CRYSTALLIZATION OF A Fe,,Ni,,B,, METALLIC ... - Science Direct

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The synthesis of novel materials by high-energy ball milling was first developed for high-temperature structural applications over 20 years ago (1). Recently, it is ...
Scripta Materialia, Vol. 34, No. 7, pp. 1081-1085, 1996 Else&r Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/96 $12.00 + .OO

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CRYSTALLIZATION OF A Fe,,Ni,,B,, METALLIC GLASS DURING BALL-MILLING Y&e1 Biro1 Materials Research Division Marmara Research Center, Ttibitak Gebze-Kocaeli, Turkey (Received June 22, 1995) (Revised September 29, 1995) Introduction

The synthesis of novel materials by high-energy ball milling was first developed for high-temperature structural applications over 20 years ago (1). Recently, it is receiving a great deal of attention as a potential technique in the production of amorphous alloys, owing to the advantages it offers over liquid quenching. Extension of the glass forming range and the potential to produce bulk amorphous alloys certainly deserve serious conside:ration. Hence, the number of alloy systems which have been amorphized through ballmilling has been increasing steadily ever since it was shown that amorphous alloy powders can be produced by mechanical alloying of a mixture of elemental powders (2) and by mechanical milling of crystalline compounds (3). The structural changes that occur during ball-milling of a glass-forming system is not limited, however, to a crystalline-amorphous phase transition. There are several studies which report at least partial crystallization of amorphous alloys during ball-milling (4-7). These investigators have ball-milled either amorphous powders and ribbons or started with originally crystalline components. In the latter case, crystallization was observed after ball milling beyond the completion of amorphization reaction (7). While the underlying mechanism for a Solid-State Amorphization Reaction (SSAR) is generally agreed upon (8), that for an amorphous-to-crystalline phase transition is still surrounded by controversy. Excessive heating of particles entrapped between colliding balls appears to be the most plausible scenario offered so far to explain crystallization during milling (9). Yet, experimental evidence has been presented to show that a. high local effective temperature can not be singled out as the only reason (6). Contamination by the milling media and/or atmosphere was also reported to lead to crystallization during milling (7). If ball-milling is to be used effectively for the synthesis of amorphous powders it is essential that the underlying mechanism for the crystallization reaction be well understood. The present work was undertaken to understand the effect of contamination by the milling atmosphere on the crystallization reaction during milling. Milling was performed with an already amorphous FeX6N&BZ8alloy which has a relatively high crystallization point (approximately 520°C). Milling conditions were selected so as to allow for contamination while measures were taken to make sure that milling was a mechanical rather than a thermal treatment. Structural changes were monitored and compared with the thermal crystallization behavior of as-cast ribbons.

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Figure I. The DSC scan of as-quenched Fe,,Ni,,B,, ribbons recorded at 40 JUmin.

ExDerimental Fe,,Ni,J3,B metallic glass ribbons were obtained by the Planar-Flow-Casting (PFC) technique. The ribbon width and thickness were 10 mm and 40 pm, respectively. As-cast ribbons were pre-cut to small pieces and ground to coarse powder with a mortar and pestle prior to milling. Milling was performed with a laboratory attritor mill run at 400 r-pm. The pre-milled powder was transferred to a stainless steel vial in ambient air. WC-Co balls were used and the ball-to-powder weight ratio was 10 : 1. The milling experiment was interrupted for 16 hrs after each (f-hour operation to avoid excessive heating. Temperature of the vial was maintained below 30°C by circulating cold water around the vial. Small amounts of powder were removed from the vial at predetermined intervals for structural characterization studies. XRD patterns were taken using a Philips diffractometer equipped with CL&.. radiation. The microstructure of the milled powders and their chemical compositions were studied using a scanning electron microscope (SEM) fitted with electron probe microanalysis (EPMA) facilities. Powder samples were metallographically prepared and etched with a 10% Nital solution. The milling experiment was discontinued after 48 hrs as the present glass could no longer be treated as a ternary alloy. The oxygen content of milled powders was found to be nearly 2 wt% after milling for 48 hrs. Results and Discussiun XRD and differential scanning calorimetry (DSC) patterns of the starting powders were identical to those of as-quenched ribbons confuming that the amorphous structure was preserved after initial grinding. The DSC curve of the unmilled powder recorded at 4OKImin shows a single exothermic peak at approximately 520°C suggesting a one-step crystallization reaction (Fig, 1). It should be noted, however, that the present glass undergoes surface crystallizatjon startingat 350°C as revealed by isothermal crystallization studies.

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26 Figure 2. XRD patterns of starting and milled powder samples: (A)tetragonal and (0) orthorhombic borides.

