PHYSICAL REVIEW B VOLUME 57, NUMBER 1 1 JANUARY 1998-I Absence of evidence of decomposition of Fe2B during mechanical grinding J. Balogh Research Institute for Solid State Physics, P.O. Box 49, H-1525 Budapest, Hungary Z. E. Horva´th KFKI Research Institute for Materials Science, P.O. Box 49, H-1525 Budapest, Hungary T. Pusztai Solid State Physics Department, Eo¨tvo¨s University, Budapest, Hungary T. Keme´ny Research Institute for Solid State Physics, P.O. Box 49, H-1525 Budapest, Hungary I. Vincze Research Institute for Solid State Physics, P.O. Box 49, H-1515 Budapest, Hungary and Solid State Physics Department, Eo¨tvo¨s University, Budapest, Hungary Received 21 April 1997; revised manuscript received 22 September 1997 The appearance of bcc Fe after grinding Fe2B for a long time has been formerly interpreted as being due to the decomposition of the thermodynamically stable intermetallic compound when the crystallite size is reduced to a few nanometers. The results of our control experiments performed by tungsten-carbide milling tools show that the appearance of bcc Fe has no relation to the grain size but should be connected to contamination when tools made of steel are used to pulverize this material. Mo¨ssbauer spectroscopy was applied to search for the appearance of bcc Fe while changes in the grain size were checked by x-ray diffraction and transmission electron microscopy. S0163-1829 98 09801-4 INTRODUCTION appearance of bcc-Fe during the milling process: i the chemical reaction14 Fe2B O2 BO2 2Fe under poor A wide range of amorphous alloys can be obtained by vacuum conditions and ii contamination from the chrome high-energy ball milling of elemental powder mixtures or steel milling tools. crystalline alloys.1­3 Free-energy considerations4 are widely Although Ref. 7 made serious efforts to exclude the pos- used to predict the composition ranges where the alloy sibility of oxidation i , the problem of contamination ii is phases formed in the solid-state process are amorphous or not considered. Though the experimental observation of nanometer-size partly or fully crystalline. In some cases, however, the ob- boron particles would not be an easy task, there is a simple servation of a reverse process has been claimed, i.e., when a way to check whether the appearance of bcc-Fe is accompa- compound phase decomposes into elemental components5­8 nied by boron segregation. In this case iron and boron are as a result of milling. In a recent paper the thermodynami- just in the right amount to form Fe cally stable intermetallic compound, Fe 2B and after a proper heat 2B, was reported7 to treatment the disappearance of bcc-Fe is expected, while in decompose into elemental boron and iron when the crystal- all other cases a change of the overall chemical composition lite size is reduced to a few nanometers by long-time me- is to be detected. This kind of control indicated a deviation chanical pulverization. In view of the amount of energy nec- from the Fe2B stoichiometry15 for samples ground by chrome essary to separate Fe2B into its components 35 kJ/mol this steel milling tools and the hypothesis of decomposition was statement is highly surprising. This energy far exceeds the questioned. In this paper we study the possibility of decom- energy release ever observed during the recovery and relax- position by presenting milling experiments under circum- ation processes of other nanocrystalline metals and alloys9 stances where iron contamination is excluded. For this aim including the nanophase alloy of FeB.10 Further questions samples have been prepared by tungsten carbide milling are raised by the fact that milling of the elemental powder tools and characterized by different methods. The grain size mixture results in the formation of an amorphous alloy.11,12 reduction was followed by x-ray diffraction XRD and Amorphous alloy formation by solid-state diffusion of the transmission electron microscopy TEM and the appearance elemental components in nanostructured multilayers was of bcc-Fe was searched by Mo¨ssbauer spectroscopy, which is also reported.13 a very sensitive tool to detect a small amount or small grain The decomposition of Fe size bcc-Fe precipitates. 