Compressed correlation functions and fast aging dynamics in metallic glasses

Abstract

We present x-ray photon correlation spectroscopy measurements of the atomic dynamics in a Zr 67 Ni 33 metallic glass, well below its glass transition temperature. We find that the decay of the density fluctuations can be well described by compressed, thus faster than exponential, correlation functions which can be modeled by the well-known Kohlrausch-Williams-Watts function with a shape exponent β larger than one. This parameter is furthermore found to be independent of both waiting time and wave-vector, leading to the possibility to rescale all the correlation functions to a single master curve. The dynamics in the glassy state is additionally characterized by different aging regimes which persist in the deep glassy state. These features seem to be universal in metallic glasses and suggest a nondiffusive nature of the dynamics. This universality is supported by the possibility of describing the fast increase of the structural relaxation time with waiting time using a unique model function, independently of the microscopic details of the system.

Revealing the fast atomic motion of network glasses

“…As shown in Fig. 1b, the relaxation dynamics measured at T ¼ 698 K (T/T g ¼ 0.93) remains stationary with tB300 s, even after B10 4 s, thus waiting for a time B70 times longer than the observed relaxation time t. These findings are also remarkably different from the correlation functions recently observed in metallic glasses at the atomic scale 8,20,21 , since these are characterized by a fast aging and a compressed shape, thus described by b41. In those systems, the dynamics has been attributed to the presence of internal stresses or excess free volume stored in the glass during the quenching, as in the case of many complex out-ofequilibrium soft materials 23 .…”

“…The relaxation dynamics of silicate glasses strongly differ from that recently reported for metallic glasses 8,20,21 . In these systems, the atomic motion appeared to be governed by the presence of internal stresses stored in the system during the quenching, suggesting a ballistic nature of the dynamics as in the case of many complex soft materials 23,69 .…”

“…Recent measurements on metallic glasses have revealed the existence of structural rearrangements at the atomic level, contrary to the common expectation of an almost arrested dynamics 8,20,21 . In those systems, the dynamics evolves from a diffusive atomic motion in the supercooled liquid, to a stress-dominated dynamics in the glass, characterized by a complex hierarchy of aging regimes.…”

“…A similar compressed/stressed and stretched/ stress-free scenario has been recently proposed for a colloidal suspension of Laponite under an external shear field 43 . Finally, it is worth noticing that the very same absence of visible physical aging at the microscopic scale has been also detected after long annealing in metallic glasses 20,21 . As in those systems, the observed stationary dynamics in our silicate glasses does not correspond to an equilibrium configuration, like in supercooled liquids.…”

“…Thanks to the instrumental developments in the collection of sparse scattering signals 18 and to an increased flux and coherence of X-ray beams, X-ray photon correlation spectroscopy (XPCS) has recently emerged as a very powerful technique able to follow the evolution of the dynamics at the atomic length scale in crystalline 19 and amorphous materials 8,[20][21][22] . Recent measurements on metallic glasses have revealed the existence of structural rearrangements at the atomic level, contrary to the common expectation of an almost arrested dynamics 8,20,21 .…”