XRD patterns of Fes,Ni,,B,, powders at selected milling times are presented in Fig. 2. After 16 hrs of milling, several difI%sepeaks were already superimposed on the former pattern. The number and intensity of such peaks increased with increasing milling time. The XRD pattern at 48 hrs clearly indicates the breakdown of the amorphous state and is typical of partially crystallized samples. Judging from the XRD patterns of samples isothermally annealed to full crystallization, two phases were identified in the partially crystallized powder samples: A compound phase which has the same structure with the tetragonal Fe,B, (Fe,Ni),B, and the orthorhombic (Fe,Ni),B phase; the former crystallizing first out of the amorphous matrix and occupying a much larger volume fraction. Since the stability of an amorphous phase correlates best with its crystallization temperature, it seems reasonable to infer that crystallization during milling is associated with a temperature increase. In their investigations regarding the effect of milling intensity on glass formation in mechanically alloyed Ni-Zr, Eckert et al. (9) observed crystallization only when the intensity was too high. They’ve interpreted their observations in’terms of local excessive heating at collision sites where the actual temperature of the powders could rise well above the average temperature of the vial. The metastable equilibrium could, in fact, be disturbed in favor of a crystallization reaction if the amorphous phase is heated above its crystallization i.emperature at collision sites. While the temperature of the vial is measured readily (7), direct measurement of the actual temperature of powders inside the vial is not possible. There have been attempts to estimate the effective temperature

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20 Figure 3. XRD pattern of a ribbon sample annealed at 390°C for 112 hrs.

of metallic powders entrapped between colliding balls. Schwarz and Koch (10) reported that the temperature of these particles could increase by approximately 38°C above the vial temperature. Following their procedure, Eckert et al. (9) estimated a temperature increase, AT, of 287°C at the highest milling intensity. This AT, when added to the average temperature of the vial, which, according to these authors, could be as high as 12O’C at high intensities, gives an effective temperature of 407°C. While this temperature may be sufficiently high for the crystallization of many glasses, it is well below the crystallization point of the metallic glass alloy used in this work. Fe,,Ni,,B,, does crystallize below 400°C merely by the growth of surface crystals, but it takes in the order of days before any crystallization can be detected at these temperatures. Figure 3 presents the XRD pattern of a ribbon sample annealed at 390°C for 112 hrs and is almost identical to the powder pattern after 48 hrs of milling. The similarity of the two patterns implies that even the average temperature of the vial would have to be at least 390°C throughout milling if thermal effects alone were to account for the crystallization reaction. However, this is known not to be true from measurements of the vial temperature which did not exceed 30°C degrees during milling. On the other hand, the possibility of individual particles having been heated to even higher temperatures when entrapped between colliding balls can not be overlooked. Since the excessively heated region of a particle will relax thermally in between two collisions, time scale of crystallization per collision is very short. It is clear from the DSC work that for any crystallization to occur during a collision which lasts only a fraction of a second, the present glass would have to be treated to well above 520°C. The crystallization behavior of this glass in the high-temperature range is well established from the metallographic analysis of salt-bath annealed samples (11). Crystal sizes in ribbons dipped momentarily into salt baths held at various temperatures ranging from 500°C to 650°C were invariably very small suggesting rather high nucleation rates. Above 600°C it was impossible to obtain partially crystallized samples, as the ribbons crystallized almost instantly. Microstructural features of milled powders were, in general, quite different from those of salt-bath treated samples. Majority of the particles were partially crystallized, only a small fraction having transformed completely to a crystalline state and a small fraction having remained amorphous (Fig. 4). Milled powders generally exhibit surface crystallization (Fig. 5), with very few exceptions, unlike salt-bath treated samples which have crystallized almost entirely in the bulk. Considering the variation in size and number of crystallites with increasing milling time, it is fair to conclude that growth was favored over nucleation during milling. While this reaction is strikingly similar to the isothermal crystallization of the present alloy around 400°C that it is so much faster can be explained only in terms of an additional mechanism which

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Figure 4. Microstructure of milled powders. (Note that the crystallized regions appear dark.)

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Figure 5. Surface crystals in powder particles after milling for I6 hrs.

accelerates crystallization during milling. Oxygen contamination, which was shown by EPMA to have approached 2 wit% after 48 hrs of milling, is claimed to be responsible. Crystallization of (Fe,Ni)B glasses are known to be very sensitive to surface oxidation which leads to local compositional changes (12). Selective oxidation of Fe, for instance, leaves behind Ni-enriched regions which undergo crystallization at much lower temperatures than the nominal glass. This could provide an explanation for the extensive crystallization the present glass has experienced during milling at temperatures well below the crystallization point. Conclusions Contamination of milled powders with oxygen apparently plays a crucial role in the crystallization of the Fe,,Ni,,B,, metallic glass during milling. This reaction is very similar, in many respects, to the isothermal crystallization of the same glass around 4OO”C,yielding the tetragonal and orthorhombic borides. Kinetic considerations, however, suggest that there ought to have been an additional factor other than temperature which accelerated crystallization during milling. Selective oxidation is believed to result in Ni-enriched surface regions which crystallize readily at temperatures which prevail during milling. Acknowledgments The author is grateful to M. Ozbey, F. Balli, M. Berk, N. Parkan, 0. Cakir, T. Gonul for technical assistance and :Drs. C. Turner, N. Durlu for stimulating discussions. This research is supported by The Scientific and Technical Research Council of Turkey. References I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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