Influence of processing on the ductile-to-brittle transition temperature of an Fe-B-Si metallic glass

The ductile-to-brittle transition temperature (T-DB) of melt-spun amorphous Fe80.3B12Si7.7 ribbons was studied as a function of their thickness (25-52 mu m), and of the speed and thermal conductivity of the casting wheel. A refined analytical technique was used to determine T-DB from measurements of the bending strain at fracture over a range of temperatures. T-DB rises linearly with thickness from about -150 degrees C for 25.2 mu m ribbons to over 100 degrees C for 42.1 mu m ribbons produced under the same conditions. The ribbon thickness for a given T-DB could be increased by 33% by increasing the conductivity of the wheel. Decreasing the wheel speed also increased the ductility. Of the theories for the embrittlement, only the one based on the increase in viscosity as a result of structural relaxation can account for all the observations. The viscosity increase due to self-annealing below the glass transition (T-g) during the late stages of casting can be calculated, and is negligible. The difference in viscosity resulting from the variation in cooling rate in the vicinity of T-g, however, is substantial, and can account for the observed changes in T-DB. Copyright (C) 1996 Acta Metallurgica Inc.