An expression for the temperature dependence of the viscosity and fragility is derived based on a simple model of the melt. According to the model, the fragility is determined by the relaxation of structural units that form the melt, and is described in terms of the bond strength, coordination number, and their fluctuations of the structural units. It is shown that the fragility of some metallic glass forming liquids such as Pd 40 Ni 40 P 20 , La 55 Al 25 Ni 20 and Zr 65 Al 10 Ni 10 Cu 15 are quite well reproduced by the model. The application of the theory to La 55 Al 25 Ni 20 has revealed that the fluctuation in the bond strength between the structural units is about 6% and that the viscous flow occurs when the bonds in about 8 adjoining structural units are broken.

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