Studies of the Glass Transition in Metallic Glasses via Fast Scanning Calorimetry

Only recently, the development of fast differential scanning calorimetry (or chip-calorimetry) has allowed us to characterise in situ the glass transition response of metallic glasses during cooling from the liquid over a wider range of time scales employing scanning rates from 100 K/s up to 50,000 K/s. The generally accepted description is that the vitrification kinetics should exhibit the same temperature dependence as the realxation time for the alpha-process. However, we have recently observed that vitrification at deep undercooling may occur with a milder temperature dependence than the alpha-relaxation [1, 2]. The slower the system is cooled the more pronounced is the decoupling between these vitrification kinetics and the atomic mobility. As a consequence, vitrification can occur at fictive temperatures lower than those which would be obtained only accounting for the alpha-process. This apparent decoupling of the time scales for the vitrification kinetics from the time scales for the alpha-relaxation process is more pronounced at deep undercooling [1] and for small sample sizes[2]. This is of most importance because, it advocates a heterogeneity of cooperative atomic rearrangements, where faster mechanisms for atomic mobility that apparently are not contributing to the alpha-relaxation process, are maintaining the undercooled liquid system in (metastable) equilibrium and delay vitrification to lower temperatures.<br/>[1] V. Di Lisio, I. Gallino, S. S. Riegler, M. Frey, N. Neuber, G. Kumar, J. Schroers, R. Busch, D. Cangialosi, Nature Commun. 14 (2023) 4698.<br/>[2] X. Monnier, D. Cangialosi, B. Ruta, R. Busch, I. Gallino, Sci. Adv. 6, eaay1454 (2020).