Nanocrystalline alloys:I. Crystallization
M. Miglierini et al.
Department of Nuclear Physics and Technology Slovak University of TechnologyIlkovicova 3,812 19 Bratislava, SlovakiaE-mail: email@example.com://www.nuc.elf.stuba.sk/bruno
Nanocrystalline alloys prepared by controlled annealing from rapidly quenched amorphous ribbons exhibit an interesting class of materials from the point of view of their magnetic properties . Resulting magnetic parameters, which are superior to those of conventional transformer steels and/or amorphous materials, are ensured by a presence of crystalline grains several nanometres in size embedded in the amorphous residual phase . Magnetic parameters of amorphous alloys are frequently deteriorated in the process of their practical employment by elevated temperature especially during prolonged operational times. On the other hand, nanocrystalline alloys are in fact already partially crystallized and from this point of view their structure is more resistant to such external effects and that is why it is more stable. Nevertheless, because the excellent magnetic behaviour of nanocrystalline alloys depends strongly on the amount and size of the crystalline grains, the process of crystallization should be known.
 K. Suzuki, A. Makino, A. Inoue, T. Masumoto, J. Appl. Phys. 70 (1991) 6232. G. Herzer, Phys. Scr. T49 (1993) 307.
The following slide shows a comparison of some magnetic parameters (magnetic permitivity me versus saturation magnetization Bs) for different types of magnetic materials used for, e.g. the production of cores of magnetic circuits. The main three types of compositions which yield nanocrystalline alloys are also listed.
Nanocrystalline Alloys - Featuresnanocrystalline alloysgood soft magnetic propertiesthermal stabilization of the structure as compared to amorphous alloys
1988: FINEMET: FeCuNbSiBYoshizawa Y, Oguma A, Yamauchi K J Appl Phys 64 (1988) 60441988: NANOPERM: FeMB(Cu) where M = Zr, Mo, Ti, Nb, Hf, Suzuki K, Kataoka N, Inoue A, et al. Mater Trans JIM 31 (1990) 7431998: HITPERM: FeCoZrB(Cu)Willard M A, Laughlin D E, McHenry M E, et al. J Appl Phys 84 (1998) 6773 A. Makino, A. Inoue and T. MasumotoMater Trans JIM 36 (1995) 924
Possible Applications of Nanocrystalline Alloysmagnetic shieldingtransformersensorsribbonscore
Preparation of Nanocrystalline Alloysproduction of an amorphous precursormixing of appropriate amounts of pure elements with subsequent meltingrapid quenching of the melt ( ~106 K/min) method of planar flow castingresult: ribbon up to several cm wide and typically about 20 mm thickcheck of composition (OES ICP) and amorphicity (XRD)
(nano)crystallizationcheck of crystallization behaviour by DSC (onset of crystallization, first crystallization peak)choice of temperature of annealingannealing (in vacuum) for typically 1 hour at the selected temperaturecharacterization of the resulting structural and magnetic propertiesplanar flow castingamorphous ribbon
Structures from a Melt Ordered structureperiodicitylong range order Disordered structureshort range orderno translation symmetryStarting material(melt)quasicrystallinecrystallineamorphousnanocrystallineConditions (quenching rate, composition, )
Characterization of Nanocrystalline Alloys structural characterizationDSC (differential scanning calorimetry)evolution of structure with temperatureXRD (X-ray diffraction)crystalline phases, relative fraction of crystallites and amorphous restTEM (transmission electron microscopy)including HREM (high resolution TEM) and XTEM (cross-sectional TEM)type and size of (nano)crystalsSTM (scanning tunnelling microscopy)including AFM (atom force microscopy)surface featuresED (electron diffraction)structural ordering of phases magnetic propertiesmagnetic measurements
57Fe Mssbauer spectroscopy (TMS + CEMS)simultaneous information on both structural arrangement and magnetic behaviour (hyperfine interactions)Miglierini M et al. J Appl Phys 85 (1999) 1014
Mssbauer spectrometry is a very sensitive tool for the study of both structural arrangement and hyperfine interactions (magnetic ordering) in nanocrystalline alloys . Tthe FINEMET-type alloys, which are very frequently studied because their macroscopic properties are beneficial for practical applications  exhibit rather complicated Mssbauer spectra. They consist of several sextets of narrow lines ascribed to different crystallographic positions in the Fe-Si lattice which are superimposed upon a broadened signal which belongs to the amorphous rest of the original precursor . Evaluation of such spectra is pretty complicated and, unfortunately, prevents from acquiring more detail information related to such phenomena as for example interfacial regions . In order to benefit from its diagnostic potential, it is useful to investigate such materials whose Mssbauer spectra are reasonably simple. This is the situation for example in NANOPERM-type alloys which crystallize into bcc-Fe, the latter being a calibration material for Mssbauer spectrometry. Thus, here we concentrate on the Fe-Mo-Cu-B system which belongs to the NANOPERM family. H. Bremers, O. Hupe, C. E. Hofmeister, O. Michele and J. Hesse: J. Phys.: Condens. Matter 17 (2005) 3197. T. Liu, Z. X. Xu and R. Z. Ma, J. Magn. Magn. Mat. 152 (1996) 365. T. Pradell, N. Clavaguera, J. Zhu and M. T. Clavaguera-Mora: J. Phys.: Condens. Matter 7 (1995) 4129. J. M. Grenche and A. Slawska-Waniewska, J. Magn. Magn. Mat. 215-216 (2000) 264.
