Evaluation of glass-forming ability for metallic glasses from time-reduced temperature-transformation diagram

An equation expressing the transformation curve for crystallization of metallic glasses has been proposed by using two parameters, viscosity and melting temperature (T-m). The Vogel-Fulcher-Tammann (VFT) equation, eta = eta (0)exp(B/(T - T-0)) where eta (0), B and T-0 (ideal glass transition temperature) are empirical parameters. was used for expressing the viscosity, A Time-reduced Temperature-Transformation (T-T-r-T) diagram was calculated using the following five quantities: reduced temperature (T-r = T/T-m) three reduced viscosity parameters (eta (0) = eta (0)/T-m, B-r = B/T-m and T-0r = T-0/T-m), and reduced critical cooling rate (R-cr = R-c/T-m) for formation of the glassy phase, The R-ct in the T-T-r-T diagram was calculated for Ni, metallic glasses and SiO2. The glass-forming ability (GFA) was estimated from T-0r - R-ct (R-c = R-c/T-m) diagrams corresponding to T-g/T-m - R-c diagrams obtained experimentally. The metallic glasses with T-0r of 0.55 are calculated to have R-cr ranging from 10(-5) to 10(0) s(-1), which agrees with the experimental data that metallic glasses with T-g/T-m of 0.6 or more have R-c of less than 10(3) K/s. The following three point, are clarified: (1) the higher GFA of metallic glass is achieved because of higher viscosity. (2) higher viscosity causes both the homogeneous nucleation frequency (l(v)(hom)) and the ratio (l(N)/l(max)), at reduced now. temperature against the maximum of l(v)(hom), to decrease, and (3) the R-cr is numerically derived from reduced viscosity parameters.