Influence of Kinetic and Thermodynamic Factors on the Glass-Forming Ability of Zirconium-Based Bulk Amorphous Alloys

Abstract

The time-temperature-transformation curves for three zirconium-based bulk amorphous alloys are measured to identify the primary factors influencing their glass-forming ability. The melt viscosity is found to have the most pronounced influence on the glass-forming ability compared to other thermodynamic factors. Surprisingly, it is found that the better glass former has a lower crystal-melt interfacial tension. This contradictory finding is explained by the icosahedral short-range order of the undercooled liquid, which on one hand reduces the interfacial tension, while on the other hand increases its viscosity. DOI: 10.1103/PhysRevLett.94.245501 PACS numbers: 61.43.Fs, 66.20.+d, 81.05.Kf, 81.10.Aj In recent years, several multicomponent alloys have been developed that can be cast in bulk amorphous form with cooling rates as low as 1 K=s [1][2][3]. Attempts have been made to explain the exceptional stability against crystallization in these systems by both kinetic and thermodynamic principles [4 -6]. From a kinetic point of view, the dynamic viscosity is an important parameter to describe the time scale for structural rearrangement of the undercooled liquid atoms for the growth of a crystal nucleus. From a thermodynamic point of view, the better glass former is expected to have a lower thermodynamic driving force for crystallization, which is given by the Gibbs free energy difference between the liquid and crystal, G. Particularly, in classical nucleation theory [7,8], the activation barrier for nucleation, G is expressed aswhere is the crystal-melt interfacial tension. It is clear from Eq. (1) that lower thermodynamic driving force and higher interfacial tension will lead to greater stability of the undercooled melt against crystallization. While a number of studies address the influence of thermodynamic and kinetic factors on the stability of undercooled liquids, no studies on their relative importance have been carried out to date. Also, there are no investigations dealing with correlations between the kinetic and thermodynamic factors to determine whether they stem from the same underlying property of the undercooled liquid. In this study, we investigate the influence of G, melt viscosity, and crystal-melt interfacial tension on the glass-forming ability of three zirconium-based bulk amorphous alloys: Zr 41:2 Ti 13:8 Cu 12:5 Ni 10 Be 22:5 (Vit1), Zr 57 Cu 15:4 Ni 12:6 Al 10 Nb 5 (Vit106), and Zr 55 Al 22:5 Co 22:5 (ZAC). These three alloys were chosen because of their widely different glass-forming abilities, but otherwise similar properties. The time-temperature-transformation (TTT) diagrams of these alloys, which give a quantitative measure of their glass-forming ability, were measured using the high-vacuum electrostatic levitation (HVESL) technique to eliminate any heterogeneous nucleation effects. A detailed description of the ESL facility is given elsewhere [9].The alloys were prepared from high purity starting materials in an arc melter. The glass-transition temperatures, T g , and the liquidus tempe...

Characteristic free volumes of bulk metallic glasses: Measurement and their correlation with glass-forming ability

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