Liquid-Like Versus Stress-Driven Dynamics in a Metallic Glass Former Observed by Temperature Scanning XPCS

X-ray photon correlation spectroscopy (XPCS) under temperature scan conditions can only be sporadically found in literature. In terms of metallic glass formers, all hitherto existing XPCS studies have been measured isothermally. In the present work, we use state of the art detector technology and the high flux of a fourth-generation synchrotron source (ESRF, ID10) to study metallic glass formers upon heating and cooling with 1 K/min through the glass, glass transition and supercooled liquid (SCL). The obtained intensity autocorrelation functions, g₂, are fitted using the Kohlrausch-William-Watts (KWW) model. High fit quality is obtained in the SCL state, allowing to precisely define the temperature-dependent relaxation time τ, and therefore the fragility. In the glass and especially the glass transition region, the conventional KWW approach fails to model the g₂ decay. Instead, we demonstrate that a multiplication of two KWW functions allows to describe the complex shape. Within the glass transition region, the fit parameters of the two separate KWW fits decouple massively. While one fit reflects non-equilibrium dynamics, showing a compressed decay and Arrhenius-like temperature dependence of the relaxation time, the other fit features liquid-like characteristics, being stretched and following a VFT-like relaxation time behaviour.<br/>We present an approach that interprets these findings as the superposition of heterogeneous liquid-like and stress-driven ballistic-like atomic motions. This work not only extends the dynamical range probed by standard isothermal XPCS, but also clarifies the fate of the α-relaxation across the glass transition and provides a new perception on the anomalous, compressed temporal decay of the density-density correlation functions observed in metallic glasses and many out-of-equilibrium soft materials.<br/>Overall, temperature scanning XPCS appears to be a highly potent method to observe rate-sensitive effects like glass transitions, phase separations, or liquid-liquid transitions, which are otherwise difficult to characterize isothermally.