Crystal nucleation in FINEMET-type alloys as evidenced by calorimetric measurements

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Materials Science and Engineering A203 (1995) Ll0 Ll 2

Letter

Crystal nucleation in FINEMET-type alloys as evidenced by calorimetric measurements A. Serebryakov, A. Gurov, N. Novokhatskaya Institute (~f Solid State Physics, Chernogolovka, Mosco~ District 142432, Russia

Received 5 April 1995; in revised form 19 June 1995

Abstract

The way that the crystallization behavior of FINEMET-type alloys changes with a decrease in the metalloid content suggests the existence of two nucleation mechanisms in primary crystallization of the alloys. Keywords: Crystallization; Nucleation; FINEMET-type alloys

1. Introduction

It has recently been found [1] that the crystallization behavior of amorphous (C077Si13.sBg.5)93.vFe4Nb2.3 alloy (A1 alloy) is very similar to the behavior of FINEMETs (FMs) [2,3]. Indeed, Fe and Nb additions to alloy A1 play the same role as Cu and Nb additions, respectively, to FMs, i.e. Fe facilitates the nucleation of crystals at the beginning of the primary crystallization whereas Nb suppresses the growth of nucleated crystals. As a result, ultrafine-grained structures may be formed when alloy A1 is annealed in a wide temperature and time ranges [1]. However, unlike FMs, where only the b.c.c, phase (~-Fe(Si) solid solution) is formed by primary crystallization, the formation of two crystalline phases can precede secondary crystallization in alloy A1. The phase, which is formed first, is a Cobased b.c.c, solid solution. The b.c.c, phase is followed by a Co-based h.c.p, solid solution. Under certain conditions, nanoscale structures with a single b.c.c. crystalline phase can be produced by primary crystallization of alloy AI [1]. The b.c.c, phase was never found to be formed by crystallization of amorphous Co Si B alloys without Fe additions. The data obtained in [1] were explained, assuming different mechanisms for nucleation of b.c.c, and h.c.p. crystals: (i) formation of Fe-rich crystal-like clusters acting as nucleation sites (precursors) for the b.c.c. phase and (ii) formation of small regions enriched in

the base element of the alloy, where nucleation of the h.c.p, phase can occur. Such regions may be formed as a result of association of atoms of alloying elements with a strong affinity for each other and clustering of the associates in a random network. An increase in the number of energetically favorable bonds can give rise to the appearance of associate-depleted "windows" in the network, which are of the same origin as the regions depleted of metalloid atoms in the computer-simulated structures of transition metal metalloid glasses [4,5]. It can be expected that the "network window" (NW) mechanism is an ordinary mechanism of the metallic crystal nucleation in amorphous alloys. If this is the case, changes in the alloy composition affecting the network stability will affect the kinetics of crystal nucleation by the NW mechanism. Moreover, changes lowering the network stability in alloys crystallizing via the precursor mechanism, as FMs probably do, can allow the NW mechanism to be operable in them. The data reported in this letter confirm the expected crystallization behavior.

2. Experimental details and results

Amorphous ribbons of alloys with the nominal composition given in Table 1 were prepared by planar flow casting in air onto a steel wheel. The behavior of the Elsevier Science S.A. S S D I 0921-5093(95)09985-9

Letter/Materials Science and Engineering A203 (1995) LIO.-.LI2

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Table 1 Nominal composition of the alloys studied

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Alloy

Composition (at.%)

AI A2 F1 F2

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alloys during primary crystallization was examined at a constant heating rate of 10 K min ~ (after rapid heating at a rate of 2 0 0 K m i n ~ up to 6 7 3 K and a exposure time of 2 m i n at this temperature) in a Perkin-Elmer DSC7 calorimeter under a flowing argon atmosphere. ,9 2,9 X-ray diffraction (XRD) patterns were taken from the free (air) side of the samples using cobalt K s radiation. The differential scanning calorimetry (DSC) curves of the F1 and F2 alloys are shown in Fig. 1. Both the composition of alloy F2 and its DSC curve are typical of FMs. The exothermic peak corresponding to the primary crystallization of alloy F1 is shifted to lower temperatures and splits into two overlapping peaks compared with that of alloy F2. The metalloid content in alloy F1 is lower than in alloy F2. The cz-Fe(Si) (110) peaks of the XRD patterns taken from the samples F1 and F2 annealed for 1 h at 813 K are shown in Fig. 2. The peak positions and intensities are slightly different; this can be easily explained by the difference in the alloy compositions. The peak halfwidths are virtually the same, which suggests that the average crystal size is about the same in both F1 and F2 samples. The insets of Fig. 2 show the XRD patterns taken from the annealed samples F1 and F2 in the angular range of the superstructure (100) reflection of the ordered (Fe3Si) phase. The reflection is hardly distinguishable in the pattern of the sample F2 (its expected

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position is indicated by a full triangle). No sign of ordering was found in the XRD pattern taken from the F1 sample, and also no phase other than the disordered or ordered (present in a tiny amount in the F2 sample) b.c.c, phase was revealed in both annealed samples. The DSC curves of the A1 and A2 alloys exhibit three exothermic peaks before the onset of the secondary crystallization (Fig. 3). The first exothermic peak is due to the formation of the b.c.c, phase and the second peak results from the h.c.p, phase formation [1]. The origin of the third exothermic peak is not understood yet. A most interesting result is that an increase in the Nb content shifts the first and second exothermic peaks to higher temperatures, the shift being larger for the second peak than for the first exothermic peak. 3. Conclusion

DSC measurements revealed that a decrease in the metalloid content drastically changes the crystallization behavior of FINEMET-type alloys in the course of their primary crystallization. The observed changes can

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Letter / Materials Science and Engineering A203 (1995) L5 L9

be regarded as showing that the transition from crystallization controlled by a single nucleation mechanism (the precursor mechanism) in FMs to crystallization controlled by two nucleation mechanisms (the precursor and NW mechanisms) in alloys with a lower (compared with FMs) metalloid content and, hence, with a lower stability of the amorphous phase against the formation of NWs. An important point is that the appearance of the second exothermic peak in the DSC curve of alloy F1 cannot be attributed to ordering in the b.c.c, phase nor to the formation of any other phase, implying that crystals of the same phase are nucleated in this alloy by two different mechanisms. Changes in the composition of alloys, whose crystallization is controlled by two nucleation mechanisms, can affect the contribution of each mechanism to the process and its kinetics, as evidenced by the DSC curves shown in Fig. 3. In conclusion, it is worth noting that metallic glasses, whose crystallization behavior is controlled by two mechanisms of primary crystal nucleation, offer new

possibilities for controlling the glass to crystal transition. This is of particular interest for tailoring nanostructured metallic materials.

Acknowledgment Financial support from the Russian Foundation for Fundamental Research (Project 93-02-02213) is gratefully acknowledged.

References [1] A. Serebryakov,V. Stelmukh, A. Gurov and N. Novokhatskaya, NanoStruct. Mater., 5 (1995) 481. [2] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys., 64 (1988) 6044. [3] Y. Yoshizawa and K. Yamauchi, Mater. Sci. Eng., A133 (1991) t76. [4] Ch. Hausleitner and J. Hafner, Phys. Rev. B, 47 (1993) 5689. [5] Ch. Hausleitner, J. Hafner and Ch. Becker, Phys. Rev. B, 48 (1993) 13119.