Formation of monophase Fe23B6-type alloy via crystallization of amorphous Fe–Ni–Nb–B system

• Amorphous Fe–Ni–Nb–B alloy system with different B and Ni/Fe content was prepared.
• Monophase (Fe–Ni–Nb)23B6 was obtained by crystallization of selected Fe–Ni–Nb–B alloy.
• Site occupancies were resolved by atomic resolution STEM and structure refinement.
• Curie temperature of (Fe–Ni–Nb)23B6 was determined as function of Ni and B content.
• Local chemical ordering at grain–matrix interface was assessed.

This study is focused on rapidly quenched Fe–Ni–Nb–B system in a wide compositional range of substitution of Fe by Ni with varying boron content. This system is known to have a good glass forming ability and to crystallize in either two distinct transformation stages or in one single step, depending on the boron content and on the ratio of Fe to Ni. On the basis of our results we have identified and quantified the formation of a complex phase (Fe–Ni–Nb)23B6 alongside with fcc-FeNi. In a specific case it was possible to obtain only a single-phase Fe23B6 type alloy and thus to explore and characterize this phase, especially as there is no prior evidence of preparing such a monophase system. Detailed characterization of composition, structure and selected magnetic properties of both phases is presented. We have proposed a model of possible local structural and chemical arrangement in (Fe–Ni–Nb)23B6 phase as well as possible building units responsible for specific features of transformation from amorphous state and for preferential formation of this phase. This was confirmed by refinement of selected X-ray patterns together with high resolution (S)TEM analysis combined with energy filtered TEM. EFTEM analysis has shown a unique interface between the Fe23B6-type phase and the remaining FeNi matrix in systems where both phases were present. On the other hand, high resolution z-contrast STEM analysis applied to a single phase system revealed atomic positions of constituent atoms. Niobium was confirmed on positions as predicted by the proposed structural model and as refined from the diffraction experiments. Conventional high resolution TEM analysis was used to characterize the Fe23B6-type grains with respect to defects and faults.