2000 Volume 41 Issue 11 Pages 1530-1537
Crystallization of the undercooled bulk metallic glass forming Zr41Ti14Cu12Ni10Be23 melt is investigated. Isothermal crystallization studies are performed between the liquidus and the glass transition temperature and a time-temperature-transformation diagram is determined. The various solidification products of samples solidified from different levels of undercooling are examined by electron microprobe. The investigations reveal the morphology and typical length scale of the microstructure as well as the primarily solidified phases after crystallizing at different degrees of undercooling. The typical length scale, the size to which one nucleation event grows, decreases continuously with increasing supercooling over 5 orders of magnitude. The number density of nuclei during primary crystallization is estimated from the microstructure. Repeated isothermal undercooling experiments are performed to investigate the scattering of the time to reach crystallization. These results suggest that at low undercooling crystallization takes place by a classical nucleation and growth mechanism. At high undercooling a diffusion controlled mechanism can explain the results. In addition, constant heating and cooling experiments are performed. Crystallization is found to be history dependent in this system. A rate of about 1 K/s is sufficient to suppress crystallization of the melt upon cooling from the equilibrium is nore liquid. During heating of amorphous samples, in contrast, a rate of about 200 K/s is necessary to avoid delectable crystallization. The crystallization temperature upon reheating depends on the minimum temperature to which the liquid was cooled prior to reheating as well as on the cooling rate by which the liquid was cooled into the amorphous state.