Role of Niobium in the nanocrystallization of a Fe73.5Si13.5B9Nb3Cu alloy

The nanostructure of a nanocrystalline Fe73.5Si13.5B9Nb3Cu alloy has been studied by means of Moessbauer spectroscopy, 1D and 3D atom probes. After 6h at 520 deg. C, the crystallized fraction of the alloy is about 53%. α-Fe(Si) nanocrystals, 10-20nm in diameter, are embedded in a retained amorphous matrix. They have a near Fe80Si20 composition and exhibit a DO3 structure. The very low number density of Cu-rich particles does not match with the hypothesis of a systematic heterogeneous precipitation of α-Fe close to Cu-rich particles. A detailed analysis of the distribution of alloying elements in the retained amorphous matrix reveals the formation of a Nb-rich shell around α-Fe(Si) nanocrystals. It is proposed that this shell is formed during the growth of nanocrystals and the decomposition of the matrix, which tends toward a Fe3B composition. The Nb-rich shell behaves as a diffusion barrier which inhibits the growth of nanocrystals. However, despite the presence of a Nb-rich shell, the junction between adjacent nanocrystals may occur, even if this phenomenon is rare