Nanocrystallization of soft magnetic Fe(Co)–Zr–B–Cu alloys

Abstract

Fe₈₈Zr₇B₄Cu₁ and Fe₄₄Co₄₄Zr₇B₄Cu₁ alloys have been melt spun to produce amorphous ribbons. Crystallization behaviour of the amorphous phase has been investigated using differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. Addition of Co in Fe–Zr–B–Cu alloy reduces the stability of the amorphous phase leading to the formation of nanocrystallites at a lower temperature. A dispersion of the nanocrystalline phase in the amorphous matrix is achieved by controlled heat treatment for optimization of magnetic properties. Coercivity, saturation magnetization and Curie temperature are found to increase on addition of Co.

Introduction

Fe-based soft magnetic alloys containing nanocrystalline precipitates embedded in an amorphous matrix are generally prepared by partial devitrification of rapidly solidified amorphous phase. The soft magnetic properties of these materials are mainly attributed to the diminishing value of magnetocrystalline anisotropy and saturation magnetostriction [1]. The strong ferromagnetic coupling of nanocrystalline phase with amorphous matrix leads to the averaging out of magnetocrystalline anisotropy and the exchange interaction forces the magnetization vector to be aligned over several grains along the field direction [2], [3]. Moreover the effective saturation magnetostriction nullifies due to the positive contribution to the magnetostriction from amorphous phase and negative contribution from nanocrystalline phase [3]. It has been found that coercivity increases 6th power of magnitude with grain size below the exchange length [4]. FINEMET, based on Fe–Si–B–Nb–Cu [5], [6]; NANOPERM, based on Fe–Zr–B–Cu [7], [8]; and HITPERM, based on Fe(Co)–Zr–B–Cu [9], [10] alloys are examples of such materials. FINEMET alloys developed by Yoshizawa et al. [5] are derived from the amorphous Fe–Si–B alloys, whereas NANOPERM and HITPERM have been developed from Fe–Zr–B glass-forming alloys [7], [9]. In both cases, Cu is added as a nucleating agent and Nb/Zr restrict the size of the nanocrystalline bcc phase.

A number of investigations have been carried out to optimize the structure and soft magnetic properties of Fe–Zr–B–Cu and Fe–Co–Zr–B–Cu alloy systems. Composition-dependent magnetic properties have been investigated using Mössbauer spectroscopy [11], [12], [13]. Kopcewicz et al [13] carried out a detailed study on structure and properties of Fe–Zr–B–Cu alloys using a special RF-Mössbauer technique which distinguishes magnetically soft nanocrystalline α-Fe particles from the magnetically hard microcrystalline α-Fe particles. Willard et al. [9], [14], [15] investigated the sequence of reaction during annealing treatment of Fe–Co–Zr–B–Cu alloys and reported that α′-FeCo (B2) nanocrystalline phase is precipitated on crystallization. Although these alloys show superior saturation magnetization and Curie temperature as compared to other nanocrystalline alloys, the coercivity and permeability are inferior.

In the present work, detailed crystallization behaviour and kinetics of crystallization of amorphous Fe–Zr–B–Cu and Fe–Co–Zr–B–Cu alloys have been investigated using differential scanning calorimeter (DSC) and X-ray diffraction (XRD). Evolution of microstructure on heat treatment above the crystallization temperature has been observed in transmission electron microscopy (TEM). Variation of magnetic properties with heat treatment temperature has been evaluated and correlated with the microstructure.