Characterization of a Co 8 4Nbi 0 B 6 Metallic Glass O. Dor, E. Manor, J. Baram yvutsrponmlihgecaUSRPONMIGEDCB Materials Engineering Department, Ben-Gurion University, POB 653, Beer-Sheva, Israel and A.N. Sembira Nuclear Research Center Negev, Israel ABSTRACT The crystallization of an amorphous melt-spun Cog 4 NbioB 6 alloy has been studied. Electrical resistivity (ER) measurements and thermal analyses (DSC, DTA and TGA) show: • a non-reversible magnetic transition, occurring in the amorphous state in the 315-333°C range; • crystallization, occurring in the 408-448°C range; • a second, reversible, magnetic transition, at 556°C on heating and 596°C on cooling, and • an additional thermal effect in the 720-750°C range, attributed to recrystallization. The crystallized structured phases, as identified by X-ray diffraction, consist of a Co-based solid solution and boride phases. The activation energy for crystalli- zation ranges between 70 kcal mole" 1 (as measured by DSC) and 80 kcal mole" 1 (as measured by ER). The Avrami exponent in the kinetic equation is 1, cor- responding to a grain boundary nucleation after site saturation and 3-D interfacial growth. The zero nucleation rate (site saturation) is consistent with the observed presence of nanocrystallites in the amorphous matrix of the as-quenched ribbons, that serve as nucleation sites. The microstructure of the fully crys- tallized ribbons is highly anisotropic, indicating a preferential, two-dimensional growth of the crystalline phases. The activation energy for recrystallization, calculated from ER measurements at elevated temperatures, is about 130 kcal mole' 1 and about 120 kcal mole' 1 if evaluated by DTA. The Avrami exponent for recrystallization is 1. The crystallized structure consists of a Co-based solid solution and the Co 3 B + CoNbB boride phases. The elastic moduli of samples annealed for 2 hours, at temperatures ranging from 400 to 1000°C, were calculated to be in the 1.0* 10 11 to 1.3*10 n Nm' 2 range. Yield stresses are in the 1-2 GPa range, and strain-to-fracture varies from 1 to 2.5%. 1. INTRODUCTION Most metallic glasses show high strength but limited ductility when fully crystallized. The embrittlement is commonly attributed to the high metalloid content of these materials. The CouNbioBe glass, with a metalloid content of only 6 at.%, shows excellent mechanical and magnetic properties when devitrified under proper annealing conditions. High strength and ductility and high coercivity qualify crys- talline Co84NbioB 6 as a new alloy for magnetic applica- tions III. It has been shown III that the bend diameter reaches a maximum when the glass is annealed between 700 to 800°C and then decreases so that, finally, after a 1-hour anneal at 1000°C, ribbons can be 147