Flux Treated Pd–Cu–Ni–P Amorphous Alloy Having Low Critical Cooling Rate

Abstract

A Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy subjected to B₂O₃ flux treatment was found to have a low critical cooling rate (R_c) of 0.100 K/s for glass formation and a large sample thickness (t_max) of 72 mm by the water quenching process. The R_c and t_max exceed largely those (R_c=1.57 K/s, t_max=40 mm) for the Pd–Cu–Ni–P alloy without the flux treatment. It is concluded that the flux treatment causes a significant increase in the thermal stability of the supercooled liquid. The glass transition temperature (T_g) remains unchanged in the fluxed state, but the crystallization temperature (T_x) increases by 7 K, leading to the extension of the supercooled liquid region defined by ΔT_x(=T_x−T_g) to 98 K which is larger than that (91 K) for the non-fluxed sample. The decrease in R_c and the increases in t_max and ΔT_x for the fluxed sample are presumably due to the suppression of heterogeneous nucleation for crystallization resulting from the increase in the degree of cleanness of the molten alloy. Besides, the Pd–Cu–Ni–P amorphous alloy has lower R_c and melting temperature (T_m) and larger t_max, ΔT_x and T_g⁄T_m values, as compared with those for Pd₄₀Ni₄₀P₂₀ alloy. The larger glass-forming ability for the Pd–Cu–Ni–P alloy is presumably due to the increase in the degree of the satisfaction of the three empirical rules for the achievement of larger glass-forming ability resulting from the more systematic change in atomic size in the order Pd>>Cu>Ni>>P and the generation of Cu–Pd and Cu–P atomic pairs with negative heats of mixing. There is no appreciable difference in the T_g, T_x and crystallization behavior between the cast 72 mm^φ amorphous ingot and melt-spun amorphous ribbon. The finding of the fluxed Pd₄₀Cu₃₀Ni₁₀P₂₀ alloy with the lower R_c and larger t_max values is promising for the future development of bulk amorphous alloys.