Correlation between fragility and cooperativity in bulk metallic glass-forming liquids

It is known that the structural relaxation in supercooled liquid and glassy metals exhibit a non-Debye type relaxation behavior and a VFT-like temperature dependence. Such behaviors are often characterized by the exponent of the Kohlrausch–Williams–Watts (KWW) function and by the value of the fragility. In the present work, a quantity NB which gives the number of structural units involved in the thermally activated viscous flow is introduced to characterize the bulk metallic glass-forming liquids. NB is defined from the Bond Strength – Coordination Number Fluctuation (BSCNF) model of the viscosity. It is shown that NB can be a new indicator that quantifies the degree of cooperativity within the bulk metallic glass-forming liquids. For several bulk metallic glass-forming liquids, it is shown that the characteristic temperature ratio T0/Tg, NB, and the stretched exponent βKWW are mutually correlated through the fragility index m derived from the BSCNF model.