Thermodynamic aspects of amorphous phase formation
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The glass-forming ability of the three alloy systems Co-Zr, Cu-Zr, and Ni-Zr has been analyzed for three distinct production routes: (1) liquid quenching, (2) vapor deposition, and (3) solidstate reaction. Using the free energy and heats of formation curves obtained from the thermodynamic characterization of the respective alloy systems, a satisfactory rationale can be obtained for amorphous phase formation by all three routes. The analysis shows that while amorphous phase formation by quenching from the high-temperature liquid is clearly dependent on factors such as quench rate and the value TG/TM, it is the low-temperature stability of the amorphous phase relative to the other crystalline structures that enables amorphous phases to be formed by both vapor deposition and solid-state reaction. The underlying free energy curves indicate the interesting possibility of a supersaturation sequence in the nucleation of an amorphous phase by solid-state reaction. The principles underlying thermodynamic characterizations are briefly discussed, and a characterization for Co-Zr is presented.
I. INTRODUCTION The recent solid-state amorphization experiments begun by Schwarz and Johnson1 now create the possibility of producing amorphous/glassy metallic alloys by at least three distinct routes: (1) quenching from the hightemperature liquid, (2) codeposition from the vapor and (3) solid-state reaction. The latter possibility clearly indicates that rapid quenching from a high-temperature phase is not a prerequisite for metallic glass formation, and the role of kinetic criterion for glass formation therefore needs to be reevaluated. A satisfactory rationale for amorphous phase formation by all three routes can be obtained by examining underlying phase stabilities and modeling amorphous phase formation in alloys produced by vapor deposition, using kinetic criteria distinctly different from those in liquid quenching. To demonstrate these principles the three Zr-based alloy systems Cu-Zr, Ni-Zr, and Co-Zr, where amorphous phases can be produced by all three routes, have been analyzed. The analysis presented here relies heavily on the thermodynamic characterization of the relevant alloy systems, two of which (Cu-Zr and Ni-Zr) have been previously reported.2 The principles behind such characterizations will first be briefly discussed and demonstrated for Co-Zr, before all three systems are analyzed for glass-forming ability (GFA) by the separate production routes listed above.
explicit mathematical synthesis of phase equilibria, first put forward by van Laar.3 The advent of computer technology has enabled the lengthy iteration processes that are often necessary to mathematically describe the phase equilibria of alloy systems to be handled with relative ease, and the modeling of phase stability pioneered by Kaufman4 is now being extensively exploited.5"7 Following generally applied principles,5~7 the free energy of each solution phase in alloy system AB can be described mathematically in the following way: where G p = the free energy o
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