Structural Arrangement and Mssbauer Spectracrystalline(ordered structure)hyperfine parametersamorphous(disordered structure)Mssbauer spectra of an ordered structure (crystallites) exhibit narrow lines which lead to single values of the spectral parameters. Due to non-unique positions of resonant atoms in a disordered structure the spectral lines are broad and, consequently, distributions P() and P(B) of the spectral parameters must be considered.
Mssbauer Spectra of Nanocrystalline Alloys (295 K)FINEMETFe73.5Nb3Cu1Si13.5B9NANOPERMFe80Mo7Cu1B12Miglierini M J Phys Condens Matter 6 (1994) 1431Miglierini M and Grenche J-M J Phys Condens Matter 9 (1997) 2303
Annealing of the Amorphous PrecursorDSC continuous heating (temperature ramp of 10 K/min)choice of annealing temperatures (B-M, A = as-quenched) => sample preparationonset of crystallization identified at Tx1
diffusion-like precrystallization effects normal grain-growth-like formation of a-Fe nanocrystallites in amorphous matrixdiffusion controlled grain-growth of already created a-Fe nanocrystallites diffusion controlled nucleation and growth-like precipitation of g-Fe(Mo)
structural relaxation Tx1 460oCMiglierini M et al. phys stat sol (b) 243 (2006) 57Fe76Mo8Cu1B15
TEM and XRDTx1 = 450 oCTx1 450oCMiglierini M et al. phys stat sol (b) 243 (2006) 57Fe76Mo8Cu1B15
Mssbauer Spectrometry300 Kevolution of Mssbauer spectra with temperature of annealing tatransmission Mssbauer spectra are plotted upside-down to enable 3D mappingtemperature of measurement 300 K and 77 KFe76Mo8Cu1B15Miglierini M et al. phys stat sol (b) 243 (2006) 5777 K
Fitting ModelFe80Mo7Cu1B12 440oC/1hMiglierini M and Grenche J-M J Phys Condens Matter 9 (1997) 2303, 2321Miglierini M and Grenche J-M Hyperfine Interact 113 (1998) 375crystalline
Transmission Mssbauer Spectrometry (295 K)bulkTx1 = 450 oC (?)Miglierini M et al. phys stat sol (b) 243 (2006) 57Fe76Mo8Cu1B15
Conversion Electron Mssbauer Spectrometry (295 K)surfaceTx1 = 450 oCMiglierini M et al. Hyperfine Int 165 (2005) 75Fe76Mo8Cu1B15
XRD Peak DecompositionTx1 = 450 oCMiglierini M et al. phys stat sol (b) 243 (2006) 57Fe76Mo8Cu1B15
Summarystructure of nanocrystalline alloys(nano)crystallitesresidual amorphous matrixinterface = surface of crystalline grains + crystal-to-amorphous matrix region
crystallizationfirst at the surfaceprogress of crystallization is more rapid at the surfaceidentification of crystalline phaseamount of nanocrystals
Mssbauer spectroscopy contributes to the study of nanocrystalline alloys from several viewpoints. First, it is possible to identify the structural arrangement from a very first look at a Mssbauer spectrum (e.g., onset and progress of crystallization). Crystalline phases are characterized by narrow and usually well separated lines whereas the amorphous residual phase exhibits broad patterns due to its disordered nature. Signal from resonant atoms located at the interfacial regions can be also distinguished. The latter two contributions are described by the help of distributions of hyperfine parameters through which information on both topological and chemical short-range order can be derived. The fraction (and/or type) of the crystalline phase(s) can be readily obtained from the spectral parameters.Second, magnetic order of the system under the study is also directly followed from changes of the spectral line shapes, viz. (broadened) doublet vs. sextet. This can be studied as a function of annealing temperature (i.e., crystalline contents), measuring temperature, and/or composition. More details can be found in another presentation.In this presentation, we have shown that the crystallization of amorphous precursors for the preparation of nanocrystalline alloys proceeds more rapidly on the surface of the rapidly quenched ribbons than in their bulk. In doing so, we have employed CEMS and TMS, respectively. The crystalline content was determined also from XRD and the results coincide well with those from TMS.The temperature of the onset of crystallization Tx1 determ