Kinetic and structural fragility—a correlation between structures and dynamics in metallic liquids and glasses

Abstract

The liquid phase remains poorly understood. In many cases, the densities of liquids and their crystallized solid phases are similar, but since they are amorphous they lack the spatial order of the solid. Their dynamical properties change remarkably over a very small temperature range. At high temperatures, near their melting temperature, liquids flow easily under shear. However, only a few hundred degrees lower flow effectively ceases, as the liquid transforms into a solid-like glass. This temperature-dependent dynamical behavior is frequently characterized by the concept of kinetic fragility (or, generally, simply fragility). Fragility is believed to be an important quantity in glass formation, making it of significant practical interest. The microscopic origin of fragility remains unclear, however, making it also of fundamental interest. It is widely (although not uniformly) believed that the dynamical behavior is linked to the atomic structure of the liquid, yet experimental studies show that although the viscosity changes by orders of magnitude with temperature, the structural change is barely perceptible. In this article the concept of fragility is discussed, building to a discussion of recent results in metallic glass-forming liquids that demonstrate the presumed connection between structural and dynamical changes. In particular, it becomes possible to define a structural fragility parameter that can be linked with the kinetic fragility.

Computational modeling sheds light on structural evolution in metallic glasses and supercooled liquids

“…Examples include a structural signature of dynamical slowdown as well as the liquid fragility in metallic glass-forming liquids, [26][27][28][29][30][31][32] anomalous thermal contraction of metallic melts in the nearest-neighbor shell, [33][34][35] possible crystallike order in MGs/liquids, [36][37][38][39] a fractal structure model of MGs, [40][41][42] advanced algorithms (such as machine learning methods) to efficiently characterize the structural basis of flow defects and dynamical slowdown in amorphous materials, 43,44 and a new structure parameter that incorporates dynamic (atomic vibration) information beyond the description of static structure/ configuration. [45][46][47][48][49] Computational research has played a key role in formulating these ideas.…”

“…27 iii) First peak height in the structure factor, S(q): Mauro et al 28,29 have studied the temperature dependence of the height of the first peak in S(q), as a measure of local structural order developed. This maximum peak intensity evolves with undercooling.…”

“…The basic idea remains that in general the kinetic fragility would be correlated with structural evolution in the liquid, 29 and a structural signature of the dynamics behavior can be understood from the picture when the liquid enters the PEL-affected regime. [20][21][22][23] Structure ordering gives rise to slowdown of dynamics, because of increased constraints and rigidity.…”

“…Specifically, the type of characteristic SRO varies from alloy to alloy and is not universal for metallic glass-forming liquids. 30,31,[63][64][65][66][67][68][69] The metric based on peak intensity in structure factors or PDFs shows considerable fluctuation when correlated with liquid fragility, 28,29,71 and the peak height may involve other effects, such as thermally enhanced asymmetry of peaks in the PDF, as will be discussed in a later section. More importantly, all these indicators do not necessarily reveal a causal link with the dynamical slowdown at the microscopic level.…”

Non‐isothermal crystallization kinetics of Al2O3‐YAG amorphous ceramic coating deposited via plasma spraying

“…With regard to amorphous materials, their dynamical properties (such as viscosity) change greatly during the glass transition. The temperature‐dependent dynamical behavior is often characterized as kinetic fragility (or fragility), which was generally believed to be connected with many properties at T g , including the variation in specific heat, elastic and configuration entropy, etc . Glass‐former liquids are called “strong” when their dynamical behavior follow an Arrhenius temperature dependence in a broad temperature range, similar to those kinetic processes such as chemical reaction rates and atomic diffusion in crystal.…”