2001 Volume 42 Issue 11 Pages 2374-2381
An equation expressing the transformation curve for crystallization of metallic glasses has been proposed by using two parameters, viscosity and melting temperature (Tm). The Vogel-Fulcher-Tammann (VFT) equation, η=η0exp(B⁄(T−T0)) where η0, B and T0 (ideal glass transition temperature) are empirical parameters, was used for expressing the viscosity. A Time-reduced Temperature-Transformation (T-Tr-T) diagram was calculated using the following five quantities: reduced temperature (Tr=T⁄Tm), three reduced viscosity parameters (η0r=η0⁄Tm, Br=B⁄Tm and T0r=T0⁄Tm), and reduced critical cooling rate (Rcr=Rc⁄Tm) for formation of the glassy phase. The Rcr in the T-Tr-T diagram was calculated for Ni, metallic glasses and SiO2. The glass-forming ability (GFA) was estimated from T0r−Rcr(Rcr=Rc⁄Tm) diagrams corresponding to Tg⁄Tm−Rc diagrams obtained experimentally. The metallic glasses with T0r of 0.55 are calculated to have Rcr ranging from 10−5 to 100 s−1, which agrees with the experimental data that metallic glasses with Tg⁄Tm of 0.6 or more have Rc of less than 103 K/s. The following three points are clarified: (1) the higher GFA of metallic glass is achieved because of higher viscosity, (2) higher viscosity causes both the homogeneous nucleation frequency (Ivhom) and the ratio (IN⁄Imax), at reduced nose temperature against the maximum of Ivhom, to decrease, and (3) the Rcr is numerically derived from reduced viscosity parameters.