Crystallization of low silicon content FeSiB metallic glasses

MaterialsScienceand Engineering, A117 (1989) 255-262

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Crystallization of Low Silicon Content F e - S i - B Metallic Glasses M. A. GIBSON and G. W. DELAMORE Department of MaterialsEngineering, Universityof Wollongong, Wollongong,New South Wales2500(Ausmdia) (ReceivedJanuary 31, 1989)

Abstract

A comparison has been made between the microstructures of conventionally solidified alloys and crystallized glassy ribbons in the Fe-Si-B system. It is shown that the types of transformation product found in hypereutectic metallic glasses are consistent with the presence of a wedge-shaped coupled zone of the stable a-Fe + Fe2B eutectic in the ternary phase diagram. Consideration of the nature and extent of the coupled zone is important for alloy selection in order to optimize processing operations and for controlled crystallization of desirable phases in the glassy matrix during subsequent heat treatment.

on the relative growth limitations of the associated primary phases as in the binary case. For a simple ternary eutectic system (Fig. l(b)) the coupled zone associated with the ternary eutectic can be visualized as a cone extending out from the eutectic point, and again the actual shape and position of the cone will be influenced by the growth characteristics of the primary phases. Few EUTECTIC

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1. Introduction Metastable extensions of equilibrium phase boundaries, T0 curves and metastable phase formation are increasingly being used to interpret the microstructures that result from rapid solidification [1-3]. There is also an increasing awareness in the literature that consideration of the nature of the coupled zone in eutectic systems is important for structural characterization of rapidly solidified crystalline alloys [4, 5] and the crystallization products of binary metal-metalloid metallic glasses [6]. The extension of the coupled zone concept to ternary systems is only geometrically more complicated than that for binary alloys and there are idealized situations that can be distinguished, depending on the nature of the ternary phase diagram. For a pseudobinary eutectic system (Fig. l(a)) which contains a eutectic trough extending across the diagram from one binary eutectic to the other, interpolation of the boundaries between the binary coupled zones results in a coupled zone wedge in three dimensions. The position and extent of this wedge are dependent 0921-5093/89/$3.50

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256 ternary systems are as simple as these examples, but the only modifications in more complex systems would be in limitations to the composition range for the coupled region due to the interaction with other phase fields; the underlying fundamentals will hold for any system irrespective of its complexity. The work presented here is an extension of an earlier investigation into the presence of coupled zones in binary glass-forming systems and reports details of the crystallization behaviour of low silicon content (10 at.% or less) Fe-Si-B alloys. This is part of a wider study on the influence of partial crystallization on the magnetic properties of ferromagnetic metallic glasses. 2. Experimental work

A range of Fe-Si-B alloys was prepared by melting together appropriate amounts of high purity components in sealed quartz tubes under argon. The alloy samples were subsequently homogenized by remelting several times on the water-cooled copper hearth of a small arc furnace. The phases present in these master alloys were determined by X-ray diffraction and their microstructure examined by optical microscopy. Metallic glass ribbons approximately 5 mm wide and 25-30 /zm thick were prepared from the master alloy buttons by planar flow casting in air using a stainless steel wheel. Dynamic differential scanning calorimetry (DSC) in a flowing argon atmosphere was used to characterize the crystallization behaviour of the glassy alloys and to determine suitable temperature ranges for isothermal annealing experiments. Isothermal annealing was carried out in a lead bath on ribbon samples wrapped in aluminium foil and coated in colloidal graphite. The crystallization products present in partially and fully transformed ribbons were examined by transmission electron microscopy (TEM) and the phases identified by electron and X-ray diffraction.

