Nanometer-scale investigation of compositional and structural evolution during crystallization of melt-spun Co84Nb10B6 metallic glass

Nanocrystalline materials are increasingly showing progress in the quest to control the properties of a solid. As the building blocks of matter approach sizes on the order of 100 nanometers or less, nearly every physical property of the material is altered. One class of nanocrystalline material that has received considerable attention in recent decades and especially in the last several years is that of the metallic glass. This research project was performed in order to obtain a better understanding of the formation of nanocrystalline structure in a Co-based metallic glass.; Ribbons of a Co₈₄Nb₁₀B₆ metallic glass were rapidly quenched by the melt spinning technique. Field Ion Microscopy (FIM), Atom Probe Field Ion Microscopy (APFIM) and Transmission Electron Microscopy (TEM) were applied to study the micro-structural and nanocompositional changes of Co₈₄Nb₁₀B₆ due to annealing.; The as-quenched metallic glass was determined to be amorphous by FIM and APFIM. Subsequent annealing at various temperatures and times produced specimens in different stages of phase separation and nanocrystallinity. Crystal nuclei in specimens annealed for one hour at 500, 550, and 600 C were constant in size (4 nm) but increased in number density with increasing temperature. Annealing at 700 C for one hour produced a fully crystalline, three-phase material. The phases consist of a nanocrystalline boride phase, a nanocrystalline Co₃Nb phase, both with average grain sizes of 20–50 nm, and a relatively pure cobalt matrix. The various phases exhibited significantly different field evaporation and imaging behavior in the FIM.; Prior to this work, it was hypothesized that boron atoms segregating to grain boundaries played a role in limiting the grain growth of this nanocrystalline material. The composition fluctuations across several grain boundaries were analyzed and there was no evidence of boron accumulation. The results of this work suggest another mechanism must be keeping the grains of this material in a nanocrystalline state. Such a hypothesis is presented at the conclusion of this work.