Papers by Keyword: Structural Relaxation

Abstract: The kinetic properties such as diffusivity and viscosity of the metal melt are the foundations to reveal the structure evolutions and the glass formation abilities during solidification of the investigated alloy, thus, to control the microstructures, defects and properties of materials. In this work, ab initio molecular dynamics simulations were utilized to investigate the kinetic and thermodynamic properties and the structural relaxations of Fe-X (X = 10-15 wt% Al, Cr, Mn and Ti, or 1-2wt% B and C) melts under various temperature and external pressure, which are in line with the interested concentration range of multi-component Fe-based alloys. The kinetics and structural relaxations are characterized by mean squared displacement, velocity autocorrelation function and self-intermediate scattering function. The thermodynamics properties including entropy and heat capacity are calculated by combining the vibrational and electronic contributions based on vibrational and electronic density of states. The predicted kinetics and thermodynamics properties under high temperature and pressure agree well with the experimental and theoretical results while the connection among structural relaxations and diffusion are revealed based on the Stokes-Einstein relation and the Hall-Wolynes (HW) relation. This work provides an insight into the structure-property relationships of metal melts, which are essential in the development of advanced multi-component Fe-based alloys.

Abstract: Electrical resistance is always related to the electronic structure of metallic glass and sensitive to structural changes, which provides a more intuitive approach to investigate structure evolution of metallic glasses upon structural relaxation. Electrical resistance relaxation of the La₅₅Al₂₅Ni₁₀Cu₁₀ bulk metallic glass was studied using the standard four-probe method. The electrical resistance of La₅₅Al₂₅Ni₁₀Cu₁₀ bulk metallic glass decreases significantly with the structural relaxation below the glass transition temperature at 445 K. During the subsequent continuous heating, the relaxed specimen shows a reduction in the resistivity decrease at the glass transition. The relaxed electrical resistance caused by the structural relaxation during the isothermal measurement equals the changes of the electrical resistance reduction in the glass transition region in the subsequent isochronal measurements. The calculated relaxed electrical resistance as a function of the annealing time can be fitted by Kohlrausche-Williams-Watts (KWW) equation. The equilibrium value, time constant and the stretched exponent for the isochronal electrical resistance measurements at 445 K are 0.0384, 2390s and 0.66, respectively. The in-situ electrical resistance data recorded in the isothermal annealing process show the same relaxation behavior with fitting parameters 0.0362, 2033 s, and 0.74, respectively.

Abstract: The Zr-Cu-Al bulk amorphous (BA) alloys, which have no long-range ordered structure, possess various properties such as high strength and toughness with compositional dependence. In the present study, in order to estimate the natures of local structure change and relaxation behavior of hyper-eutectic BA alloys during annealing, positron annihilation measurements and density measurement have been performed for hyper-eutectic Zr-Cu-Al BA alloys with annealing. The enthalpy relaxation and structural relaxation was also measured by differential scanning calorimetry (DSC) measurement. These results show that the relaxation process of free volume containing in hyper-eutectic BA alloys is different from that in hypo-eutectic alloys.

Abstract: In order to investigate the relationship between the bonding nature and the cooperative relaxation, a comparative study of the relaxation behavior in polymeric and metallic glass forming systems has been performed based on the Bond Strength–Coordination Number Fluctuation (BSCNF) model developed by the authors. In the present work, we studied the correlations between the fragility m, the Vogel temperature T₀, the degree of molecular cooperativity N_B, and the Kohlrausch exponent β_KWW. The results show that T₀ and N_B increase, whereas β_KWW decreases systematically with the increase of m. Reflecting the difference of the interatomic interactions of the materials considered, the analysis by the present study reveals that the value of N_B in ion-conducing polymers is about 5 times larger than that in metallic systems, and for each system, the material dependence of β_KWW is clearly seen in the fragility index m and the cooperativity N_B.

Abstract: An brief overview of the selected thermo-physical measurements realized in the VILA laboratories for the glass industry and for the fundamental research of glass is presented. Among the routine measurements realized for the glass industry the thermodilatometry for measuring the glass transition temperature, and linear thermal expansion coefficients of glass and metastable glassforming melt are described in detail. The fact that the glass transition temperature is not a single valued physical quantity is stressed in connection with the measurement time temperature schedule. The probably most important quantity related to the glass production technology is the viscosity. Its measurement in the range extending ten orders of magnitude is described. The combination of the falling ball method, the rotation viscosimetry and the thermomechanical analysis is needed to cover the above viscosity range. Among the methods used in the fundamental research of glass structure and properties the study of structural relaxation is overviewed. Here the own method of combined viscous flow and structure relaxation TMA measurement is described in detail.

