Dynamic crossover in metallic glass melt detected by NMR

The structure and dynamics of melts have critical influence on the glass formation and solidification of crystals, which attracts broad interest to clarify the underlying mechanisms. In this paper, the quadrupolar spin-lattice relaxation rates of ⁶³Cu and ⁶⁵Cu nuclei in a Au ₅₀Cu _25.5Ag _7.5Si ₁₇ metallic glass melt are studied by using a home-designed high-precision high-temperature NMR equipment. The relaxation rates follow the Arrhenius behavior at high temperatures above 870 K, but become non-Arrhenius at low temperatures, which indicates a non-Arrhenius dynamical crossover. The crossover temperature T A is about 2.3 times of the glass-transition temperature ( T_g=378 K). Based on the analysis of Knight shift and magnetic relaxation rates, Cu and Si atoms are found to tend to form stronger covalent-like bonds below T A . Such an increase in bonding strength may promote the connectivity of energetically preferred short-range clusters, which is responsible for the observed sluggish dynamics.