverse alloy compositions, it has been observed that changes in electrical resistivity which begin instantly with the onset on isothermal annealing do not follow diffusion kinetics. Mossbauer experiments also show that neither long range diffusion nor the formation of a new phase occurs as a result of annealing. It is concluded that viscous flow is the mechanism for structural relaxation induced by isothermal annealing and short range ordering during this process is also a consequence of the viscous flow process.

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

The paper is devoted to a comprehensive review of the Directional Structural Relaxation model developed recently. The model treats homogeneous plastic flow of metallic glasses as a result of structural relaxation oriented by the external stress. The analytical expressions for the macroscopic plastic strain rate due to irreversible structural relaxation of metallic glasses are derived. A comparison of the predictions of the model with the experimental data is given. It is concluded that the model provides a good basis for the understanding of homogeneous plastic flow of metallic glasses under various experimental conditions.

Isothermal strain recovery as a result of reversible structural relaxation of metallic glasses

Strain recovery of melt spun metallic glasses Fe 84B 16 and Ni 77.5Si 7.5B 15 predeformed by tensile creep at temperatures 453<T<633 K has been investigated. The determined kinetic recovery law allowed to propose and verify a model of strain recovery as a result of reversible atomic rearrangements in the two-well relaxation centers with a small asymmetry of the potential. The model is shown to explain the main strain recovery regularities. An inter-relationship between reversible and irreversible components of structural relaxation of metallic glasses is discussed.

Isothermal creep of metallic glasses: a new approach and its experimental verification

An extended version of the theory of structural relaxation in metallic glasses under the action of external stress is presented. The theory explains the structural relaxation as a set of irreversible noncorrelated two-stage elementary shear atomic rearrangements with continuously distributed activation parameters in distinct regions of structure—relaxation centres. In loaded samples structural relaxation results in an accumulation of plastic deformation in accordance with magnitude and orientation of the applied stress. General equations are obtained for the creep strain rate as a result of the irreversible directed structural relaxation. Specially designed experiments were performed to test the theory. It is shown that the theory gives an adequate interpretation of experimental creep kinetics except for the time interval that covers 10 1–10 2 s after loading. This initial creep stage is supposed to result from reversible atomic rearrangements in relaxation centres with a symmetrical two-well potential.

Determination of activation energy spectra of irreversible structural relaxation of metallic glasses using non-isothermal stress relaxation data

A new method for determination of activation energy spectra of irreversible structural relaxation of metallic glasses using the kinetics of torque relaxation under linear heating has been developed. The method is simple, has acceptable accuracy, and lacks some limitations inherent to the known spectrum reconstruction methods. An experiment was carried out to determine the real structural relaxation spectrum of a Co-based metallic glass by this method. The form of this spectrum can indicate that plastic flow under intensive structural relaxation is realized via a set of atomic rearrangements in regions with `faulty' and `good' short range order.

Kinetics of isothermal creep in metallic glasses including the statistical distribution of activation parameters

A generalized theoretical model is proposed for the structural relaxation of metallic glasses under load. Structural relaxation is treated as a set of irreversible, uncorrelated, two-stage atomic displacements in some regions of the structure, the “relaxation centers.” In loaded samples structural relaxation acquires a directional character, leading to the buildup of plastic deformation in accordance with the magnitude and orientation of the applied mechanical stress. General equations are obtained for creep kinetics including a continuous statistical distribution of the principal activation parameters. These equations are compared with the results of a special experiment. The model is found to provide an adequate interpretation of the observed creep kinetics, except for the first 101–102 seconds after loading. It is argued that the initial stage of creep is determined by reversible atomic realignments in relaxation centers having symmetric two-well potential.

Quantitative description relaxation of metallic glasses macroscopic properties

In metallic glasses (Fe, Ni, Co) 80(Si, B) 20 relaxation of heat capacity, tensile deformation, internal stresses, magnetic hysteresis loop and Curie temperature are investigated. Relaxation of these properties is described in terms of Tool-Narayanaswamy glass transition model. It is shown, that the model satisfactorily describes structural relaxation in metallic glasses.

