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

Abstract The crystallization process of amorphous Fe 74 Si 14 B 12 , Fe 73.5 Cu 1 Si 13.5 B 12 , Fe 74 Nb 3 Si 14 B 9 and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 ribbons was investigated by annealing the ribbons under vacuum at 520°C for 6 h. Only Nb containing samples show a nanocrystalline state consisting of α-Fe(Si) nanograins embedded in a remaining amorphous phase. The amorphous ribbons were then submitted to a nitriding thermochemical treatment at 520°C for 6 and 18 h under ammonia. It is observed that crystallization occurs prior to nitrogenation. Upon nitrogenation, a reactive diffusion process of nitrogen is observed, which appears to be slower in the partially crystallized ribbons, due to the presence of a remaining amorphous phase. In the crystalline regions, the α-Fe(Si) phase is disproportionated into α-Fe and probably Si–N precipitates. Then Fe 4 N is formed when nitrogenation proceeds further.