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 Mössbauer spectroscopy, 1D and 3D atom probes. After 6 h at 520 °C, the crystallized fraction of the alloy is about 53%. α-Fe(Si) nanocrystals, 10-20 nm 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.