Viscoelasticity and viscosity of Pd–Ni–Cu–P bulk metallic glasses
Introduction
Recently, new multicomponent bulk metallic glasses with low critical cooling rates (down to 1 K/s or even less) have been developed, such as Zr- and Pd-based materials [1], [2], [3]. In these alloys the temperature interval ΔTx of supercooled liquid, defined by the difference between the glass transition temperature (Tg) and onset of crystallisation (Tx), can exceed 100 °C. Consequently, the mechanical response of these bulk metallic glasses can be investigated from the low temperature range up to high temperatures where a viscous flow is observed.
Various authors have reported data concerning the mechanical properties of new Zr- or Pd-based bulk metallic glasses, at room temperature or in a large temperature range [4], [5], [6], [7], [8], [9]. There is a clear tendency for the tensile strength to increase with increasing Young's modulus (E). The bulk amorphous alloys have higher tensile strength, higher hardness and lower Young's modulus than any kind of crystalline alloys.
At high temperature, in the supercooled liquid region, a large temperature dependence of the viscosity is observed in Pd- or Zr-base alloys, larger than that of SiO2 glass. This viscosity decrease enables a superplastic deformation. Inoue et al. [2], [6], [10] have shown in Pd-base alloys that this viscosity depends strongly on strain rate and that results can be fitted using a characteristic time τ in a stretched exponential. An adequate procedure to characterise the relaxation time and its temperature dependence is to undertake dynamic measurements using a large frequency range, including low frequencies. For instance, ultrasonic measurements give information on elasticity and not on relaxation phenomena, which are rejected at high temperature, where crystallisation can occur. In order to determine the dynamic modulus of glassy materials, mechanical spectroscopy is a very suitable technique, since only a very low stress level is used and physical parameters of the relaxation process can be determined. A lot of work has been reported on this subject on polymers or oxide glasses [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], but significantly less on metallic glasses. Schröter et al. [22] have investigated the temperature evolution of the dynamic modulus in a Pd40Ni40P20 and Zr65Al7.5Cu27.5Ni10 (all numbers indicate at.%). The studied temperature range was fairly limited. In addition, the rheological behaviour is described using a phenomenological model that ignores physical concepts related to nanometric scale structure and cannot be used, for instance, to properly describe the influence of testing parameters on other mechanical properties (viscosity or creep resistance).
Perez et al. [19], [23], [24], [25] proposed a theory for the mechanical response of amorphous materials, based on ideas well established in physical metallurgy. This theory was successfully applied for amorphous polymers, molecular glasses and oxide glasses. One of the purposes of the present work is to check the ability of this model to describe both viscoelasticity and flow behaviour in a bulk metallic glass. The Pd43Ni10Cu27P20 alloy was selected due to the large range of its supercooled liquid region [3].
Section snippets
Experimental procedure
The Pd43Ni10Cu27P20 metallic glass was prepared from the pure metals (Pd, Ni, Cu) and elemental phosphorus, by induction melting under argon atmosphere in a quartz tube and, finally, water quenched. To reduce heterogeneous nucleation, all samples were subsequently treated in B2O3 flux at 1300 °C. Ingots have a cylindrical shape (diameter: 5 mm, length: 40 mm). Specimens for X-ray diffraction, differential scanning calorimetry (DSC) and mechanical spectroscopy were machined from theses ingots.
Preliminary DSC results
In order to determine the characteristic temperature Tg (glass transition temperature) and Tx (onset of crystallisation), heating scans were performed with various heating rates using DSC. The heating trace at 1 K min−1 is shown in Fig. 1. The glass transition (endothermic phenomenon), starting at Tg=295 °C is followed by exothermic crystallisation, starting at Tx=367 °C. Consequently the supercooled liquid region corresponds to the range ΔTx=Tx−Tg=72 K. Like in other amorphous materials, glass
Discussion
The present experimental results demonstrate the occurrence of two relaxation phenomena: a β relaxation at low temperature and a main α relaxation at higher temperature. The peak temperature depends on the testing frequency. Tα is close to Tg when mechanical spectroscopy experiments are carried out at 1 Hz.
Conclusion
Measurements of the dynamic shear modulus vs temperature in a broad frequency range was performed in the Pd43Ni10Cu27P20 bulk metallic glass. Two relaxation phenomena are observed: at high temperature, in the dynamic glass transition region, the classical α relaxation occurs. This relaxation corresponds to the onset of the supercooled liquid region with the occurrence of long-range movements. At lower temperature, a sub-Tg (or β) relaxation appears, which could be attributed to the existence of
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