Atom Probe Studies of Nanocrystallization of Amorphous Alloys

The nanocrystallization processes in Fe-Si-B-Nb-Cu and Fe-Nd-B(-Cu-Nb) amorphous alloys have been studied by transmission electron microscopy (TEM) and a three dimensional atom probe (3DAP). Cu additions are effective in refining the nanocrystalline microstructures of an Fe74.5Si13.5B8Nb3Cu1 alloy and an Fe3B/Nd2Fe14B nanocomposite microstructure. This is because Cu atoms form a high density of clusters prior to the crystallization reaction, which serve as heterogeneous nucleation sites for the primary crystals. However, Cu additions are not effective in refining α-Fe/Nd2Fe14B nanocomposite microstructures, because the nucleation of the crystallites occurs during solidification from a melt in this system. Introduction The primary crystallization reaction of amorphous alloys often leads to the evolution of nanocrystalline microstructures which gives rise to excellent magnetic or mechanical properties. Some of the nanocrystalline materials produced by this route, e.g. Fe-based nanocrystalline soft magnetic materials [1,2] and Fe-Nd-B based nanocomposite hard magnetic materials [3,4], are currently being considered for various commercial applications. Since the magnetic properties in nanocrystalline soft magnetic materials and nanocomposite hard magnetic materials are originated from the nanosized particles, refining the grain size in the final microstructure is particularly important. In addition to optimizing the composition of the initial amorphous alloy, many attempts have been made on controlling the grain size by microalloying. For example, nanocrystalline microstructure is obtained from the Fe-Si-B amorphous alloy only with a combined addition of Nb and Cu [1]. Cu additions to Fe-Zr-B amorphous alloys are also known to be effective in refining the nanocrystalline microstructure [2]. However, Cu additions do not appear to be effective in improving the hard magnetic properties of α-Fe/Nd2Fe14B nanocomposite, while Nb additions are beneficial to the hard magnetic properties [5]. On the other hand, the present authors recently reported that a small amount of Cu addition substantially reduces the grain size of Fe3B/Nd2Fe14B nanocomposite microstructure [6,7], and a combined addition of Cu and Nb is more effective in the grain size reduction [8]. Among various microalloying elements added to Fe-based nanocrystalline alloys, the effect of Cu is particularly interesting as it works in many Fe-based amorphous alloys in refining the microstructure after nanocrystalliztion. The purpose of this paper is to overview the microalloying effects of Cu and Nb in the nanocrystalline microstructure evolution of Fe-Si-B based soft magnetic materials and Fe-Nd-B based hard magnetic materials. Fe-Si-B-Nb-Cu Nanocrystalline Alloy A unique feature of the crystallization process of an Fe73.5Si13.5B9Nb3Cu1 alloy is the formation of Cu clusters in the pre-crystallization stage [9]. Figures 1 show 3DAP elemental maps of Cu within analyzed volumes of 10 × 10 × 40 nm (a) in the as-melt-spun specimen, (b) in the specimens annealed for 5 min at 400°C and (c) in the specimen annealed for 60 min at 400°C [9]. In the as-melt-spun specimen, Cu distribution is uniform, confirming that the as-melt-spun Proc. Inter. Conf. Solid-Solid Phase Transformations '99, May 24 28, 1999, Kyoto 2 specimen is a chemically homogeneous solid solution. In the specimen annealed for 5 min at 400°C, heterogeneous distribution of Cu atoms is apparent, indicating that clustering of Cu atoms occurs. After annealing for 60 min at 400°C, clustering of Cu atoms is observed more clearly. Separate HREM observation results confirmed that no crystallization occurs up to 60 min at 400°C [9], thus we can conclude that the clustering observed in this analysis occurs in the amorphous phase. The number of atoms in each cluster is in the range of 50 to 100, and the size of the clusters is approximately 3 nm. The density of the Cu clusters estimated from the analyzed volume is in the order of 10 m. The concentration of the Cu cluster has been estimated to be approximately 12 at.% Cu initially, but it increases as the clusters grow in size. The structure of these clusters was not identified by HREM, because the HREM image did not give any fringe contrast corresponding to any crystalline structure at these stages. Based on the extended x-ray absorption fine structure (EXAFS) measurement results, Ayers et al. [10] reported that Cu atoms form clusters having near-fcc symmetry from the very early stage of the heat treatment. Thus, the Cu clusters observed in the 3DAP data are believed to have a fcc-like short range ordered structure, but they do not appear to be distinct fcc-Cu in the initial stage. Such Cu clustering appears to be common in Cu containing Fe based amorphous alloys. For example, Suzuki et al. [2] reported that the addition of 1 at.% of Cu in Fe90Zr7B3 amorphous alloy causes a reduction of the grain size leading to better soft magnetic properties in the final nanocrystalline microstructure. Clustering of Cu in this system was also confirmed prior to the crystallization reaction by AP analysis [11]. 3DAP elemental maps near a Cu precipitate in the Fe73.5Si13.5B9Nb3Cu1 alloy annealed at 550 °C for 10 min are shown in Fig. 2. The Cu precipitate is in direct contact with the α-Fe(Si) particle, which suggests that Cu clusters directly serve as heterogeneous nucleation sites for the α-Fe(Si) primary crystals. Since the number density of the Cu clusters is ~10 m, if all α-Fe primary crystals are heterogeneously nucleated at the site of the Cu clusters without coalescence during growth, nanocrystalline microstructure having average grain size of ~10 nm can be readily formed. The presence of intergranular amorphous phase hinders the coalescence of the primary particles. Fe-Nd-B Based Nanocomposite Alloys Figures 3 (a) (c) show bright field TEM micrographs of Nd4.5Fe77B18.5, Nd4.5Fe76.8B18.5Cu0.2 and Nd4.5Fe75.8B18.5Nb1Cu0.2 alloys annealed at 660°C for 10 min, respectively [13,14]. The (a)