Analysis of Cooperativity in Metallic Glass Forming Liquids

The relation between fragility and cooperativity of atomic motion in bulk metallic glass forming liquids is studied based on the bond strength-coordination number fluctuation model. The model describes the temperature dependence of the viscosity in terms of the mean values of the bond strength, coordination number and their fluctuations of the structural units that form the melt. According to the model, the cooperativity increases with the increase of fragility. The model estimates that the magnitude of the cooperativity NB extends approximately from 7 to 60 structural units, depending on the material. The temperature dependence of NB for different metallic glass forming systems reveals that NB increases with the decrease of temperature. The relation between NB and diffusivity of atoms is discussed briefly.