large non-faceted dendrites of an iron-rich solid solution in a eutectic matrix. As the boron content in the alloys increases, the dendritic primary phase is replaced by faceted particles of Fe2B, again in a eutectic matrix, much of which has a complex regular morphology. Dynamic DSC traces for melt-spun ribbon samples of these alloys are shown in Figs. 4 and 5. For low boron content alloys, two crystallization peaks are observed, the first substantially smaller in height and broader than the second. These two peaks merge together as the boron content is increased by the gradual shift of the first peak to higher temperatures while the position of the second peak remains virtually insensitive to changes in composition. For medium range boron contents (Fe78Si4B18 and Fe75Si10B15), the low temperature peak disappears and an additional peak appears on the high temperature side of the main peak. For high boron contents, only a single peak is observed. Supplementary DSC experiments up to 1273 K, conducted in a Stanton Redcroft instrument, showed no additional peaks in the traces up to this temperature. TEM photomicrographs of fully transformed ribbon samples in the Fe96_xSi4Bx series isothermally annealed at 723 K are shown in Fig. 2. The microstructure of FeszSi4Bt4 (Fig. 2(f)) consists of a large volume fraction of non-faceted a-Fe dendrites in an a-Fe + Fe2B eutectic matrix. The morphology of the eutectic is fine and irregular. As the boron content increases, the volume fraction of the primary a-Fe dendrites decreases, becoming zero in Fev4Si4B22which has a fully coupled structure of a-Fe + Fe2B. A similar sequence was found for the Feg0_xSil0B x series as shown in Fig. 3. Non-faceted a-Fe dendrites in a eutectic matrix were observed in both FesoSil0Blo and FeTsSiloB12 (Figs. 3(e) and 3(f) respectively), while the primary phase in Fe75SiioB15 is non-faceted Fe3Si dendrites in an Fe3Si + Fe2B eutectic (Fig. 3(g)) [7]. Fe72SiloBj8 is fully coupled Fe3Si + Fe2B; no primary phase of any kind was observed in the microstructure (Fig. 3(h)).

3. Results

The as-cast microstructures of the master alloys for the Fe96_xSi4Bx and Fe90-xSil0Bx series of alloys investigated are shown in Fig. 2 and Fig. 3 respectively and are very similar to those observed in the binary Fe-B system. Hypoeutectic alloys (Fe82Si4B14 and Fe80Sil0Bl0) contain

4. Discussion

There is limited information on the characteristics of the Fe-Si-B ternary phase diagram. The X-ray investigations of Aronsson and Engstrom [8] show that most of the established glassforming region in this system [9] is contained in

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an essentially two-phase field consisting of an iron-rich solid solution and F%B (Fig. 6); it should be noted, however, that there is a disorderorder transition from b.c.c, a-Fe to f.c.c. Fe3Si in the iron-rich phase with increasing metalloid content. Thermal analysis [10] on bulk alloy specimens indicated the presence of a trough in the liquidus surface extending from the Fe-B eutectic out into the ternary diagram. These results are in good agreement with the position of the eutectic trough determined by De Cristofaro et al. [11] from metallographic observations, at least for low to medium silicon contents (less than 15 at.% Si). All the available data indicate that the Fe-Si-B system behaves like a pseudobinary

eutectic within the glass-forming range, at least for low silicon contents (10 at.% Si or less); beyond this region the presence of other phase fields introduces complications. The microstructures of the master alloys for both series in the present work show a transition from a non-faceted primary to a faceted primary in a eutectic matrix with increasing boron content, indicating that both series span the eutectic trough. The crystallization behaviour of the amorphous phase in both alloy series was also similar, as shown by the results of the DSC analysis (Figs. 4 and 5). Thermal stability appears to be a strong function of composition in the glass-

Fig. 2. Optical and TEM photomicrographs of (a)-(e) as-cast master alloys and (f)-(j) fully crystallized (annealed at 723 K for 65 h) glassy ribbons for the Fe96_/Si4Bx series: (a) Fe82Si4B~4 (non-faceted iron-rich primary dendrites in a pseudobinary eutectic matrix); (b) FesoSi4B16(small faceted Fe2B primary crystals in a largely complex regular a-Fe + Fe2B eutectic matrix); (c) FevsSi4Bis (Fe2B in a eutectic matrix); (d) Fe76SiaB20(Fe2Bin a eutectic matrix); (e) Fe74Si4B22 (Fe2B in a eutectic matrix); (f) F%2Si4Bu ribbon (a-Fe dendrites and two-phase matrix product); (g) FesoSi4BL~,ribbon (ct-Fe dendrites in an a-Fe + Fe2B matrix); (h) FeTsSi4Bis ribbon (as for (g)); (i) Fe76Si4B20ribbon (as for (g)); (j) Fe74SiaB22ribbon (fully coupled a-Fe + Fe2B eutectic).

