Heterogeneous influences on the crystallization of Pd₄₃Ni₁₀Cu₂₇P₂₀

Abstract

The influence of ingot purity as well as B₂O₃ fluxing on the crystallization of undercooled Pd₄₃Ni₁₀Cu₂₇P₂₀ is studied in a differential scanning calorimeter. B₂O₃ fluxing strongly influences the isothermal crystallization behaviour at temperatures above the nose in the time-temperature-transformation diagram. The ingot purity, in contrast, affects the isothermal crystallization at low temperatures but leaves the crystallization at high temperatures relatively unaltered. Thermal analysis, together with microstructure investigations of samples processed and prepared under different conditions, suggests two sources of heterogeneous nucleation. One source is surface impurities, which can be passivated by fluxing with B₂O₃. The other consists of impurities from the ingot materials homogeneously distributed within the sample. Crystallization at high temperatures is controlled by the formation of one nucleus, preferentially formed at the surface, which can be suppressed to a large extent by B₂O₃ fluxing. At low temperatures the impurities from the ingot material act as nuclei and their number defines the crystallization kinetics. Thus, the B₂O₃ flux treatment significantly improves the glass-forming ability whereas the ingot purity has much less influence on the critical cooling rate.