RESUMEN
Nonexponential relaxations are universal characteristics for glassy materials. There is a well-known hypothesis that nonexponential relaxation peaks are composed of a series of exponential events, which have not been verified. In this Letter, we discover the exponential relaxation events during the recovery process using a high-precision nanocalorimetry, which are universal for metallic glasses and organic glasses. The relaxation peaks can be well fitted by the exponential Debye function with a single activation energy. The activation energy covers a broad range from α relaxation to ß relaxation and even the fast γ/ß' relaxation. We obtain the complete spectrum of the exponential relaxation peaks over a wide temperature range from 0.63Tg to 1.03Tg, which provides solid evidence that nonexponential relaxation peaks can be decomposed into exponential relaxation units. Furthermore, the contribution of different relaxation modes in the nonequilibrium enthalpy space is measured. These results open a door for developing the thermodynamics of nonequilibrium physics and for precisely modulating the properties of glasses by controlling the relaxation modes.
RESUMEN
The influence of physical aging on the ß relaxation in La60Ni15Al25 bulk metallic glass has been investigated by mechanical spectroscopy. The amplitude of the ß relaxation (ΔGâ³) decreases while its relaxation time (τ(ß)) increases during aging. We find that, as in organic glasses, the changes of ln (τ(ß)) and ln (ΔG(max) ) are linearly correlated with ln (τ(ß)) = b - a ln (G(max)â³). This behavior is discussed in term of the asymmetric double-well potential (ADWP) model, with U and Δ the energies characterizing the ADWP. It is suggested that during aging the ratio U/Δ remains approximately constant, with a value close to the coefficient describing the linear correlation between ln (τ(ß)) and ln (G(max)â³)(U/Δ ~ a). Moreover, the evolution versus aging time of ΔG(max) can be described by a simple stretched exponential equation giving values of τ(aging) consistent with tan(δ) measurements during aging. The very similar behavior of the ß relaxation during aging in metallic glasses and organic material strongly suggests a common nature for this relaxation.
RESUMEN
Studying complex relaxation behaviors is of critical importance for understanding the nature of glasses. Here we report a Kovacs-like memory effect in glasses, manifested by non-monotonic stress relaxation during two-step high-to-low strains stimulations. During the stress relaxation process, if the strain jumps from a higher state to a lower state, the stress does not continue to decrease, but increases first and then decreases. The memory effect becomes stronger when the atomic motions become highly collective with a large activation energy, e.g. the strain in the first stage is larger, the temperature is higher, and the stimulation is longer. The physical origin of the stress memory effect is studied based on the relaxation kinetics and the in-situ synchrotron X-ray experiments. The stress memory effect is probably a universal phenomenon in different types of glasses.
RESUMEN
Hightemperature deformation around the glass transition temperature Tg and the dynamic mechanical behavior of La30Ce30Al15Co25 metallic glass were investigated. According to dynamic mechanical analysis (DMA) results, La30Ce30Al15Co25 metallic glass exhibits a pronounced slow ß relaxation process. In parallel, strainrate jump experiments around the glass transition temperature were performed in a wide range of strain rate ranges. The apparent viscosity shows a strong dependence on temperature and strain rate, which reflects the transition from nonNewtonian to Newtonian flow. At low strain or high temperature, a transition was observed from a nonNewtonian viscous flow to Newtonian viscous flow. It was found that the activation volume during plastic deformation of La30Ce30Al15Co25 metallic glass is higher than that of other metallic glasses. Higher values of activation volume in La30Ce30Al15Co25 metallic glass may be attributed to existence of a pronounced slow ß relaxation. It is reasonable to conclude that slow ß relaxation in La30Ce30Al15Co25 metallic glass corresponds to the "soft" regions (structural heterogeneities) in metallic glass.
RESUMEN
We study the mechanical relaxation spectrum of Pd42.5Ni7.5Cu30P20 metallic glass. The effect of aging on the relaxation behavior is analyzed by measuring the internal friction during consecutive heating runs. The mechanical relaxation of the well-annealed glass state is modeled by fitting susceptibility functions to the primary and secondary relaxations of the system. The model is able to reproduce the mechanical relaxation spectrum below the glass transition temperature (sub-Tg region) in the frequency-temperature ranges relevant for the high temperature physical properties and forming ability of metallic glasses. The model reveals a relaxation spectrum composed by the overlapping of primary and secondary processes covering a wide domain of times but with a relatively narrow range of activation energies.
RESUMEN
The dynamics of Pd-based metallic glass-forming liquids (Pd(40)Ni(10)Cu(30)P(20), Pd(42.5)Ni(7.5)Cu(30)P(20), Pd(40)Ni(40)P(20), and Pd(30)Ni(50)P(20)) was studied by mechanical spectroscopy and modulated differential scanning calorimetry (MDSC). We found that the change in composition has a significant effect on the α relaxation dynamics; the largest difference corresponds to an increase of the glass transition temperature Tg of â¼ 15 K, for materials in which 30% Ni was substituted by 30% Cu (i.e., from Pd(40)Ni(40)P(20) to Pd(40)Ni(10)Cu(30)P(20)). We also found that all Pd-based metallic glasses have very similar fragilities, 59 < m < 67, and Kohlrausch stretched exponents, 0.59 < ßKWW < 0.60. It is interesting that the values of m and ßKWW correlate well with the general relation proposed by Böhmer et al. for nonmetallic glass formers (Böhmer, R.; et al. J. Chem. Phys. 1993, 99, 4201-4209), which for the observed ßKWW values predicts 58 < m < 61. From a linear deconvolution of the α and ß relaxations, we find that the substitution of the Ni with Cu induced a large change in the time constant of the Johari-Goldstein relaxation, τß. The activation energy, Uß, of the ß relaxation was largely independent of chemical composition. In all cases, 25 < Uß/RT < 28, a range in agreement with results for other glass formers (Kudlik, A.; et al. Europhys. Lett. 1997, 40, 649-654 and Ngai, K. L.; et al. Phys. Rev. E 2004, 69, 031501). From the heat capacity and mechanical loss, estimates were obtained for the number of dynamically correlated units, Nc; we find significantly larger values for these metallic glass-forming liquids than Nc for other glass-forming materials.
RESUMEN
The relaxational dynamics in metallic glasses (MGs) is investigated by using mechanical spectroscopy. The spectra show that in MGs there are two relaxations: (i) the α relaxation, linked to the glass transition, as observed in other classes of amorphous materials; and (ii) the ß relaxation, well observed below the glass transition, with an intensity strongly dependent on the MG composition, the nature of which has been linked to the local microstructure of MGs. For the investigated MGs we find that the intensity and relaxation time of the ß relaxation depends, in a reproducible fashion, on the thermal history of the samples. During aging experiments, the intensity decreases (as well as the τß) with a time dependence described well by a stretched exponential, with an exponent ß(aging) independent of the driving frequency. Moreover, we find that the activation energy Uß and the peak temperature Tßp of the ß relaxation follow the approximate relationship: Uß ≈ 31.5RTßp (for driving frequency 1 Hz), indicating that the high temperature limit of the peak frequency is approximately the same for all the MGs investigated. Finally, the frequency separation of the α and ß processes in the mechanical loss spectra for La-and Pd-based metallic glasses is tested against the prediction of the Coupling Model.