Microstructural Characterization

An important part of the recent developments corresponding to nanostructured materials is related to those obtained by controlled crystallization, either by annealing the amorphous single phase or by decreasing the cooling rate from the liquid of metallic systems. Typically, in these nano-structures, precipitate sizes range between 5 and 50 nm embedded in an amorphous matrix with nanocrystal volume fractions of 10 to 80%, which means particle densities of 1022 to 1028 m-3.

We present the most relevant aspects of the nano-crystallization process of Fe-based nanocrystalline alloys as a two-phase system, namely a-Fe or a-Fe(Si) grains embedded in a residual amorphous matrix, which, being ferromagnetic, results in a material with extremely good soft magnetic properties. The microstructural analysis of the primary crystallization of Fe-rich amorphous alloys usually has been done by using conventional techniques such as differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission Mossbauer spectroscopy (TMS), and transmission electron microscopy (TEM). In this way, through the combined structural analysis of these techniques, useful information, such as the dependence of the microstructure upon time and temperature of treatment, can be obtained.

The kinetics of metastability loss of the disordered system above glass transition, i.e., under less than equilibrium conditions, is a key subject, because it provides new opportunities for structure control by innovative alloy design and processing strategies. Several examples include soft and hard magnets and high-strength materials [30-32]. Most studies focus on the crystallization onset as a measure of kinetic stability under heat treatment and recognise the product phase selection involved in nucleation and the role of competitive growth kinetics in the evolution of different microstructural morphologies [33, 34]. Differential scanning calorimetry has become quite effective as a means of studying the nature of nanoscale structures and their stability. It has been established that the initial annealing response allows one distinguish between a sharp onset for a nucleation and growth or a continuous grain growth of pre-existing grains [35]. Kinetic data on the transformation often are obtained from this technique.

Figure 1 shows the DSC curves of the as-prepared amorphous alloy (Fe73 5Cu1Nb3Si175B5, trademark Finemet), as well as of that of alloy previously annealed for 1 hour at 703 K and 763 K, respectively [36]. For the as-prepared alloy, the calorimetric signal shows some relaxation before the exothermic nanocrystallization process, as well as with the Curie temperature of the amorphous phase (Tqmc ~ 595 K). When the sample has been annealed at 703 K, relaxation is no longer apparent in the calorimetric signal, the Curie temperature is shifted about 15 K, to higher values, and the nanocrystallization process is slightly advanced in temperature (2 K). On further increasing the annealing temperature, the calorimetric signal shows no clear changes in the Curie temperature of the annealed sample with respect to annealing at 703 K. However, a clear shift of the nanocrystallization onset toward higher temperatures (40 K) can be observed, as well as a significant decrease of its area [36]. Consequently, DSC measurement permits evaluation of the maximum temperature to prevent partial crystallization during previous heating and the maximum heating rate to control the temperature of the sample by analyzing the transformation peak connected to the primary crystallization.

Calorimetric measurements, however, give information about microstructural development. Microstructure determination by the use of several techniques (XRD, TMS and TEM) allows us to complete the understanding of the mechanisms of the primary crystallization. Transmission Moss-bauer spectroscopy has the main advantage of giving local information of an active element (Fe nuclei in these alloys). Because Fe is present both in the nanocrystalline precipitates and in the disordered matrix, TMS provides information on local ordering in both phases, which can be correlated with the changes in the short-range order of the amorphous phase and with the composition of nano-crystalline phase. X-ray diffraction and TEM provide a

Figure 1. DSC signals of Fe73 5Si13 5B9Cu1Nb3 alloy obtained after heating at 40 K/min. For the as-quenched amorphous alloy (solid line); after 1 h isothermal annealing at 703 K (dashed line) and at 763 K (dotted line). Reprinted with permission from [37], M. T. Clavaguera-Mora et al., Progress in Materials Science 47, 559 (2002). © 2002, Elsevier Science.

Figure 1. DSC signals of Fe73 5Si13 5B9Cu1Nb3 alloy obtained after heating at 40 K/min. For the as-quenched amorphous alloy (solid line); after 1 h isothermal annealing at 703 K (dashed line) and at 763 K (dotted line). Reprinted with permission from [37], M. T. Clavaguera-Mora et al., Progress in Materials Science 47, 559 (2002). © 2002, Elsevier Science.

close look at the developed microstructure and permit the characterization of the precipitates, showing their morphology and grain-size distribution.

Figure 2(a-c) shows a TEM image of a Finemet alloy after heating at 763 K at three annealing times (3, 10, and 30 minutes), while Figure 2(d) corresponds to the case (b) with increasing contrast, which allows the shape and size of the precipitates to be determined [37]. As can be seen, the microstructure is characterized by a homogeneous, ultrafine grain structure of a-Fe(Si), with grain sizes around 10 nm and a random texture, embedded in a still amorphous matrix. The formation of this particular structure is ascribed to the combined effects of elements as Cu (which promotes the nucleation of grains) and Nb, Ta, Zr, Mo,—(which hinders their growth) and their low solubility in a-Fe(Si) [38-40]. Nevertheless, the size and morphology of the nano-crystals in these alloys, as well as their distribution, could be analyzed by the application of local probe techniques. These techniques, such as scanning tunneling microscopy (STM) and atomic force microscopy (ATM) provide three-dimensional (3D) topographic images at the nanometer level [41, 42] and represent powerful tools to study the surface properties and structures of metals and alloys.

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