Effect of Co addition on the atomic ordering of FeCo-phase in nanocrystalline Fe81-xCoxNb7B12 alloys (x=20.25, 27, 40.5, 54, 60.75): An anomalous diffraction and Mossbauer study

The influence of Co addition on the atomic ordering of bcc FeCo-phase in partially nanocrystalline Fe81-xCoxNb7B12 alloys (x = 20.25, 27, 40.5, 54, 60.75) was investigated using synchrotron radiation based x-ray diffraction (SR-XRD) and Fe-57 Mossbauer spectroscopy (MS). The results indicate that as the Co content in the alloy increases, there is a decrease in the thermal stability against crystallization of the amorphous phase. In the pure amorphous state, topological ordering as well as compositional homogeneity of the amorphous phase increases with Co addition (x >= 40.5). In the partially nanocrystalline state (isothermally annealed at 823 K for 1 h), the anomalous diffraction measurements close to Fe K-edge provide unambiguous evidence of atomically ordered alpha'-FeCo phase (CsCl type) as the nanocrystalline ferromagnetic phase by the presence of (100) super-lattice reflection for the alloys with x >= 40.5. However, alloys with x <= 27 present a disordered structure as confirmed by the absence of superlattice reflection. The long range order parameter (S) possess maximum value for the alloys close to equi-atomic composition (x = 40.5 and 54). By predominant low hyperfine field contribution (close to 33.7 T), Mossbauer spectroscopy measurement also validates the atomically ordered FeCo nanocrystalline phase in Fe81-xCoxNb7B12 alloy with x >= 40.5. (C) 2013 AIP Publishing LLC.