Nanocrystallization process of the Fe69.5Cu1Nb3B9Si13.5Cr4 FINEMET-type alloy: an AFM study

Abstract

The different devitrification stages of Fe69.5Cu1Nb3B9Si13.5Cr4 (F-Cr4) amorphous alloy were studied by atomic force microscopy (AFM) on samples annealed at 400, 600 and 670 °C for different treatment times. The AFM images showed no topographic changes in the alloys treated at 400 °C, indicating that only structural relaxations took place at this annealing temperature. Samples thermally treated at 600 °C showed hemispherical features uniformly distributed on the surface, suggesting initiation of the nanocrystallization process. The nanocrystals formed had a size distribution of 15–40 nm, which is slightly affected by the annealing time. At 670 °C the crystallization phenomena occurred with the formation of large crystals and an increase in roughness. The corrosion behaviour of the nanocrystalline alloys in 2 M HCl solution was correlated with the AFM studies. The potentiodynamic experiments for these alloys showed a significant decrease of the current density peak at the active–passive transition in relation to the F-Cr4 amorphous alloy. The results indicated that the corrosion resistance and passivating ability of the alloys are improved by the thermal treatment. Copyright © 2000 John Wiley & Sons, Ltd.