First Stages of Nano-Crystallization of Amorphous Fe 75.5 Cu 1 Nb 3 Si 12.5 B 8 Studied by the Positron Annihilation Lif
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ABSTRACT The first stages of the nano-crystallization process of amorphous Fe 755CuNb 3Si 125Bs into a nano-crystalline structure are investigated by the positron annihilation lifetime technique. Samples have been isothermally annealed at 643 K for times varying between 600 and 10' seconds. The positron lifetime spectra have been analyzed allowing for three lifetimes. The shortest and the longest lifetime, T, = 150 ± 2 ps and T3 = 1500-2000 ps respectively, are attributed to annihilation of positrons in the amorphous phase and to the formation and annihilation of ortho-positronium at the surface of the stacked foils and did not change significantly upon the annealing. The intermediate positron lifetime C2 increased from 324 ps to 387 ps. The intensity of this component increased from 5 to 15%. Comparison with resistivity measurements indicates that the change of this lifetime component occurs at an early stage in the crystallisation process, i.e. when the fraction of crystalline material is on the order of 103. The increase of T 2 is attributed to positrons annihilating in a region with lower average density surrounding the small crystallite.
INTRODUCTION The FeCuNbSiB alloy is a typical example of a material with a nanometer-sized grain structure showing extremely good magnetic properties such as a high permeability and a high saturation magnetization [1]. For this reason the crystallization from the amorphous phase into the nano-crystalline structure, which takes place at a temperature around 800 K, has intensivily been studied. Examples of experimental techniques commonly used to this aim are X-ray diffraction, isothermal resistivity measurements [2] and positron annihilation. Positron annihilation techniques have shown to be very powerful for the study of open-volume defects in solids [3]. In particular the positron annihilation lifetime (PAL) technique is capable of detecting small changes in the structure and density of this type of defects. This has been demonstrated by Schaefer et al. [4] who attributed observed positron lifetimes and intensities to selected positron trapping sites, such as vacancy-sized free volumes and microvoids, at the intersections of the crystallites produced by compacting techniques. The application of PAL for annealing studies starting from the amorphous phase has been demonstrated by Sui et al. [51, Kristiakova et al. [6], and Novotny and Zemcik [7]. All measurements reported so far, however, concern the formation of a considerable fraction of the crystalline phase. In this article we present the results of PAL experiments on Fe.75 .CuNb3 Si, 2 5Bs during annealing at a relatively low temperature (643 K), by which the very early stages of the development of nano-crystals can be studied. 245 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society
EXPERIMENTAL SamRle preparation The amorphous alloy used in these experiments was produced in ribbon of 20 mm width and 24 pm thickness by the melt-spinning technique. The composition of the alloy is Fe 75.5Cu 1Nb3S' 12.5B,
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