Recent progress in understanding high temperature dynamical properties and fragility in metallic liquids, and their conn
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The advent of containerless processing techniques has opened the possibility of high quality measurements of equilibrium and metastable liquids. This review focuses on the structure and dynamics of metallic liquids at high temperature. A clear connection between structure, viscosity, and fragility has emerged from recent containerless experiments and molecular dynamics simulation studies. The temperature-dependent changes of liquid structures are smaller for the stronger liquids. The onset of cooperativity usually occurs above the liquidus temperature at a characteristic temperature TA, where the dynamics change from Arrhenius to non-Arrhenius behavior; this is accompanied by the onset of development of more spatially extended structural order in the liquids. Several metrics for fragility, consistent with the traditional fragility parameter, can be developed from the structural and dynamical properties at high temperature. It is becoming increasingly evident from theory and experiments that the fundamental properties that determine fragility are the repulsive part of the interatomic potential and the anharmonicity.
I. INTRODUCTION
Among the three states of matter, the two extreme cases of crystalline solids and gases are fairly well understood. In their ideal state (sufficiently low densities) gases represent complete atomic disorder, while an ideal crystal is an example of perfect spatial order of the atoms in three dimensions. Theories can be relatively easily formulated for well-defined states with complete order or disorder. Liquids and amorphous solids (glasses if they are formed from the liquid) are more difficult to understand since they are intermediate between these two idealized extremes. Although lacking the long-range order of crystals, they do maintain significant order in the short- and medium-length (atomic) scales. In principle, if the first order liquid/vapor to solid phase transformation (i.e., nucleation and growth) is arrested, any liquid/vapor can be frozen into an amorphous solid.1 This is most commonly accomplished by rapidly cooling (or quenching) the liquid/vapor2 although other approaches include high mechanical deformation3 and high energy ion irradiation.4 The ease of forming an amorphous solid varies enormously from one material to another. Among the elemental liquids, covalent-bonded materials, such as group III, IV, and V are relatively easy to synthesize into Contributing Editor: Himanshu Jain a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2017.253
glasses,5 especially when there is a decrease in volume during melting. In comparison, transition metals (with 3d, 4d, 5d electrons) are much more difficult to make amorphous. Thus far only a handful of transition metal elements (Co,6 Fe,7 Nb,7 Mo,7 W,7 Ta,7,8 V8) have been produced in the amorphous state. Since extremely high cooling-rates, between 107 and 1014 K/s, are required, elemental glasses have only been synthesized in the form of
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