Dynamic heterogeneity, cooperative motion, and Johari–Goldstein $$\beta $$-relaxation in a metallic glass-forming material exhibiting a fragile-to-strong transition

Abstract

We investigate the Johari-Goldstein (JG) b-relaxation process in a model metallic glass-forming (GF) material (Al90Sm10), previously studied extensively by both frequency-dependent mechanical measurements and simulation studies devoted to equilibrium properties, by molecular dynamics simulations based on validated and optimized interatomic potentials with the primary aim of better understanding the nature of this universal relaxation process from a dynamic heterogeneity (DH) perspective. The present relatively low temperature and long-time simulations reveal a direct correspondence between the JG b-relaxation time tJG and the lifetime of the mobile particle clusters tM, defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the a-relaxation time ta and the lifetime of immobile particle clusters tIM. Moreover, we find that the average diffusion coefficient D nearly coincides with DAl, of the smaller atomic species (Al), and that the hopping time associated with D coincides with tJG to within numerical uncertainty, both trends being in accord with experimental studies indicating that the JG b-relaxation is dominated by the smaller atomic species and the observation of a direct relation between this relaxation process and rate of molecular diffusion in GF materials at low temperatures where the JG b-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that Al90Sm10 exhibits fragile-to-strong (FS) glassformation, as found in measurements of many other metallic GF liquids, but this fact does not greatly alter the nature of DH in this material and the relation of DH to dynamical properties. 10

Crossover in dynamics in the Kob-Andersen binary mixture glass-forming liquid

“…Our results are consistent with those obtained in [27], and the change in behavior of χ 4 (t) observed for the 2 : 1 and 3 : 1 KA-BMLJ model reported in [28]. In recent work on a metallic glass model [25], a clear peak in the χ peak 4 values is observed, along with a specific heat maximum, which are described as manifestations of a fragile to strong crossover. As described later and in [27], no evidence of a specific heat maximum is found in the present model.…”

“…The remaining time scales, (D/T ) −1 (which has been scaled to match the magnitude of the others at one reference temperature), t * , t peak n M , and t peak L , also exhibit the same temperature dependence, which is milder than that of τ α [35,55]. We note that recent work on a metallic glass former indicates that t peak n M is larger than t * ∼ t peak L and is equal to the time scale associated with the Johari-Goldstein process [25], although all these time scales exhibit a milder temperature dependence than the alpha relaxation time τ α . Such a distinction is not apparent from our present results.…”

“…In recent years, the fragile to strong crossover in dynamics has also been reported in a variety of other glass formers, including phase change alloys [19,20] used in memory devices, and several metallic glasses [21][22][23][24][25]. In many of these instances, though not all, the fragile to strong crossover has been shown to be accompanied by a heat capacity maximum.…”

“…We next consider the statistics and morphology of clusters of particles that move in a correlated fashion, which have been a subject of considerable study [25,28,35,[48][49][50][51][52][53]. In particular, the time dependent size of spatially correlated clusters of mobile and immobile particles, as well as string-like correlated mobile particles were investigated in [35], as also the morphologies of such clusters.…”

“…The decoupling of the diffusion time scale and the alpha relaxation time scale have been investigated extensively ( [42,[55][56][57][58][59][60][61][62][63][64][65][66][67][68] and references therein), in the context of the breakdown of the Stokes-Einstein relation, employing τ α as being proportional to the viscosity [42]. We consider the breakdown of the Stokes-Einstein relation in order to investigate whether it reveals any indication of the crossover in dynamics around T M CT , as reported in [25]. We plot the diffusion coefficient D (obtained from mean squared displacement plots shown in C) vs. τ α in Fig.…”