NANOCRYSTALLINE FE73.5CU1NB3SI13.5B9 OBTAINED BY DIRECT-CURRENT JOULE HEATING - MAGNETIC AND MECHANICAL-PROPERTIES

Nanocrystalline ferromagnetic materials obtained by proper annealing of amorphous ribbons display excellent soft magnetic properties. However, the technological application of these nanostructured ferromagnets is hindered by their high brittleness. Direct-current Joule heating in vacuo is exploited to increase both the magnetic permeability and the ductility of nanocrystalline Fe73.5Cu1Nb3Si13.5B9. The development of the nanostructure in samples submitted to different Joule-heating currents for the same time (t = 60 s) has been analysed by means of X-ray diffraction, and is associated with magnetic permeability and reduced fracture strain measurements. Measurements of electrical resistance performed in the course of the heat treatment allows us to single out an activated process of atomic rearrangement occurring in the amorphous phase. Fracture strain data show that the alloy's embrittlement is mainly to be ascribed to this process rather than to nanocrystallization. It is shown that further improvement in the alloy's ductility can be obtained by inducing fast nanocrystallization in samples submitted to high electrical currents.