We produced amorphous Fe–Si–B soft-magnetic powder using water atomisation. During annealing the powder particles developed a nanocrystalline structure, and annealing at over 700°C led to the formation of ferrite and boride phases. Here we present a high-magnification electron backscatter diffraction (EBSD) mapping analysis of the powder particles, in combination with a field-emission-gun scanning electron microscopy (FEGSEM) analysis. Some of the problems associated with the preparation of the powder particles for the EBSD analysis as well as the drift problems occurring during the EBSD mapping are reported.Amorfni Fe-Si-B magnetni prahovi su bili proizvedeni vodenom atomizacijom. Za vrijeme procesa žarenja čestice praha dobiju nanokristaliničnu strukturu. Žarenje iznad 700 °C prouzroči nastanak feritne i boridne faze. U ovom članku je prikazana EBSD analiza područja kod velikog uvećanja prašnih čestica u kombinaciji s FESEM. Istaknuti su problemi pripreme čestica praha za EBSD analizu kao i problem pomicanja uzorka tokom EBSD analize

B. Šuštaršič

M. Godec

M. Jenko

English

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M. GODEC, B. [U[TAR[I^, M. JENKOA HIGH-MAGNIFICATION EBSDMAPPING ANALYSIS OF Fe–Si–B POWDER PARTICLESReceived – Prispjelo: 2005-12-19Accepted – Prihva}eno: 2007-05-14Orginal Scientific Paper – Izvorni znanstveni radINTRODUCTIONOf the group of common soft-magnetic materialsthat includes high-purity iron, silicon steels, iron-nickeland iron-cobalt alloys, and ferrites, one material is in-creasing in terms of commercial importance: the amor-phous or nanostructured soft-magnetic material basedon Fe–Si–B. Nanostructured, soft-magnetic materialsderived from rapidly solidified Fe–Si–B alloys exhibitexcellent soft-magnetic properties due to the crysta-llisation of nanoscale -iron grains in an amorphous ma-trix 1, 2.They are usually produced in the form of thin rib-bons, using either melt-spinning or planar-flow castingtechniques; however, by using gas- and water-atomi-sation procedures, fine powder particles can be obtai-ned. When these particles are combined with a bindinginsulation material they are described as soft-magneticcomposites or SMCs 3.In order to achieve the nanocrystalline structure,careful control of the annealing treatment is required 4.Various studies have been published [5,6 relating to thecharacterisation of Fe–Si–B-based SMCs, but none ofthem have made use of electron backscatter diffraction(EBSD) as an analytical tool for the phase identification.For an introduction to the determination of micro-tex-tures using EBSD the reader is referred to 7,8. In thisstudy we present a high-magnification EBSD mappinganalysis of Fe–Si–B-based powder particles in combi-nation with a field-emission-gun scanning electron mi-croscopy (FEGSEM) analysis.EXPERIMENTALPROCEDUREThe experimental Fe–Si–B powders were preparedby water-atomisation using a Davy McKee-type D5/2pilot water-atomiser. A charge of 10 kg was prepared,melted in an induction-heating furnace and rapidly coo-led in the water spray produced by a nozzle of dimen-sions 1,2 dia. x 1,05 mm and at a pressure of 20 MPa.After applying a sieve-separation technique, the finestfraction (< 45 µm) was obtained with the followingchemical composition (in weight percent): Fe 88,1 %, Si6,84 %, B 3,14 %, Ni 1,45 %, C 0.025 % and Al 0,12 %.Samples of water-atomised powder were air an-nealed from 300 to 900°C in increments of 100°C. Theannealed powders were then hot mounted in Bakelite,followed by a grinding-and-polishing procedure. Thelast step in the preparation of the samples for the EBSDanalysis was colloidal silica emulsion polishing (OPS),METALURGIJA 47 (2008) 1, 9-12 9We produced amorphous Fe–Si–B soft-magnetic powder using water atomisation. During annealing the pow-der particles developed a nanocrystalline structure, and annealing at over 700°C led to the formation of ferriteand boride phases. Here we present a high-magnification electron backscatter diffraction (EBSD) mappinganalysis of the powder particles, in combination with a field-emission-gun scanning electron microscopy(FEGSEM) analysis. Some of the problems associated with the preparation of the powder particles for the EBSDanalysis as well as the drift problems occurring during the EBSD mapping are reported.Key words: EBSD, soft-magnetic materials, Fe–Si–B-based powder, nanocrystalline structure, amorphousFe-Si-B ~estice praha studirane EBSD analizom podru~ja kod velikog uve}anja. Amorfni Fe-Si-B ma-gnetni prahovi su bili proizvedeni vodenom atomizacijom. Za vrijeme procesa `arenja ~estice praha dobiju na-nokristalini~nu strukturu. @arenje iznad 700 °C prouzro~i nastanak feritne i boridne faze. U ovom ~lanku jeprikazana EBSD analiza podru~ja kod velikog uve}anja pra{nih ~estica u kombinaciji sa FESEM. Istaknuti su pro-blemi pripreme ~estica praha za EBSD analizu kao i problem pomicanja uzorka tokom EBSD analize.Klju~ne rije~i: