Abstract
Understanding and controlling physical aging, that is, the spontaneous temporal evolution of out-of-equilibrium systems, represents one of the greatest tasks in material science. Recent studies have revealed the existence of a complex atomic motion in metallic glasses, with different aging regimes in contrast with the typical continuous aging observed in macroscopic quantities. By combining dynamical and structural synchrotron techniques, here for the first time we directly connect previously identified microscopic structural mechanisms with the peculiar atomic motion, providing a broader unique view of their complexity. We show that the atomic scale is dominated by the interplay between two processes: rearrangements releasing residual stresses related to a cascade mechanism of relaxation, and medium range ordering processes, which do not affect the local density, likely due to localized relaxations of liquid-like regions. As temperature increases, a surprising additional secondary relaxation process sets in, together with a faster medium range ordering, likely precursors of crystallization.
Highlights
Understanding and controlling physical aging, that is, the spontaneous temporal evolution of out-of-equilibrium systems, represents one of the greatest tasks in material science
X-ray photon correlation spectroscopy (XPCS) data were collected at the position of the first sharp diffraction peak (FSDP), q0 1⁄4 2.81 Å À 1, during long subsequent isotherms at selected temperatures below Tg
Lines are fits with the Kohlrausch–Williams–Watts model g2ðtÞ À 1 1⁄4 c Á fq2ðta; TÞexp1⁄2 À 2ðt=taðta; TÞÞbðta;TÞ, where all parameters depend both on temperature and on the annealing time, ta, at any given T: ta(ta,T) is the structural relaxation time, b(ta,T) the shape parameter and fq(ta,T) is the nonergodic plateau of f(q, t) associated to the trapping of the particles in the nearest neighbours cage before their escape during the a relaxation[30]
Summary
Understanding and controlling physical aging, that is, the spontaneous temporal evolution of out-of-equilibrium systems, represents one of the greatest tasks in material science. A widespread use of out-of-equilibrium materials is at this day limited by the lack of a detailed understanding of the mechanisms ruling their physical aging This comprehension is for instance fundamental for a proper exploitation of the outstanding mechanical, physical and chemical properties of metallic glasses (MG)[1,2]. A similar behaviour has been recently associated to elastically interacting activated events in yield stress and jammed materials[19,20,21] and could be related to the existence of microscopic elastic heterogeneities[22,23] and atomistic free volume zones[24] This behaviour suggests the existence of a complex mechanism ruling the atomic dynamics, previously unreported in macroscopic studies[2] or in any current theory for glasses[18,25]. We observe the surprising thermal activation of a secondary relaxation process at high temperature in the decay of the density fluctuations, not reported in previous studies, which appears related to the onset of the crystallization
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