Microstructural study of nanocrystalline Fe-(Cu-Nb)-Si-B ribbons obtained by a nitriding thermochemical treatment

The crystallization process of amorphous Fe74Si14B12, Fe73.5Cu1Si13.5B12, F e74Nb3Si14B9 and Fe73.5Cu1NB3Si13.5B9 ribbons was investigated by annealing the ribbons under vacuum at 520 degrees C for 6 h. Only Nb containing samp les show a nanocrystalline state consisting of alpha-Fe(Si) nanograins embe dded in a remaining amorphous phase. The amorphous ribbons were then submit ted to a nitriding thermochemical treatment at 520 degrees C for 6 and 18 h under ammonia. It is observed that crystallization occurs prior to nitroge nation. Upon nitrogenation, a reactive diffusion process of nitrogen is obs erved, which appears to be slower in the partially crystallized ribbons, du e to the presence of a remaining amorphous phase. In the crystalline region s, the alpha-Fe(Si) phase is disproportionated into alpha-Fe and probably S i-N precipitates. Then Fe4N is formed when nitrogenation proceeds further. (C) 2000 Elsevier Science B.V. All rights reserved.