Ch. Cunat

A thermodynamic theory of relaxation based on a distribution of non-linear processes

Abstract Relaxation phenomena are explained by an irreversible thermodynamic approach accounting for dissipation processes in the non-linear range. Perturbated systems are treated by means of fluctuation thermodynamics to depict their spontaneous evolutions. A ‘universal’ master curve is derived to explain various recovery processes.

Determination electrochimique, par pile a electrolyte solide CaF2, des proprietes thermodynamiques de formation des alliages binaires (Ca, Cu) entre 800 et 1300 K

Abstract Thermodynamic quantities of formation of the Caue5f8Cu system intermetallic phases are, for the first time, directly determined by means of electromotive force measurements. The alloy sample is prepared, in situ, on a copper point electrode by electrolysis. The activity of calcium in Caue5f8Cu alloys is directly compared with that of pure calcium (liquid or solid) produced in the same conditions on an inert (iron) electrode.

Complex behaviour of specific heat during structural relaxation of Fe73Co12B15 metallic glass

Abstract The non-isothermal DSC thermograms of the metallic glass Fe73Co12B15 were investigated. The complex analysis of the structural relaxation anomalies of the apparent specific heat of metallic glass samples was performed. The influence of ageing, pre-annealing and more complicated heat treatments, as well as of the heating rate, on the saturation of the structural relaxation enthalpy was demonstrated. The relaxation exotherm is principally connected with the rapid cooling of the sample. The relaxation endotherm represents the retarded approach to a certain heat treatment (non-isothermal crossover effect). All results can be generalized and interpreted by the DNLR (distribution of non-linear relaxation) model.

Relaxation parameters to simulate the changes in magnetostriction in amorphous magnetic alloys

Abstract We propose a method for deducing the parameters of the non-linear relaxation distribution model from isothermal measurements. Following this thermodynamic theory, isothermal relaxations can be described as isokinetic processes over a reduced time scale, taking into account non-linear effects, i.e. integrating the sample history. The proposed method leads to the definition of three characteristic parameters of this new time scale. In addition, the model predicts a universal shape of relaxation spectrum for frozen-in fluctuations. Applying this method to magnetostriction changes in Vitrovac® 6025 (Co67Fe4Mo1.5Si16.5B11), we are able to simulate various thermal histories starting from high temperature equilibrium in the liquid state. We found good agreement with experiments. Free-energy spectra are determined.

Etude par effusiometrie de torsion des proprietes thermodynamiques en phase solide des laitons binaries (Cu, Zn)

Abstract The Gibbs thermodynamic variables which we have determined for thirteen brass alloys (Cu, Zn) are in good accord with the literature. The experimental conditions were as close as possible to those of sublimation in static vacuum. The application of the test of the third law in β-brass shows that our free enthalphy measurements are in agreement with the measurements of specific heat by Moser. The primary solid solution α copper of the binary brasses shows quasi-regular behaviour over the whole composition range. Our measurements seem to indicate the existence of a β′/(α + γ) eutectoid at a temperature less than 100°C. The high degree of order of the γ phase is seen in the large negative values of excess entropy.

An extended review of structural relaxation models with the mutual correlation of their parameters

Abstract Both the metallic and chalcogenide glasses possess the exothermal, endothermal and λ-shaped effects of the apparent specific heat cp,a(T) in the differential scanning calorimeter. The kinetics of structural relaxation of Kissinger, Moynihan and distribution of non-linear relaxation models are used to interpret these three phenomena. Because of the consistency of the models, their parameters are mutually related. The activation enthalpies ΔH Kiss ∗ (T a , t a ) and ΔH M ∗ (T a , t a ) were predicted from the material parameters ΔH DNLR ∗ , ΔS DNLR ∗ and KDNLR and the heat treatment history of the sample.

A new concept of effective temperature to correlate isothermal and anisothermal continuous temperature-time-transformation curves-application to crystallization of FeNiB amorphous alloys☆

Abstract In discussing the additivity rule of reaction times we consider the following points. 1. (1) We recall the mathematical origins of this rule and show the limits of its applicability. 2. (2) We show that the empirical Grange and Kiefer method can be considered to be a first approximation of this law. 3. (3) We examine Cahns improvements to extend the range of the additivity rule of anisothermal kinetic reactions and observe that they require some new experimental information in addition of the knowledge of isothermal temperature-time-transformation curves. 4. (4) We propose a new concept of effective reaction temperatures and effective reaction times. Thus the additivity rule can be extended to non-isothermal kinetics. We apply this method to predict anisothermal onsets of crystallization for twenty Feue5f8Niue5f8B amorphous alloys.

Theoretical analysis of stress relaxation measurements by bend tests for (Fe50Ni50)1−xBx glasses: alloying effects on aging

Abstract We deduce the parameters of the non-linear relaxation distribution model (DNLR) from isothermal measurements. Following this thermodynamic theory, we present a theoretical application of stress relaxation experiments by bend tests of (Fe50Ni50) 1−xBx quenched metallic glasses. From radius of curvature measurements we deduce the three characteristic parameters of the DNLR model: the activation enthalpy ΔH+, the activation entropy ΔS+ and the non-linear parameter Knle. We can thus determine the effect of the boron concentration on the aging of these alloys.

Instability and relaxation of electrodeposited Ni75P25 amorphous alloys: experimental aspects quantitative theoretical predictions of isothermal and anisothermal aging using the cunat non-linear relaxation distribution model

Abstract The aging of electrodeposited Ni75P25 alloys is studied by means of expansion relaxation experiments. The interpretation proceeds within the framework of the non-linear relaxation distribution model. Relaxations appear non-linear and the initial distribution of densities suggests positive and negative excess density regions. For as quenched samples Cunat showed that the initial distribution is related to positive excess densities regions.

A relaxation theory to explain the rheology of elasto-plastic materials

Abstract Rheological behaviour is described by an approach of relaxation using irreversible thermodynamics. Compared with structural relaxation or inelastic relaxation for low stresses, already depicted in a companion paper [these Proceedings, p. 196], elasto-plastic behaviour at high stress levels is characterized by a large non-linearity induced by the applied strain rate.