Low Frequency Internal Friction Induced by Structural Relaxation of Metallic Glasses

A quantitative model of visco-plastic damping induced by structural relaxation of metallic glasses is developed. The model is shown to be consistent with the experiment.

The kinetics of stress-oriented structural relaxation in metallic glasses

A theoretical model of the homogeneous flow of as-cast metallic glasses is proposed. Plastic flow within the framework of the model is viewed as a totality of local irreversible shear rearrangement with distributed activation energies. The equations for the macroscopic quantities measured in the experiment are derived.

X-ray-diffraction study of structural relaxation in metallic glasses

The effect of structural relaxation of amorphous ${\mathrm{Zr}}_{67}$${\mathrm{Ni}}_{10}$${\mathrm{Cu}}_{23}$ was studied by measuring the structure factor with x-ray diffraction. The changes in the radial distribution of the atoms upon thermal relaxation are described quantitatively by a model initially proposed for amorphous ${\mathrm{Pd}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{20}$. The model describes the structural changes by two orthogonal atomic processes. Radial atomic motion changes the density of the glass, while the short-range order of the glass is enhanced or reduced by local atomic motion which conserves the density of the sample. This local shear motion of the atoms is correlated over 2.5 \AA{} in ${\mathrm{Zr}}_{67}$${\mathrm{Ni}}_{10}$${\mathrm{Cu}}_{23}$, while for ${\mathrm{Pd}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{20}$ the correlation length is larger than 7 \AA{}. Structure and chemical bonding in the purely metallic and the metal-metalloid system most likely determine these length scales.

Structural relaxation of metallic glasses studied by Mössbauer spectroscopy

We present microscopic evidence of structural relaxation in the purely metallic glass Zr67Ni10Cu23 doped with 0.5 at.% of57Fe. The Mossbauer spectra under different heat treatments were analyzed by a six-parameter model. Two regimes of structural relaxation, irreversible and reversible relaxation, are clearly distinguished and the latter involves different atomic processes over different temperature regions.

Diffusivity and viscosity during structural relaxation in metallic glasses

Experimental data on structural relaxation in metallic glasses are discussed in terms of atomistic models, in particular the free volume model. It is shown that it is essential to discriminate between the behaviour of properties controlled by atomic mobility (viscosity and diffusivity) and other physical properties. The linkage between the two types of behaviour, through the free volume model, is demonstrated. The emphasis is on recent developments in the insight into the mechanism controlling diffusivity and viscosity in metallic glasses.

On the Kinetics of Structural Relaxation in Metallic Glasses

On the Kinetics of Structural Relaxation in Metallic Glasses

Structural Relaxation in Metallic Glasses by Means of Positron Lifetimes Methods

Structural Relaxation in Metallic Glasses by Means of Positron Lifetimes Methods

A quantitative study on reversible structural relaxation of metallic glasses

Two phenomenological theories were investigated for describing the reversible structural relaxation in metallic glasses: the activation-energy spectrum model of two-level systems and the diffusion model based on the free volume theory, where the driving force is thought to be compositional ordering of different types of atoms. Two kinetic curves were measured and compared with those calculated from the models. For comparison the whole annealing procedure performed in the experiment was taken into account by using a new evaluation technique. In the temperature range investigated (160-270 degrees C) for Ni60Fe20P20 the diffusion picture seems to describe the data better.

Atomic dynamics of metallic glasses and icosahedral alloys (quasicrystals)

The most recent results from the investigation of the atomic dynamics of amorphous and icosahedral alloys using neutron inelastic scattering techniques are briefly presented. Short wavelength collective excitations, low energy modes, and structural relaxation in metallic glasses are covered. The dynamics of the disorder-dominated metastable rapidly quenched icosahedral alloys is contrasted with the first very detailed results obtained most recently for the phonon density of states and the dispersion of acoustic phonons in stable slowly cooled icosahedral alloys.