Vitrification decoupling from α-relaxation in a metallic glass, X. Monnier (a), D. Cangialosi (a,b), B. Ruta (c), R. Busch (d) and I. Gallino (d), Sci. Adv. 6, eaay1454 (2020);
https://doi.org/10.1126/sciadv.aay1454. (a) Donostia International Physics Center, San Sebastián (Spain) (b) Centro de Física de Materiales (CSIC- UPV/EHU), Sebastián (Spain)
(c) Université Lyon, Universitè Claude Bernard Lyon 1, CNRS (France) (d) Chair of Metallic Materials, Saarland University, Saarbrücken (Germany)
 S. Hechler et al., Phys. Rev. Mater. 2, 085603I (2018).  B. Ruta et al., Phys. Rev. Lett. 125/5, 05570127 (2020).
PRINCIPAL PUBLICATION AND AUTHORS
function. This quantity was measured for the same alloy in the deep undercooled state on approaching the glass transition using X-ray photon correlation spectroscopy (XPCS) [1-2]. These experiments demonstrate that the atomic dynamics in the deeply undercooled state are heterogeneous and involve microscopic relaxation processes that are much faster than the structural α-relaxation, which reflects their multicomponent nature. This implies a variety of different atomic motions exhibiting various timescales that are profoundly decoupled from the α-relaxation. Combining XPCS data with FSC data revealed that these faster motions do not involve appreciable atomic rearrangement connected to the α-relaxation but are responsible for maintaining the system at equilibrium. The implication of this new finding is that the vitrification is delayed to lower temperatures compared to what would be expected exclusively for the α-relaxation.
The origin of this decoupling reflects the fact that the melt comprises atoms of various kinds and of very different sizes, in this case gold, copper, silver, palladium and silicon. When the large atoms, like gold, are frozen and essentially immobile, the smaller atoms, like silicon and copper, can still move around and need extra time to freeze themselves into their energetically preferred positions. Until then, the system continues to exhibit liquid-like features. It is only when the smaller atoms finally freeze that the liquid fully vitrifies and, therefore, the associate glass transition temperature, called the fictive temperature Tf, is lower than that expected of the α-relaxation process.
XPCS was employed at beamline ID10 to attain data for microscopic dynamics in terms of structural relaxation times at deep undercooling in the range from 1 to 1000 s (Figure 50). The freezing process was studied separately using a commercial FSC calorimeter by means of two studies, one to determine the vitrification kinetics (star symbols) and the other to probe the atomic mobility of the BMG former (green circles). The first is based on the detection of the limiting Tf (i.e., the temperature at which a glass formed after cooling at a given rate would be at equilibrium). The second is based on a step
response analysis, which provided information on the atomic mobility via the temperature and frequency dependence of the complex- specific heat. The employment of FSC enables characterisation over a wide range of timescales (down to 0.001 s) that are inaccessible by other techniques. Moreover, the data of the dynamic glass transition temperature as obtained from the step response analysis (Tg,dyn in Figure 50) is directly correlated with the results of the XPCS investigations of the α-relaxation, and were used in modelling the kinetic slowdown of the liquid dynamics during cooling (VFT-fit in Figure 50).
The results of Figure 50 show that the cooling rate dependency of the vitrification kinetics, as identified by Tf, is decoupled from the temperature- dependence of the α-relaxation. In particular, the former follows milder temperature-dependence in comparison to the α-relaxation that induces a vitrification delay to lower temperature. This has great implications on the properties of glasses and implies a reconsideration of the glass transition mechanism. It also suggests a new way of tuning the properties of glasses, like mechanical, magnetic or optical properties, which are intimately connected to Tf.
Fig. 50: Timescale for vitrification and α-relaxation extracted from FSC and XPCS data as a function of inverse temperature. Data from dynamic mechanical
spectroscopy (DMA) and enthalpy recovery after ageing (DSC) complemented the work. Inset: Optical photograph of the sample furnace site of a FSC sensor.
The size of the active area (bright circle) is 0.5 mm in diameter.