Amorphous Phase Separation in a Bulk Metallic Glass of Negative Heat of Mixing.

摘要

Amorphous phase separation in metallic glass (including bulk metallic glass) has been a controversial issue in the past several decades. There are reports saying that amorphous phase separation occurs in Pd-Ni-P, which has a negative heat of mixing among its constituent elements. However, there are also as many reports claiming that phase separation is absent in amorphous Pd-Ni-P alloys. The lack of direct experimental evidence makes the issue to be difficult to be resolved.;To solve this problem, differential scanning calorimetry (DSC), high resolution transmission electron microscopy (HRTEM), high angle annular dark field (HAADF) in scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy (EDX) have been employed. Intermediate thermal annealing is introduced before an undercooled Pd41.25Ni41.25P 17.5 melt is cooled down to become a solid amorphous specimen.;A-type, B-type, and C-type specimens of composition, Pd41.25 Ni41.25P17.5, have been prepared via three different cooling paths. It was found that amorphous phase separation indeed occurs in C-type specimens. Results suggest that there may be unique short range orders in amorphous/liquid Pd41.25Ni41.25P17.5 , which are responsible for the phase separation. Experimental arrangements were made to study the occurrence of spinodal reaction in undercooled molten Pd41.75Ni41.75P17.5 alloys as a function of time. The lower bound of the duration of the spinodal decomposition at a temperature of ≈625 K is about 200 s and the time constant R of the spinodal decomposition at a temperature of ≈625 K is 0.002 s-1.;A-type and B-type specimens have similar crystallization behavior. At low temperature, it starts with the formation of a spherical core and then eutectic crystallization takes over. At higher temperatures, an additional phase in the shape of a cube appears. In annealed C-type specimens, cores and cubic precipitates are also found. However, the composition profile of the cores is different and the number of nucleation events versus time has peculiar characteristics. The crystallization behavior of Pd40Ni 40P20 BMG was studied for comparison. It again starts out with the formation of a core, but with a composition profile different from those of A-type and B-type specimens.