Mechanical Alloying of FeCo Nanocrystalline Magnetic Powders

Journal of Electronic Materials, Nov 2004 by Li, H F, Ramanujan, R V

Mechanical alloying of the Fe^sub 50^Co^sub 50^ equiatomic-magnetic alloy from elemental powders has been studied. Two milling speeds of 200 rpm and 300 rpm were used to process these powders. The as-milled powders were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). The mixing of Fe and Co was completed in 200 min at a milling speed of 300 rpm; however, an increase in saturation magnetization was observed up to 10-h milling, indicating an increase in compositional homogeneity as a function of milling time. These findings were also reflected in the XRD results. During the milling of Fe and Co at 300 rpm, an increase of powder size was observed after 100-min milling. Further milling at 300 rpm led to a reduction in powder size; the decrease of powder size was more effective when milling was conducted at 200 rpm. This was attributed to a difference in the milling mechanisms dominating at these two speeds. The TEM observation showed that a banded microstructure was observed in the as-milled powders. The banded structure consists of grains, many of which show texture effects. After further milling, the banded structure became finer, then randomly arranged, and finally disappeared.

Key words: Mechanical alloying, magnetic material, nanomaterial, transmission electron microscopy (TEM)

INTRODUCTION

Mechanical alloying is a versatile technique and has been successful to process a variety of commercially useful and scientifically interesting materials, such as intermetallics and amorphous, nanocrystalline, and nanocomposite materials.1-5 The FeCo-based magnetic alloys with compositions of 25-50 at.% Co in Fe have the highest moment per unit mass or volume of any magnetic material; thus, they have attracted considerable research effort.6 It is well known that good, soft magnetic properties have been obtained in HiTperm alloys, which contain equiatomic Fe and Co as well as other metalloid elements.7 The HiTperm alloys were prepared by crystallization from melt-spun amorphous precursors and have a thermally stable microstructure of high-density nanoprecipitates embedded in amorphous matrix. Mechanical alloying of FeCo-based alloys from elemental powders can simplify the processing of FeCo-based alloys and lead to the formation of nanostructures; it also has the potential advantage of being amenable to processing of complicated shapes and large-sized electronic devices. Therefore, mechanical alloying of FeCo-based alloys is potentially useful in the processing of magnetic materials.6'8

Previous investigations have reported mechanically alloyed Fe-Co systems.9-16 The effect of milling media on the phases obtained in the Fe-Co equiatomic mixture of mechanically alloyed elemental powders was significant, as discussed by Gonzalez et al.9 Two milling methods, cyclic and conventional operation, were tested to optimize the milling process; and the cyclic milling method was found to be more effective to obtain a smaller-grained structure.10 The mixing of Fe and Co by ball milling was considered to be the result of Co diffusion into the α-Fe while Fe did not diffuse into the ε-Co.13-15 Generally for long enough milling time, high-energy milling could process the nanocrystalline Fe-Co alloy.9"16 However, the magnetic properties of these nanocrystalline Fe-Co alloys were not soft; the reason was considered to be the internal strain induced by milling. Based on previous work,9-16 to process FeCo alloys with good, soft magnetic properties, the following points are considered to be important: (a) the microstructure of mechanically alloyed powders and its effects on the magnetic properties; (b) strain relaxation and microstructural evolution after heat treatment; and (c) the effects of elemental additions on microstructures. In this paper, the microstructural evolution of as-milled Fe^sub 50^Co^sub 50^ powders will be described. The alloying process and magnetic properties will be discussed.

EXPERIMENTAL

Starting from elemental powders Fe (99.9%, 22 mesh) and Co (99.9%, 100 mesh), mechanical alloying was conducted in hardened steel vials with hardened steel balls (about 10 mm in diameter) using an F5 planetary ball miller (Fritsch Pulverisette, Germany, model F5). Argon gas was used to minimize the oxidation during milling. The speed was initially set to be 300 rpm; after 5-h milling, two speeds 300 rpm and 200 rpm were selected for further milling and for comparison purposes. Reverse mode after every 5-min milling was used during the milling process to decrease the sticking of powders to the balls and vials and also to make the milling more effective. Four groups of milling were selected. The milling time and speeds are listed in Table I. The weight ratio of balls to powders was set to be 10:1 for the beginning of each group of samples. A small sample of powders was retrieved after milling for different periods of time before reaching the longest milling time. For example, in the first group, elemental powders were filled into the vial with the weight ratio of balls to powder 10:1; after milling for 25 min, a small amount of powders was retrieved, but the remaining powder underwent further milling; then after milling for 50 min, powders were again retrieved. The as-milled powder was then characterized using scanning electron microscopy (SEM, JEOL scanning electron microscope 5410, Japan Electron Optics, Tokyo), x-ray diffraction (XRD, SHIMADZU 6000 Lab X diffractometer, Cu target, Kyoto, Japan), transmission electron microscopy (TEM, JEOL 200 kV transmission electron microscopy), x-ray energy-dispersive spectroscopy (EDX, OXFORD, Oxon, UK), and vibrating sample magnetometry (VSM, vibrating sample magnetometer 736, Lake Shore Cryotronics, Westerville, OH). For the TEM sample preparation, the as-milled powders were mixed with epoxy and filled into a copper tube; then, the copper tube was cut into disks; the disks were then ground slightly, dimpled, and ionmilled. For the magnetic properties testing, the as-milled powders were filled into a small holder (VSM attachment); the magnetic hysteresis loops were obtained under a maximum field of 10 k Oer (3.2 × 10^sup 6^ A/m). The powder size was directly determined by measuring the diameter of powders from representative SEM micrographs; generally, a size of more than 200 powders was measured using Analytical Imaging Station image analysis software (Image Research, Inc., Brock University, Canada).