Quantitative evaluation of critical cooling rate for metallic glasses

Evaluation of critical cooling rate Rc for glass formation was carried out quantitatively for metallic glasses. A fundamental equation proposed for oxide glasses was modified for metallic glasses. In the calculation both Gibbs free energy of the molten alloys and the effect of the differences in atomic size were taken into account. The calculation was carried out for three groups: pure metals, typical glass-forming systems, and the latest metallic glasses with a large glass-forming ability found after 1990. The calculated results were in agreement with previous data. As examples, the Rc for Ni metal, Co- and Pd–Cu-based metallic glasses in the present work were 9.1×108, 1.2×105 and 1.6×10−2 K/s, respectively, instead of 3×1010,3.5×105 and 1×10−1 K/s in the previous work. The following points are clarified: (1) Tm2/η characterizes Rc as is typical for the Pd–Cu-based metallic glass with a low melting point as well as high viscosity at the melting point; (2) negative heats of mixing and atomic size mismatch above about 12% affect the reduction of Rc from 10−2 to 10−7 and from 10−1 to 10−2; and (3) the reduction of Rc is understood as a stabilization of liquid state.