2B was deduced from the appear- ance of bcc-Fe in the x-ray-diffraction pattern and in the EXPERIMENT Mo¨ssbauer spectra, while the segregation of elemental boron was not verified experimentally. However, besides the direct Fe2B ingots were pulverized in a Fritsch ``Pulverisette 7'' decomposition there are two other possibilities to explain the type planetary ball mill supplied with tungsten carbide vials 0163-1829/98/57 1 /29 4 /$15.00 57 29 © 1998 The American Physical Society 30 BRIEF REPORTS 57 pears already after 4 h milling time at I 8. The intensity of the WC lines increases as the milling time is increased and all the well-resolved lines belong to WC when the maximum milling intensity (I 10) is applied for 24 h. Broadening of the lines belonging to Fe2B are already apparent after a 12 h milling at I 8. The 002 and 121 reflections at 42.5° and 45.0° angles, well resolved in the unmilled sample, overlap to form a broad asymmetric peak after a one day milling time. This feature is a raw indication that the one day milling at I 8 in our experimental setup produced a grain size com- parable to the one which was claimed7 to destabilize Fe2B. However, since WC has several lines close to those of Fe2B, the correct evaluation of the average grain size of the Fe2B crystallites is not easy. A reliable distinction between contri- butions from grain-size reduction, deformation-induced stress, and amorphous phase formation cannot be made. The average grain size calculated from the line broadening of the 121 peak neglecting other contributions to the line width is about 2 nm in the case of the two samples milled for one and for two days at I 8. Further milling results in the gradual disappearance of the distinguishable Fe2B lines and an amor- FIG. 1. XRD pattern of Fe phouslike broad background is formed under the WC lines as 2B pulverized with tungsten carbide milling tools for 0, 4, 12, 24, and 48 h at I 8 milling intensity and it can be seen in Fig. 1 in case of the one day milling at for 24 h at I 10 milling intensity. Line positions of Fe I 10. 2B, WC, and bcc-Fe are indicated in the bottom. To get a more reliable value of the average grain size TEM measurements were made on the samples milled for and balls. The vials were sealed under argon atmosphere. one and two days at I 8. Dark-field TEM micrograph taken Eight 4 g balls and ball to powder mass ratio 30 were ap- with the strongest Fe2B ring at d 0.2 nm of the sample plied. The milling intensity (I) could be varied on a ten milled for one day is shown in Fig. 2. The TEM micrograph grade scale, where the maximum setting is 700 rev/min. Each time starting with a fresh ingot, milling times from a few hours up to several days were applied. The XRD measurements were carried out on a Philips EXPERT diffractometer. The spectra from the powder samples were collected in parafocusing geometry, the Cu K radiation was selected by a curved pirolitic graphite analyzer placed in the two- arm. The powder samples were measured in a Si single-crystal sample holder. The TEM study of the samples was made with a Philips CM20 electron microscope with TWIN objective lenses and a Noran Voyager energy dispersive spectrometer EDS ana- lyzer with HPGe detector. Bright-field and dark-field imag- ing techniques as well as selected-area electron-diffraction patterns and EDS spectra were used to detect various crys- talline phases in the samples. For the specimen preparation a special method based on ion milling16,17 was used. The pow- der grains mixed with araldite powder were filled into the slot of a Ti disk in between two small pieces of silicon wafer. The epoxy was then hardened by polymerization in a furnace of 440 K for 1 h. After mechanical grinding to about 50 m thickness the final thinning was performed by ion milling. 57Fe Mo¨ssbauer spectra were recorded by a standard con- stant acceleration spectrometer using 25 mCi 57Co Rh source and fitted with the usual six-line pattern of Lorenzian line shape. RESULTS AND DISCUSSION XRD patterns of the samples milled for different times at two different milling intensities are shown in Fig. 1. Besides FIG. 2. Electron micrograph of the sample ground for 24 h at the lines of Fe2B, the pattern of tungsten carbide WC ap- I 8 milling intensity. 