Crystal nucleation in amorphous (Au 100 − yCu y) 77Si 9Ge 14 alloys

Abstract

Melt-spun alloys of the series (Au 100 − yCu y) 77Si 9Ge 14 are found to be amorphous for y ≤ 37.5 and partially or fully crystalline for y = 50 and y = 75. Because, unlike most metallic glasses, these alloys exhibit well separated glass transition and kinetic crystallization temperatures, crystallization can be studied in the fully relaxed amorphous phase. Isothermal calorimetric analysis of the devitrification kinetics of the amorphous alloys indicates sporadic nucleation and a constant growth rate. A steady state nucleation frequency is established after a transient period, except for the y = 37.5 alloy, where the kinetics are dominated by growth on quenched-in nuclei. In the y ≤ 25 alloys, we find that the classical theory of homogeneous nucleation is consistent with the observations, including transient effects. The analysis of the crystallization kinetics indicates that slow crystal growth rates play an important role in glass formation in these alloys. As the copper content of the alloys is increased, the reduced glass transition temperature increases, but, surprisingly, glass formation is more difficult at high copper contents. This is interpreted to be a consequence of a different nucleus composition.