Linking Structure to Fragility in Bulk Metallic Glass-Forming Liquids

Using in-situ synchrotron X-ray scattering, we show that the structural evolution of various bulk me- tallic glass-forming liquids can be quantitatively connected to their viscosity behavior in the super- cooled liquid near Tg. The structural signature of fragility is identified as the temperature dependence of local dilatation on distinct key atomic length scales. A more fragile behavior results from a more pronounced thermally induced dilatation of the structure on a length scale of about 3 to 4 atomic diameters, coupled with shallower temperature dependence of structural changes in the nearest neigh- bor environment. These findings shed light on the structural origin of viscous slowdown during undercooling of bulk metallic glass-forming liquids and demonstrate the promise of predicting the properties of bulk metallic glasses from the atomic scale structure