57 BRIEF REPORTS 31 when the I 10 milling intensity is applied, no sign of bcc-Fe can be detected either. The amount of the alloy phase increases during further milling and a broad hyperfine field distribution comprises about 80% of the spectra. Since be- sides the WC lines no sharp peak appears in the XRD pat- tern, this alloy phase is most probably an amorphous phase formed by solid-state reaction of Fe2B and WC. It is also obvious that the composition of the sample will be changing gradually with further milling and a steady state of the sample can hardly be attained because of the continuously milled-in wear debris. However, due to the huge mass ab- sorption of W, following the process by Mo¨ssbauer spectros- copy becomes increasingly troublesome. A detailed investi- gation of the milling intensity dependence of the alloying rate of Fe and WC or experiments with milling tools made of different kinds of iron-free materials, e.g., agate or zirconia, should reveal if besides the obviously present contamination effect a hypothetical decomposition process also exists. CONCLUSION FIG. 3. Mo¨ssbauer spectra of the Fe The notion that intermetallic compounds may decompose 2B ingot and the samples pulverized by tungsten carbide milling tools for different times at to their elemental components when the grain size is reduced I 8 milling intensity and for 24 h at I 10. Positions of the two below a critical value is widely accepted, though direct trac- outermost lines of the sextet expected for bcc-Fe are indicated by ing of the appearance of both components is usually not very arrows. easy5 and in many cases6­8 not performed. Therefore chemi- cal reactions either with the wear debris or with residual shows large WC particles dark spots spread among the gases or contamination from the milling tools leading to the smaller Fe2B crystallites white spots . The large dark spots appearance of one of the elemental components might be are rich in tungsten and the matrix is rich in iron as found by misinterpreted. When direct tracing of the appearance of EDS. The diameter of the Fe both components is not possible, there are at least two indi- 2B particles varies between 2 and 20 nm with an average of about 4.5 nm measured manu- rect ways to check the decomposition theory. By heat treat- ally. This is in reasonable agreement with the x-ray results, ing the milled sample an increase of the grain size and re- and shows that a slight broadening of diffraction lines can covery of the thermodynamically stable intermetallic come from other than grain-size effects. A similar TEM mi- compound is expected. The other possibility is to follow the crograph was observed on the sample milled for two days. process of grain-size reduction by applying milling tools This indicates that no further grain size reduction can be made of different materials. In the case of Fe2B we per- expected and the minimum value is reached at a one day formed both kinds of experiments to determine if the appear- milling time at I 8. ance of bcc-Fe when the samples are pulverized by Cr-steel Mo¨ssbauer spectra of some samples milled for different tools is a product of decomposition. The first kind of check times are shown in Fig. 3. No bcc-Fe can be observed in any has shown15 that the bcc phase contains a significant amount of the measured spectra within a 1 at % accuracy. The spec- of Cr and does not disappear in heat treatments. Mo¨ssbauer tra can be fitted by two sextets representing Fe spectroscopy, x-ray diffraction, and transmission electron 2B with hy- perfine fields of 23 and 24 T and isomer shifts of 0.11 mm/s microscopy results reported here show that Fe-based bcc relative to -Fe, in good agreement with those of the un- structure does not appear when the Fe2B compound is milled milled ingot and with additional sextets most probably due by WC tools. Though the possibility that the process of de- to an alloy phase of Fe composition is overwhelmed by an alloying process with the 2B and WC, which was not aimed to be identified. The atomic fraction of iron atoms in the Fe milled in wear debris cannot be unambiguously excluded, the 2B phase is 52 5 and 37 5 percent of all iron atoms in the results of the two series of measurements cast serious doubt samples milled at I 8 for one and two days, respectively. on the interpretation that Fe2B is decomposed into elemental According to the TEM picture this fraction has grain sizes in components when the grain size is reduced to the nanometer the range of 2­20 nm and one would expect the appearance range. of bcc-Fe if Fe2B were destabilized below a 4 nm grain size. One cannot exclude the theoretical possibility that an alloy ACKNOWLEDGMENTS phase is formed from decomposed elemental Fe and B. In this case, however, the rate of alloying with WC should be This work was supported by the Hungarian Scientific Re- equal to or higher than the rate of decomposition. 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