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

Abstract

The ductile-to-brittle transition temperature ( T DB) of melt-spun amorphous Fe 80.3B 12Si 7.7 ribbons was studied as a function of their thickness (25–52 μm), 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. DB rises linearly with thickness from about −150°C for 25.2 μm ribbons to over 100°C for 42.1 μm 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 vicosity 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.