Thermodynamic and kinetic fragilities of Mg-based bulk metallic glass-forming liquids
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H-J. Fecht Institute of Micro and Nanomaterials, Faculty of Engineering, Ulm University, 89081 Ulm, Germany; and Research Center Karlsruhe, Institute of Nanotechnology, 76021 Karlsruhe, Germany (Received 18 February 2008; accepted 18 July 2008)
The thermodynamic and kinetic fragilities of two near-eutectic Mg-based bulk metallic glass (BMG)-forming liquids, Mg61Cu28Gd11 and Mg59.5Cu22.9Ag6.6Gd11, were investigated using high-precision differential scanning calorimeter (DSC). The thermodynamic fragility denoted as F3/4 was determined by evaluating the temperature dependence of the excess entropy Sex. The heating rate dependence of the relaxation time at the glass transition temperature was investigated to measure the kinetic fragility. A positive correlation between the thermodynamic and kinetic fragilities could be established in Mg-based BMG-forming liquids on the basis of Adam-Gibbs equation in contrast to a number of other BMGs.
I. INTRODUCTION
A new type of amorphous materials, bulk metallic glasses (BMGs), have been attracting tremendous attention because of their potential applications based on unique properties such as supreme strength and excellent erosion resistance.1–4 Their high thermal stability against crystallization and liquid/liquid phase separation in the undercooled liquid state makes them attractive for the research in liquid science. Angell proposed the classical fragility concept5 for the liquid defined in terms of the deviation of the relaxation time temperature dependence from simple Arrhenius behavior. In this framework, the strongest glass-forming liquids (SiO2 and GeO2) showed near-Arrhenius behavior indicated as the fragility parameter D* of about 100, whereas the most fragile systems (polycarbonates and polyvinyl) exhibited a large departure from the Arrhenius law with D* of 2. The steepness index, m(Tg), defined as m ⳱ |d log /d(Tg /T) |T⳱Tg,, was also employed to evaluate the kinetic fragility. The m value varies from 16 for the strongest liquids to about 200 for the other extreme.6 BMG-forming liquids could be classified into intermediate liquids, which research proved in some BMG-forming systems with D* of 10 to 30 and m of 30 to 90.7–18 In more recent studies,19 the concept of thermodynamic fragility was introduced based on the temperature dependence of the excess en-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0347 2816
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 10, Oct 2008 Downloaded: 14 Mar 2015
tropy, Sex. Martinez and Angell19 found a connection between thermodynamic and kinetic fragilities for a series of glass-forming liquids, including molecular, covalent, and ionic glass formers. Although, it was challenged by the research in polymer and some molecular liquids.20 In this work, we report on investigations of the fragilities of two new types of Mg-based BMG-forming liquids, Mg61Cu28Gd11 and Mg59.5Cu22.9Ag6.6Gd11. Since Inoue’s report in 1992,21 Mg-based BMGs have been extensively investigated because
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