Thermodynamic, kinetic and structural aspects of the formation of nanocrystalline phases in Fe73.5−xNixCu1Nb3Si13.5B9 alloys

Formation of nanocrystalline phases in Fe73.5−xNixCu1Nb3Si13.5B9, x=10,20,30,40 at.%, has been studied by TEM, ED, X-ray diffraction and Mössbauer spectroscopy. In the case of the FeCuNbSiB (Finemet) alloy after aging at temperatures 773–873 K the formed crystalline phase consists uniquely of nanoscale α-(FeSi) phase stable for a relatively long aging time. In the FeNiCuNbSiB system with Ni content up to 10 at.% the nanocrystalline phase formed at 750–780 K remains stable against prolonged annealing. With increasing Ni content (above 10 at.%) the forming nanocrystalline microstructure is unstable; in the initial stages the decomposition involves only the nanocrystalline phase. The results obtained indicate that Ni causes the change of chemical composition of the original FeCuSi clusters as observed in classical Finemet to FeNiSiCu. Upon annealing the nanocrystalline phase, in dependence on the Ni content, decomposes very rapidly into Fe3Si and Fe3NiSi1.5 due to the diffusion of Ni which destabilizes the nanocrystalline microstructure. While it is generally assumed that the decisive factor for the formation of the nanocrystalline phase in this system is represented by the presence of Cu atoms, systems are known, however, where the nanocrystalline phase is formed also in alloys without Cu. The combined results of the methods used suggest that the most important factor influencing the formation of the nanocrystalline phase is the existence of medium-range ordered clusters with definite chemical composition in the as-quenched amorphous structure which increase in size upon annealing and act as embryos in the nucleation process.