MECHANICAL-BEHAVIOR OF BOTH SIDES OF AN AMORPHOUS FE78B14SI8 ALLOY RIBBON AS DETERMINED FROM MINIATURIZED DISK-BEND TESTS

A miniaturized disk-bend test was used to examine the mechanical properties of the "air" and "wheel" sides of melt-spun ribbons, 30 mum thick, of an amorphous Fe78B14Si8 alloy. The specimens were 3 mm in diameter and were tested clamped at their perimeters. Deformation was non-linear, with the load rising to a maximum value signifying the onset of fracture, Crack propagation was accompanied by substantial elongations at lower loads. The displacement of the center of the specimen at peak load was used as a measure of its ductility. The wheel side is consistently harder, stronger and more ductile than the air side. We propose that this is due to the numerous regions of low free volume at the wheel side, produced when air is entrapped between the melt and the wheel, combined with local enrichment of boron and/or silicon in these same regions. These strong regions are interspersed with those that solidified at the most rapid rate and which have the largest free volume, highest ductility, but lowest hardness and strength. The heterogeneous distribution of free volume at the wheel side invests it with its high strength and ductility; the air side has properties between these two extremes. The role of ribbon surface topography in the initiation of fracture is also discussed.