The shear viscosity of liquid Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 has been measured. At the liquidus temperature we find an extremely high viscosity of 2.5 Pa s, favoring glass formation. At deep supercooling the time scales for the diffusion of small and medium sized atoms as reported in the literature decouple from the internal relaxation time as probed by our viscosity measurements. Similarly, crystallization from the supercooled liquid state can be described with an effective diffusivity that scales with the viscosity at high temperatures and is Arrhenius-like at deep supercooling. [S0031-9007(99)08631-7] PACS numbers: 61.43. -j, 64.70.Pf, 66.10.Cb, 66.20. + d Monatomic and binary metallic liquids at the melting point have viscosities h of about 10 23 Pa s and diffusivities on the order of 10 29 m 2 s 21 [1]. Their temperature dependencies can be described by apparent activation energies Q of less than 0.5 eV [2]. By containerless processing or fluxing techniques, these liquids may be supercooled below their melting points, but critical cooling rates to bypass crystallization and form a glass are typically 10 7 to 10 9 K s 21 .Recently, diffusion [3-6] and viscosity measurements [7-9] as well as crystallization studies [10][11][12] in the deeply supercooled liquid of multicomponent bulk metallic glass (BMG) forming alloys have found considerable attention. Still, only limited data are available for transport coefficients in the equilibrium or slightly supercooled liquid of BMG forming alloys. However, the temperature range between the liquidus temperature, T liq , and the temperature with the minimum time to crystallization, T n , is decisive for the glass forming ability of a liquid upon constant cooling [13]. In the following we will present results from viscosity measurements and crystallization studies on Zr 41.2 Ti 13.8 Cu 12.5 Ni 10.0 Be 22.5 (V1) and discuss the time scales for viscous flow and crystallization along with published results on atomic transport.We have designed a high temperature Couette viscometer to measure rheological properties of Zr-based glass forming alloys in the viscosity range from 10 22 Pa s to 10 3 Pa s. The viscometer's concentric cylinder shear cell is machined from graphite and is mounted vertically inside a high vacuum induction furnace. The outer cylinder is attached to a static torque sensor and the temperature is measured with a thermocouple mounted inside the cylinder wall. Spatial temperature variations within the shear cell are less than 64 K as observed by optical pyrometry. Under continuous rotation of the inner cylinder the static torque on the outer cylinder is proportional to the viscosity of the liquid. The proportionality constant can be calculated from the rotational frequency and the geometry of the shear cell [14].Results obtained from the high temperature viscometer as well as from beam bending rheometry [9] are summarized in Fig. 1. At the liquidus temperature, T liq 1026 K, we find a viscosity of 2.5 Pa s with a slope [2] of 2.0 eV, both of which are...

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The critical cooling rate as well as the thermal stability are measured for a series of alloys in the Zr-Ti-Cu-Ni-Be system. Upon cooling from the molten state with different rates, alloys with compositions ranging along a tie line from (Zr 70 Ti 30 ) 55 (Ni 39 Cu 61 ) 25 Be 20 to (Zr 85 Ti 15 ) 55 (Ni 57 Cu 43 ) 22.5 Be 27.5 show a continuous increase in the critical cooling rate to suppress crystallization. In contrast, thermal analysis of the same alloys shows that the undercooled liquid region, the temperature difference between the glass transition temperature and the crystallization temperature, is largest for some compositions midway between the two endpoints, revealing that glass forming ability does not correlate with thermal stability. The relationship between the composition-dependent glass forming ability and thermal stability is discussed with reference to a chemical decomposition process. 3 GFA ͑represented by critical cooling rate͒ scales with the reduced glass transition temperature T rg defined as the glass transition temperature T g divided by the liquidus temperature T l . This correlation has been confirmed in many experiments ͑see Ref. 4 for summary͒. Thermal stability in metallic glasses is usually quantified by measuring the temperature difference ⌬T between the glass transition and the first crystallization event upon heating at a constant rate. For some systems, it has been demonstrated that larger values of ⌬T tend to be associated with lower values of critical cooling rate R c . 5,6 As a result, the thermal stability has served as an indicator of GFA in these alloys.In recent years, the crystallization of Vit1 has been extensively examined.7-11 Several studies of this alloy have revealed a tendency to undergo chemical decomposition in the undercooled liquid, 7-10 which has a direct influence on the subsequent nucleation and growth of crystalline phases. Since the decomposition occurs at a temperature close to T g , the isothermal crystallization behavior of Vit1 for low undercooling is quite different from its behavior when deeply undercooled.11 In addition, a study by Schroers et al. 12 involving constant heating and cooling experiments has shown that Vit1 crystallizes in a different manner upon heating from the amorphous solid than upon cooling from the melt. In this study, a cooling rate of approximately 1 K/s was required to bypass crystallization during cooling, whereas a heating rate of 200 K/s was necessary to avoid crystallization of a detectable volume fraction. Chemical decomposition has also been observed in a series of alloys which lie along the tie line between ͓Vit1͑-a͔͒, Vit1, Vit1a, Vit1b, Vit1c, and Vit4, i.e., alloys with xϭ13-19, respectively. In order to determine the GFA of the alloys, continuous cooling rate experiments have been performed to establish R c for each. These results are compared with ⌬T and T rg values obtained through calorimetric methods, and a chemical decomposition process in the undercooled liquid is used to explain how a disparity betw...

Crystallization behavior and equilibrium viscosity of a series of alloys in the Zr-Ti-Cu-Ni-Be system are studied using multiple techniques to determine the various contributions to glass-forming ability. Lowtemperature time-temperature-transformation diagrams of alloys whose compositions lie at equally spaced points along the tie line from Zr 38.5 Ti 16.5 Cu 15.25 Ni 9.75 Be 20 to Zr 46.25 Ti 8.25 Cu 7.5 Ni 10 Be 27.5 are measured during isothermal annealing of initially amorphous specimens. Surprisingly, for all investigated alloys, a primary quasicrystalline phase forms at a rate which varies substantially with alloy composition. Subsequent constant heating measurements, x-ray-diffraction patterns obtained after various states of annealing, beam bending viscosity results, and previous thermal analysis are all used to describe the influences on crystallization in this series. The description of both the kinetic and thermodynamic aspects of crystallization allows for an explanation of the crystallization mechanism. In addition, it explains why, in this series, thermal stability is greatest in those alloys with the poorest glass-forming ability. Overall, the investigations reveal that simple criteria like thermal stability or high viscosity fail to predict the glass-forming ability in complex bulk glass-forming systems.

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