Nanocrystallization kinetics and magnetic properties of the melt spun amorphous (Fe_0.5Co_0.5)₇₇Si₁₁B₉Cu_0.6Nb_2.4 alloy

Highlights

• Kinetics of crystallization in cobalt Co-Finemet alloy was investigated.

• Variable activation energy for crystallization implies a complex multi stage reaction.

• Values of scan rates used to calculate the activation energy could affect the results.

Abstract

Kinetics of crystallization in an amorphous (Fe_0.5Co_0.5)₇₇Si₁₁B₉Cu_0.6Nb_2.4 (at.%) alloy was investigated using differential scanning calorimetry (DSC). Transformed fraction as a function of temperature was obtained by accurate DSC measurement and the experimental data analyzed with Vyazovkin model-free kinetic method. Reconstructed form of the experimental kinetics model, g(α), clearly showed the crystallization mechanism do not belongs to a single model but almost follows the Avrami-Erofe’ev. Magnetic coercivity and hysteresis loss values of the annealed samples at 823 K were 7.5 A m⁻¹ and 1.2 J m⁻³, compared to 17.1 A m⁻¹ and 37.1 J m⁻³ for as spun samples. Magnetic measurements show the annealing reduces both coercivity and hysteresis loss at all frequencies.

Introduction

Fe-based amorphous and nanocrystalline alloys have been a research interest since the discovery of Finemet composition (Fe_73.5Si_13.5B₉Cu₁Nb₃) in 1988 [1]. They show very good magnetic softness, corrosion resistance, and electrical properties and have found wide practical applications [1], [2], [3], [4], [5]. For a suitable chemical composition, nanocrystalline structure could be obtained by proper heat treatment of amorphous precursor [6]. For the Finemet composition, final structure is consist of bcc (Fe)Si or (FeCo)Si (for Co-Finemet) nano-grains randomly dispersed in the remained amorphous matrix [7], [8]. Alloy composition has a crucial role in the formation of nanocrystalline structure. Copper segregates at the initial stage of crystallization that multiplies the nucleation. Niobium with a large atomic radius (0.146 nm) controls the reaction kinetics by retarding the growth of the nano-grains [9], [10], [11], [12]. The composite microstructure of primary crystals distributed in the amorphous matrix makes a great contribution to the magnetic properties. Averaging out of the anisotropies in the nano-grains lowers the overall anisotropy leading to an excellent magnetic softness. This is explained in terms of the random anisotropy model by Herzer [13]. However, controlling the crystallization process and kinetic parameters are essential factors for obtaining the desired structure and properties [14].

Many attempts have been made to improve these materials by optimizing the alloy compositions [15], [16], heat treatment conditions [17], or by adding the alloying elements [18], [19], [20]. Modifying the chemical composition is a way to tune the structure by a change in the transformation kinetics. For example, the percentage of copper in the alloy controls the size and distribution of nano-grains in the Finemet alloys [15], or substitution of iron by cobalt reduces the crystallization onset temperature [18]. For Finemet-type alloys, partial replacement of iron with cobalt also plays a crucial role in broadening its applications, which requires much higher performance of soft magnetic materials. Accordingly, a large number of research results have proven the improvement in high-temperature and high frequency magnetic properties of the Co-Finemet alloys [21], [22], [23]. The incorporation of cobalt is not only expected to improve ferromagnetic properties but also to play a dual role as a nucleating element in place of copper in Finemet systems [12].

Since, the kinetic parameters are important factors determining the structure and properties of nano-crystalline alloy, present work aims to study the kinetics of cobalt containing Finemet alloy (Fe_0.5Co_0.5)₇₇Si₁₁B₉Cu_0.6Nb_2.4 (at.%). The alloy composition derived by replacing of iron with cobalt into the modified Finemet composition of Fe₇₇Si₁₁B₉Nb_2.4Cu_0.6 [24]. According to Ohnuma et al. [15], the modified composition has an improved magnetic flux density in comparison to the original composition Fe_73.5Si_13.5B₉Cu₁Nb₃. Vyazovkin isoconversional method [25], [26], [27] was selected for kinetic calculations due to its accuracy and reliability.