Dynamic crossover in metallic glass melt detected by NMR

Abstract

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 Au50Cu25.5Ag7.5Si17 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 TA is about 2.3 times of the glass-transition temperature (Tg=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 TA. Such an increase in bonding strength may promote the connectivity of energetically preferred short-range clusters, which is responsible for the observed sluggish dynamics.