Abstract:It is difficult to formulate the statistical mechanical theory of liquids and glasses, because phonons, which are the basis for the statistical mechanics of lattice dynamics in crystals, are strongly scattered and have a very short lifetime in liquids and glasses. Instead computer simulation and the ''free-volume'' theory are most frequently used in explaining experimental results on metallic glasses. However, both of them suffer from serious problems, as discussed in this article. We propose an alternative ap… Show more
“…It was found that the development of icosahedral order fails to explain the observed diffusivity and energy changes in Cu 50 Zr 50 . The present work develops a more general view that ideally-packed atomic clusters are formed at the expense of clusters with excess or deficient free-volume during structural relaxation of a metallic glass, which supports earlier work by Egami et al 24 The proposed model also explains slowed diffusivity in compositions and alloy systems that do not show a propensity for forming icosahedrally-coordinated clusters and provides a framework for understanding why the formation of networks of icosahedral clusters is correlated with slowed dynamics.…”
The atomic structure of the supercooled liquid has often been discussed as a key source of glass formation in metals. The presence of icosahedrally-coordinated clusters and their tendency to form networks have been identified as one possible structural trait leading to glass forming ability in the Cu-Zr binary system. In this work, we show that this theory is insufficient to explain glass formation at all compositions in that binary system. Instead, we propose that the formation of ideally-packed clusters at the expense of atomic arrangements with excess or deficient free volume can explain glass-forming by a similar mechanism. We show that this behavior is reflected in the structural relaxation of a metallic glass during constant pressure cooling and the time evolution of structure at a constant volume. We then demonstrate that this theory is sufficient to explain slowed diffusivity in compositions across the range of Cu-Zr metallic glasses.
“…It was found that the development of icosahedral order fails to explain the observed diffusivity and energy changes in Cu 50 Zr 50 . The present work develops a more general view that ideally-packed atomic clusters are formed at the expense of clusters with excess or deficient free-volume during structural relaxation of a metallic glass, which supports earlier work by Egami et al 24 The proposed model also explains slowed diffusivity in compositions and alloy systems that do not show a propensity for forming icosahedrally-coordinated clusters and provides a framework for understanding why the formation of networks of icosahedral clusters is correlated with slowed dynamics.…”
The atomic structure of the supercooled liquid has often been discussed as a key source of glass formation in metals. The presence of icosahedrally-coordinated clusters and their tendency to form networks have been identified as one possible structural trait leading to glass forming ability in the Cu-Zr binary system. In this work, we show that this theory is insufficient to explain glass formation at all compositions in that binary system. Instead, we propose that the formation of ideally-packed clusters at the expense of atomic arrangements with excess or deficient free volume can explain glass-forming by a similar mechanism. We show that this behavior is reflected in the structural relaxation of a metallic glass during constant pressure cooling and the time evolution of structure at a constant volume. We then demonstrate that this theory is sufficient to explain slowed diffusivity in compositions across the range of Cu-Zr metallic glasses.
“…2(b)). Such a misfit supports an idea of Egami [45,46] on the existence of a large fraction of an anti-excess volume (V) in this alloy which redistribution is responsible for large difference in DH and low difference in DV values between the large and small samples.…”
“…As illustrated in Fig. 5(a), upon loading at stress P which is below yield stress, the coalescence of the loosely packed regions and the densely packed regions (i.e., the positive free volume sites and the negative free volume sites) 31, 32 could occur, leading to the volume contraction and a permanent deformation, as indicated by the decreased relaxation enthalpy and increased density of the post-elastostatic compressed specimens as shown in Figs 2 and 4, respectively. As also being emphasized in the reported cyclic cryogenic rejuvenation 3 , It can be concluded that structural heterogeneity 33 plays a crucial role in optimizing the performance of amorphous materials.Figure 5Schematic of the atomistic mechanism in the relaxation-to-rejuvenation transition and the existence of loosely packed regions and densely packed regions in bulk metallic glasses, i.e., the positive free volume sites and the negative free volume sites.…”
The relaxation of amorphous materials, i.e., aging, would largely endanger their performances in service. Here we report a mechanical relaxation-to-rejuvenation transition of a Zr35Ti30Be27.5Cu7.5 bulk metallic glass (BMG) in elastostatic compression at ambient temperature, thus provide an accessible way to tailor the mechanical properties of amorphous materials. To unravel the structural evolution underlying the observed transition, atomistic simulations parallel with the experimental tests on a typical model glass system Zr60Cu40 were performed, which successfully reproduced and thus upheld the experimentally observed mechanical relaxation-to-rejuvenation transition. The variations of coordination number and atomic volume during the transition are evaluated to indicate a de-mixing tendency of the constituent atoms in the rejuvenation stage. This de-mixing tendency largely explains the difference between mechanical rejuvenation and thermal rejuvenation and reveals a competitive relationship between activation enthalpy and activation entropy in the stress-driven temperature-assisted atomic dynamics of BMG, such as diffusion and plastic deformation etc.
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