Thermodynamic approach to quantitative assessment of propensity of metallic melts to amorphization
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Section I: Basic and Applied Research
Thermodynamic Approach to Quantitative Assessment of Propensity of Metallic Melts to Amorphization A.I. Zaitsev and N.E. Zaitseva
(Submitted May 30, 2004) On thermodynamic grounds, it was found that key properties that control the capacity of molten metallic alloys for transition into an amorphous state are the excess (configurational) entropy and heat capacity of the liquid. Chemical short-range order in liquids exhibiting various tendencies to glass formation was analyzed on the basis of the associated solution theory and the results of detailed thermodynamic research on a wide set of alloys. An interrelation was established between the association, characteristics of molten alloys (viscosity , activation energies of viscous flow, and crystallization) that determine the possibility of amorphization and characteristics of glassy state stability (glass transition point, Gibbs energy, and enthalpy of crystallization). It was demonstrated that the magnitude of the key functions is completely determined by the covalent constituent of chemical interaction between components and depends mainly on the entropy terms of association reactions. The prospects for developing the quantitative criteria of amorphization on the basis of the entropy of association was discussed. It was also shown that the suggested approach based on taking into account the specificity of chemical interaction between components can be useful for prediction of physical, chemical, and mechanical properties of solid amorphous metallic materials.
1. Introduction The amorphous state is rather new for metallic alloys, and it is likely its potentialities have not yet been completely revealed. This view is confirmed by the fact that since the discovery of the amorphization phenomenon in metallic liquids, the search for new glass-forming systems and even for defined compositions within a specific system are being carried out mainly by the trial-and-error method under the guidance of a large number of, as a rule, indirect criteria. The most frequently used criteria can be divided into three basic groups: thermodynamic, kinetic, and structural.[1] The first are founded on characteristics like the position of liquidus lines, the coordinates of deepest eutectics, enthalpy of formation, changes in thermodynamic functions accompanying crystallization of pure liquid components, and their mixtures, etc. Although each of these types of features is derived from the thermodynamic properties of a glass forming liquid and determined ultimately by the nature of the interparticle interaction between its components, none of them has generality or sufficiency. For example, the Al-La and Ni-Zr melts with the composition corresponding to the deepest eutectics fall far short of having the extreme tendency to transfer into amorphous state.[2,3] Kinetic criteria of glass-forming capacity, from which a sole quantitative characteristic of the amorphization process—the critical cooling rate—was derived, are based primarily on the regularities
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