THERMAL EVOLUTION OF FE 62.5CO6NI7.5ZR6 NB2CU1B15 METALLIC GLASS

Thermal evolution of Fe62.5Co6Ni7.5Zr6Nb2Cu1B15 amorphous alloy prepared by one-roll melt-spinning technique was studied by XRD and DTA. The crystallisation process, occurring in several steps, can be summarised as follows: a --> a' + alpha-Fe --> a' + alpha-Fe + gamma-Fe --> alpha-Fe + gamma-Fe + ZrB(1)2, where a and a' are amorphous phases, and a' can be indexed as a gamma-Fe (fcc) structure, with a crystalline order on an average distance of 8 Angstrom. The metallic glass demixed on quenching, but component phases tended to mix by exchanging Fe atoms in a temperature range overlapped with the first crystallisation, which yields alpha-Fe nanocrystals (approximate to27 Angstrom). Higher temperature exo-peaks correspond mainly to a recrystallisation of the phases formed at lower temperature. It was found that this alloy has nanocrystalline structure also after heating at a well higher temperature than first crystallization. Even after the last exo-peak, the average crystallite size (D) was considerably smaller than that found in the literature for analogous metallic glasses; D values for our alloy were comparable to those of nanocrystalline phases of other systems heat treated below the temperature of exothermal DTA peaks. Extensive oxidation above 600degreesC, even at a low oxygen content (c(o2) less than or equal to 2 ppm), led to a marked modification of the surface layer: two zirconia polymorphs were identified on the surface of the ribbons, and the ratio of gamma-Fe to alpha-Fe content increased with respect to the bulk. Differences in thermal evolution between outer layer and bulk are attributed to a different phase composition and non-uniform distribution since the as-quenched stage