Kinetics of Nanocrystallization in an Amorphous Metal Alloy

Introduction

Soft magnetic materials are ferromagnetic substances that are weakly magnetized in a magnetic field and thus exhibit slight hysteresis. They are used in new innovative electrical devices such as compact low-loss switch-mode power supplies, magnetic amplifiers, transformers and chokes

Ferromagnetic materials can acquire good soft magnetic properties when ultrafine microcrystalline structures are formed. Fe-Si-crystallites with a grain size limit of 10−15 nm are a good example. Such structures are produced when amorphous Fe-Si-B alloys, to which small amounts of Cu and Nb have been added, crystallize [1].

The originally proposed alloy has the nominal composition Fe_73.5Si_13.5B₉Cu₁Nb₃ (at%). Rapid solidification yields an amorphous metallic glass in the form of a ribbon with a thickness of about 20 µm.

The nanocrystalline structure is obtained by annealing between 500 and 600 °C. In this process, Fe-Si grains are produced with a typical particle size of 10−15 nm in an amorphous matrix. The distance between the crystallites is about 1 to 2 nm [1].

Knowledge of the crystallization kinetics of amorphous alloys offers the possibility of producing specific crystalline structures and hence for optimizing the properties of the alloys. 

The processes involved in the crystallization of amorphous metal alloys are complex. This makes it impossible to describe the nanocrystallization using a model-fitting method such as the Avrami crystallization model [2]. For this reason, in this article, model free kinetics is used to investigate the crystallization of the nanocrystallites. A detailed discussion of the results and a comparison with different crystallization models have been published in reference [2].

Experimental Details

Sample preparation

The Fe_73.5Si_13.5B₉Cu₁Nb₃ alloy was prepared by arc melting under an argon atmosphere using high purity metals. Amorphous ribbons with a cross-section of 1 mm by 20 µm were prepared by melt spinning under a protective argon atmosphere. The amorphous nature of the ribbons was confirmed by X-ray diffraction analysis. 

Methods

DSC measurements were performed from room temperature to 700 °C at heating rates between 5 and 80 K/min using a METTLER TOLEDO DSC 1. The samples were measured in 70-µL platinum crucibles; the sample mass was about 20 mg. The DSC furnace was continuously purged with 20 mL/min nitrogen.

Results

Interpretation of the DSC curves

The DSC curves are displayed in Figure 1. They show three exothermic events. The first is a very small, broad peak between 390 and 500 °C in the curve measured at 80 K/min. It probably corresponds to the Curie temperature of the amorphous phase and Cu clustering, which occurs before the nanocrystallization of the FeSi nanograins.

Conclusions

The crystallization process of the metastable metal alloy used in this study is a complex process that takes place in several steps. Nanocrystallites are formed that largely determine the magnetic properties of the material. In the DSC curve, the crystallization is observed as a broad asymmetric exothermic peak that moves to higher temperatures at higher heating rates. The apparent activation energy as a function of the conversion is determined by performing measurements at different heating rates and evaluating the results using the Advanced Model Free Kinetics, AMFK, software.

The apparent activation energy changes significantly with conversion. This allows conclusions to be drawn about the complex crystallization mechanism. Furthermore, using AMFK, predictions can be made about the course of crystallization. 

_{Kinetics of Nanocrystallization in an Amorphous Metal Alloy | Thermal Analysis Application No. UC 305 | Application published in METTLER TOLEDO Thermal Analysis UserCom 30}