Abstract: In this paper we have studied changes in the structure and properties of amorphous metallic alloys in annealing. We have identified the regularity of plasticity decreasing. We have determined that processes decreasing plasticity are thermally activated. The ratio of the alloy components affects the shape of the plasticity-temperature curves. Based on experimental results we have proposed an energy model of the plasticity formation in alloys in annealing, We have determined the relation between a crystallization temperature and the concentration of cobalt in ribbon metallic glasses. We have constructed diagrams of the time-temperature stability which allow establishing acceptable exploitation modes of amorphous metallic alloys and products made from them.

Abstract: In the present work, a detailed study of the structural relaxation through the complex inductance response by using Inductance Spectroscopy (IS) of Fe-based amorphous ribbons obtained by as quenching ultra-rapid technique, as a function of frequency form 4 to 400 kHz and under thermal treatment during 10, 20, 40, 60, 120 and 180 min, is presented. The analysis of experimental results of IS plots of real (L ́) and imaginary (L ́ ́) inductance show evidence of magnetization processes associated with domain walls: At low fields and low frequencies, L ́ showed a plateau, followed by a dispersion with a relaxation character, the relaxation frequency is about 100 kHz (domain wall bulging). For higher fields, the inductance value depended on the field amplitude (domain wall displacement). As the frequency increased, all the curves merged into the low field plot and exhibited the same relaxation dispersion. The dependence of magnetization processes on the domain wall is determined and interpreted on the basis of the domain wall motion equation. In this way the structural relaxation associated with domain dynamic is discussed and we can establish criteria for the design of certain filters at low frequencies (from 4 to 50 kHz).

Abstract: Firstly, this paper reminds the reader of some basic facts about the glassy state, then of the various ways to produce amorphous metals with particular emphasis on the route of vitrification from the melt. Vitrification of an undercooled melt is the most important route from the viewpoint of the application of metallic glasses. We compare diffusion in some metallic glasses with related crystalline metals. Glassy metals, also called metallic glasses, comprise conventional [1] and bulk metallic glasses [2,3]. We remind the reader of the major experimental techniques for diffusion studies in metallic glasses. The paper then reviews our current understanding of diffusion in glassy metals (see also [4,5,6]), including conventional as well as bulk metallic glasses and undercooled melts. We cover the temperature dependence of diffusion in metallic glasses and discuss the spectrum of activation parameters of glassy metals and its difference to the corresponding one of crystalline metals. We mention the pressure dependence and the isotope effect and we discuss tracer diffusion and viscosity diffusion for a bulk metallic glass and its undercooled melt. Finally we mention computer simulations of atomic jump processes. The diffusion mechanism in metallic glasses differs from that in crystalline metals and involves thermally activated, highly collective (chain-like or caterpillar-like) diffusion jumps. Finally, we mention diffusion along shearbands in a plastically deformed glassy metal.

Abstract: Structural relaxation through isothermal annealing below the glass transition temperature was conducted on a Zr_64.13Cu_15.75Ni_10.12Al₁₀ bulk metallic glass. Differential scanning calorimetry was used to quantify enthalpy differences between the as-cast and relaxed samples, which were then related to average free volume differences. The influence of structural relaxation on plasticity was examined. While the free volume decreasement can be clearly observed between the as-cast and relaxed samples, structural relaxation is not accompanied by severe embrittlement.

Abstract: Structural relaxation an creep strain recovery of the amorphous metallic glass Fe40Ni41B19 after the longtime loading at different annealing temperatures below the glass transition temperature have been studied using anisothermal differential scanning calorimetry (DSC) and dilatometry (TMA). It has been demonstrated that structural relaxation effects depend on the stress-annealing temperature of the amorphous ribbon. The influence of the stress-annealing temperature on the rearrangement of deformation defects in the amorphous alloy Fe-Ni-B was analyzed. The properties of local disordering in flow defects were analyzed by means of their activation energy spectra. In the samples stress-annealed at higher temperatures, only the relaxation centers favourably oriented to the external stress contribute to the creep strain recovery process.