We studied structural relaxation in the bulk metallic glass-forming alloy Zr46.8Ti8.2Cu7.5Ni10-Be27.5 on different time and length scales, with emphasis on the supercooled liquid state. Using X-ray photon correlation spectroscopy, we determined the microscopic structural relaxation time covering timescales of more than two decades in the supercooled liquid region, down to sub-second regime. Upon heating across the glass transition, the intermediate scattering function changes from a compressed to a stretched decay, with a smooth transition in the stretching exponent and characteristic relaxation time. In the supercooled liquid state, the macroscopic and microscopic relaxation time, as well as the melt viscosity all exhibit the same temperature dependence. This points to a relaxation mechanism via intrinsic structural relaxation of the majority component Zr, with its microscopic timescale controlling both the stress relaxation and viscous flow of the melt.

In glasses, secondary (β-) relaxations are the predominant source of atomic dynamics. Recently, they have been discovered in covalently bonded glasses, i.e., amorphous phase-change materials (PCMs). However, it is unclear what the mechanism of β-relaxations is in covalent systems and how they are related to crystallization behaviors of PCMs that are crucial properties for non-volatile memories and neuromorphic applications. Here we show direct evidence that crystallization is strongly linked to β-relaxations. We find that the β-relaxation in Ge15Sb85 possesses a high tunability, which enables a manipulation of crystallization kinetics by an order of magnitude. In-situ synchrotron X-ray scattering, dielectric functions, and ab-initio calculations indicate that the weakened β-relaxation intensity stems from a local reinforcement of Peierls-like distortions, which increases the rigidity of the bonding network and decreases the dynamic heterogeneity. Our findings offer a conceptually new approach to tuning the crystallization of PCMs based on manipulating the β-relaxations.

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