Nanostructure and compositional segregation of soft magnetic FeNi\u2010based nanocomposites with multiple nanocrystalline phases

P Ohodnicki,M.E Mchenry,E.J Kautz,Y Krimer,A Devaraj,A Leary,N Aronhime,Y Yu

doi:10.1557/s43578-020-00066-5

Abstract

Soft magnetic metal amorphous nanocomposite alloys are produced through rapid solidification and thermal annealing yielding nanocrystals embedded within an amorphous precursor. Similar free energies in Co‐rich and FeNi‐based alloy systems result in multiple nanocrystalline phases being formed during devitrification. Studies of multi‐phase crystallization processes have been reported for Co‐rich alloys but relatively few have investigated FeNi‐based systems. A detailed characterization of compositional partitioning and microstructure of an optimally annealed FeNi‐based MANC (Fe70Ni30)80Nb4Si2B14 alloy is presented through complementary high‐resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). HRTEM demonstrates orientation relationships between FCC and BCC nanocrystals, suggesting heterogeneous nucleation of nanocrystals in the amorphous matrix or a cooperative mechanism of nucleation between BCC and FCC nanocrystallites. APT results show evidence for (i) the segregation of Fe and Ni between nanocrystals of different phases, (ii) B partitioning to the amorphous phase, and (iii) an Nb‐enriched shell surrounding nanocrystals.

Highlights

Metal amorphous nanocomposite (MANC) alloy systems are of significant interest for a range of emerging soft magnetics applications which include, but are not limited to, inductors, transformers, and rotating electrical machinery such as motors [1]
Existing commercial alloy systems produced in bulk quantities are based upon the quaternary FeNbSiBCu system and contain only a single transition metal ferromagnetic element (Fe) such that the MANC microstructure obtained is comprised of a nanocrystalline BCC-derivative Fe–Si DO3 phase surrounded by an amorphous matrix [2]
Despite these initial studies reporting the structural characterization of multi-phase crystallization processes, detailed high-resolution transmission electron microscopy (HRTEM) and compositional segregation analysis has not been reported for FeNi-based MANC alloys to date

Summary

Introduction

Metal amorphous nanocomposite (MANC) alloy systems are of significant interest for a range of emerging soft magnetics applications which include, but are not limited to, inductors, transformers, and rotating electrical machinery such as motors [1]. Newer alloy systems are being explored that have substantially modified compositions with a significant content of other ferromagnetic transition metal elements such that other close-packed nanocrystalline phases (e.g., FCC, HCP) become similar or lower in free energy as compared with BCC-derivative phases These alloys are based upon (i) high Co-containing alloys in which nanoscale BCC, FCC, and HCP (as well as highly faulted close-packed) grains can be found in varying proportions and (ii) FeNi alloys for which nanoscale FCC grains become a significant or dominant crystalline phase after optimized annealing treatments [3, 4, 5, 6, 7, 8, 9]. As observed for the FeNiZrB-based alloys, a greater volume fraction of BCC relative to FCC nanocrystals could be observed at lower annealing temperatures Despite these initial studies reporting the structural characterization of multi-phase crystallization processes, detailed high-resolution transmission electron microscopy (HRTEM) and compositional segregation analysis has not been reported for FeNi-based MANC alloys to date.

Experimental Results and Discussion

Conclusions

Materials and Methods