A Kinetic Transition from Low to High Fragility in Cu-Zr Liquids

Researchers have reported that Cu-Zr liquids are kinetically strong at the best glass-forming compositions. Here we systematically study the temperature dependence of viscosity and diffusion of Cu-Zr liquids using molecular dynamics simulations, and the results illustrate that the better glass formers are actually more fragile close to the glass transition. There is a kinetic transition from low to high fragility when the optimal glass-forming liquids are quenched into glass states. This transition is associated with the more rapid decrease of the excess entropy of the liquids above and close to the glass transition temperature, T_g, compared to other compositions. Accompanied by the transition to high fragility, peaks in the thermal expansivity and specific heat are observed at the optimal compositions. Furthermore, the Stokes–Einstein relation is examined over a wide composition range for Cu-Zr alloys, and the results indicate that glass-forming ability closely correlates with dynamical heterogeneity.