J.C. Qiao

Relaxation of Bulk Metallic Glasses Studied by Mechanical Spectroscopy

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 β 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 β relaxation depends, in a reproducible fashion, on the thermal history of the samples. During aging experiments, the intensity decreases (as well as the τβ) with a time dependence described well by a stretched exponential, with an exponent β(aging) independent of the driving frequency. Moreover, we find that the activation energy Uβ and the peak temperature Tβp of the β relaxation follow the approximate relationship: Uβ ≈ 31.5RTβp (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 β processes in the mechanical loss spectra for La-and Pd-based metallic glasses is tested against the prediction of the Coupling Model.

Characteristics of the structural and Johari-Goldstein relaxations in Pd-based metallic glass-forming liquids.

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 ∼ 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 < βKWW < 0.60. It is interesting that the values of m and βKWW 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 βKWW values predicts 58 < m < 61. From a linear deconvolution of the α and β relaxations, we find that the substitution of the Ni with Cu induced a large change in the time constant of the Johari-Goldstein relaxation, τβ. The activation energy, Uβ, of the β relaxation was largely independent of chemical composition. In all cases, 25 < Uβ/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.

Effect of physical aging on Johari-Goldstein relaxation in La-based bulk metallic glass

The influence of physical aging on the β relaxation in La60Ni15Al25 bulk metallic glass has been investigated by mechanical spectroscopy. The amplitude of the β relaxation (ΔG″) decreases while its relaxation time (τ(β)) increases during aging. We find that, as in organic glasses, the changes of ln (τ(β)) and ln (ΔG(max) ) are linearly correlated with ln (τ(β)) = b - a ln (G(max)″). 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(β)) and ln (G(max)″)(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 β relaxation during aging in metallic glasses and organic material strongly suggests a common nature for this relaxation.

Aspect ratio effects on the serration dynamics of a Zr-based bulk metallic glass

Based on the constant strain rate compressive experiments to a Zr-based bulk metallic glass, the aspect ratio effects on the statistical properties of serration is systematically investigated. In the plastic deformation state, as the aspect ratio decreases, the stress drop magnitude increases dramatically and the reloading time in one serration event increases significantly. Moreover, distribution of the elastic energy density of each serration event is predicted, which obeys the squared exponential decay law. The size effect law is adopted to describe the serration dynamic-related parametric variation with different aspect ratios. It is noted that the scaling exponent reduces with the increase of the aspect ratio. In general, the effect of aspect ratio on the serration flow could be explained by the shear-band stability index.

Modeling of the Sub-Tg Relaxation Spectrum of Pd42.5Ni7.5Cu30P20 Metallic Glass

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.

Slow β relaxation in La-based metallic glasses based on mechanical spectroscopy measurements

Dynamic mechanical relaxations of La-based metallic glasses were investigated by mechanical spectroscopy. In the framework of the mixing enthalpy of constituent atoms, it was found that β relaxation was less evident by the addition of Cu to replace Ni in the LaCuNiAl glassy alloy. By introducing Cu into the LaNiAl metallic glass, the mixing enthalpy was less negative, which led to weaker β relaxation of the metallic glasses. The a relaxation of the La-based metallic glasses could be described by a Kohlrausch-Williams-Watts (KWW) function with a Kohlrausch exponent βKWW around 0.5. It should be noted that physical aging above the glass transition temperature Tg induced a decrease of β relaxation intensity in the La-based metallic glass.