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Fig. 2. (continued)

forming region close to the iron-rich boundary whereas it becomes relatively insensitive to composition at higher boron contents. The poor thermal stability of the low boron content alloys is associated with the easy crystallization of the non-faceted iron-rich primary phase, this process

becoming progressively more difficult as the boron content increases, as can be seen from Figs. 2(f)-2(i); the corresponding trend for crystallization of primary Fe2B is not observed, presumably because of nucleation and/or growth difficulties experienced by the faceted Fe2B

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Fig. 3. Optical and TEM photomicrographs of (a)-(d) as-cast master alloys and (e)-(h) fully crystallized glassy ribbons for the Fe90_xSiloBx series: (a) FesoSijoBi0 (non-faceted a-Fe dendrites in an a-Fe+Fe2B matrix); (b) Fe78SijoB12 (almost entirely eutectic); (c) Fe75Si~0B15 (faceted Fe:B primary in a eutectic matrix); (d) Fe72SiloB18 (Fe2B crystals in a eutectic matrix); (e) FesoSiioBl0 (ribbon annealed for 75 h at 723 K showing a-Fe dendrites in an a-Fe+FezB eutectic matrix); (f) FevsSi~0B~2 ribbon annealed for 90 h at 723 K (as for (e)); (g) Fe75SijoBj5 ribbon annealed for 180 h at 723 K (primary non-faceted Fe~Si in rash Fe3Si + Fe2B eutectic matrix); (h) Fe72Si10B~8ribbon annealed for 240 h at 723 K (fully coupled Fe3Si + Fe2B eutectic).

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Fig. 3. (continued)

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phase, leading to a skewed coupled zone in the phase diagram. This is apparent when the microstructures of the master alloys solidified from the liquid are compared with those obtained by crystallizing the glass. FesoSi4B16, Fe78Si4B18, Fe76Si4B20 and Fe75Si10B15 all have master alloy microstructures which contain faceted Fe2B primary whereas the primary phase that forms from the amorphous state is a non-faceted ironrich solid solution, indicating that these compositions lie on the hypoeutectic side of the coupled zone at lower temperatures. Similarly, Fe74SiaB22 and Fe72Si10B]8 have a large volume fraction of faceted Fe2B primary in the master alloy but are fully coupled when crystallized from the glass and therefore must lie within the coupled zone at this temperature. Taking into consideration all the published dynamic DSC data for the Fe-Si-B system together with the present results [9, 11-23] (Fig. 7), it appears that there is a general trend for the two-step crystallization behaviour in low silicon content alloys, which has been shown to be associated with the crystallization of a non-faceted primary, to extend 2-3 at.°/c beyond the projection of the eutectic trough and approximately parallel to it into the faceted primary phase field. This suggests the presence of a coupled zone wedge extending from the stable a-Fe + Fe2B eutectic in the Fe-B binary system into the ternary diagram and skewed towards the faceted FezB phase, although the precise composition and temperature range are not known at present. For silicon contents greater than 10 at.% the situation becomes complicated by the formation of additional phases [24]. It is well established that there is a metastable eutectic in the Fe-B system between a-Fe + Fe3B

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[25] and this eutectic has also been observed in certain Fe-Si-B glasses [11, 17, 19, 20]. Competition between stable and metastable eutectics and the interaction of their associated coupled zones have been demonstrated in the AI-Fe system [26, 27] and seems equally likely in ternary systems. In fact, recent work [28] has established that certain glassy alloys in the Fe-Si-B system can crystallize to either the stable or the metastable eutectic depending on the annealing temperature, presumably owing to the relative nucleation and growth kinetics of the two products under the imposed conditions, although from this preliminary work the precise nature of the interaction of the coupled zones is not yet clear. There was no evidence of the metastable eutectic in the current study. Information on the nature and extent of the coupled zone in the Fe-Si-B system has practical

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Fig. 7. Compilation of published [11-23] DSC data for Fe-Si-B glasses [small symbols), together with results from this investigation (large symbols): • , small low temperature peak (e.g. Fe82SiaBI4, Fig. 4); *, small high temperature peak (e.g. FevsSiaB18,Fig. 4); ,,, single peak (e.g. Fe76Si4B20, Fig. 4).

significance since slightly hypereutectic metalmetalloid alloys are, in general, the easiest glass formers [9, 29-31]; economic mass production of high-quality strip is therefore simpler if such compositions can be used. Research by Hasegawa et al. [32] has also shown that heat treatment produces approximately 1 vol.% of crystalline precipitates in the amorphous matrix, leading to an improvement in the high frequency magnetic properties of Fe-B metallic glasses, and that it is primary a-Fe rather than Fe2B which is most effective in this regard. The presence of a skewed

262

coupled zone wedge in the Fe-Si-B system therefore allows the selection of hypereutectic compositions for improved glass formability when quenching from the melt, while retaining the possibility of producing a random distribution of primary a-Fe particles on heat treatment, thereby enhancing the high frequency magnetic properties of the resulting material.

Acknowledgments We gratefully acknowledge financial support for this work from the Australian Research Council. We also thank Dr. R. K. Day, Dr. J. B. Dunlop and Dr. C. P. Foley for the high temperature DSC work.

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