EBSD, meki magnetni materiali, Fe-Si-B prahovi, nanokristalini~ne strukture, amorfnostISSN 0543-5846METABK 47(1) 9-12 (2008)UDK: 621.318.1; 669.018.95; 621.38=111M. Godec, B. [u{tar{i~, M. Jenko, Institute of Metals and Technology,Ljubljana, Sloveniawhich lasted for 5 minutes. It should be noted that it isvery important to wipe the sample surface with cottonwool immediately after the OPS polishing and then toclean the surface in an ultrasound bath. The surfaces ofthe samples were sputtered with a 1-nm-thick carbonlayer using a GATAN Precision Etching Coating Sys-tem PECS 682. The samples for the high-magnificationSEM observation were etched in 2 % nital.The microstructures of the powders were analysedusing a JEOL JSM 6500F field-emission-gun electronmicroscope equipped with an EBSD INCA CRYSTAL300. The accelerating voltage of the primary electronbeam for the backscatter images was 15 kV, the aperturediameter was 30 µm, the probe current was 0,08 nA andthe working distance (WD) was 6 mm. The EBSD spotanalysis and mapping were performed using a probecurrent of 7 nA, a sample tilt angle of 70,5 °, a WD of 15nm and an aperture of 50 µm.RESULTSXRD (X-ray diffraction) and HRTEM (high-reso-lution transmission electron microscopy) analysesshowed that the water-atomised Fe–Si–B powder had anamorphous structure. However, during air-annealing theamorphous structure changed to a crystalline one. At anannealing temperature of 500°C the first isolated crys-talline phases began to appear. These crystalline phaseshave a typical star-pattern shape and grow with two,three or four branches of dendrites (Figure 1).The EBSD spot analysis showed that the first crystal-line phase was -Fe. Higher annealing temperatures ledto a nanocrystalline structure, which was fully devel-oped at 700°C. Figure 2 (a) shows a backscatter electronimage of a cross-section of a powder particle annealed at800°C. The microstructure consists of light- anddark-grey grains. The average grain size of both graintypes is approximately 300 nm. Using EBSD mapping,shown in Figure 2 (b), acquired at the same site of inter-est as the backscatter image, two phases, -Fe and Fe2B,were detected. The brighter grains in the backscatter im-age correspond to the boride phase and the darker grains10 METALURGIJA 47 (2008) 1, 9-12M.GODEC et al.: A HIGH-MAGNIFICATION EBSD MAPPING ANALYSIS OF Fe–Si–B POWDER PARTICLESFigure 1. Secondary electron image shows Fe-Si-B powderparticle cross-section. Powder particles wereair-annealed at 500 °C for 15 minutes. The sam-ple surface was polished and etched by 2 % ni-tal.Figure 2. Cros-section images of Fe-Si-B powder particleannealed at 800 °C (a) Backscatter electron ima-ge, (b) EBSD map and (c) Pattern Quality mapto the ferrite phase. Both images coincide very well inspite of the slight drift that takes place during the longacquisition time. Additionally, a pattern-quality mapwas produced. Because the pattern-quality map dependson the orientation of the crystals, the one in Figure 2 (c)exhibits the grain structure. In Figure 2 (c) one can seethat the boride phase has a slightly poorer pattern qualitythan the ferrite phase; this is due to the different crystalstructure. In Figure 3, the Kikuchi patterns of bothphases are shown. The orientations of the -Fe and theFe2B crystallites in the polycrystalline aggregate areshown inFigure 4. The textures of both phases are presentedseparately, as the normal direction orientation map inthe colour-key mode. It is clear that there are no pre-ferred orientations of either phase. The powder particleswere rapidly solidified during the water atomisationand, as a result, no texture is to be expected.DISCUSSIONThe main problem encountered duringhigh-resolution EBSD mapping is drift. There are, how-ever, two kinds of drift. The first is sample-holder-stagedrift, which can be overcome to some extent by waitinga longer time before the data acquisition, and the othersort of drift is electron-beam drift, which is more or lessalways present during longer data-acquisition times. Upuntil now, no commercial software has been availablefor correcting the drift during data acquisition. The rea-son is that the sample is highly tilted and therefore it isalmost impossible to make a drift correction. There are,however, some solutions that are able to speed up theprocess of EBSD mapping. One is to use a higher accel-erating voltage, which results in faster EBSD acquisi-tion as well as sharpening the Kikuchi bands. Unfortu-nately, we had to decrease the beam’s accelerating volt-age due to charging of the sample, despite the1-nm-thick carbon layer on the surface. A thicker con-ducting layer would have had an adverse effect on theKikuchi pattern’s quality, resulting in the need for a lon-ger acquisition time. There might be some possibilitiesto use an ion-etching procedure, but the powders wouldhave to be embedded in some soft, conductive materialwith a similar sputtering rate.In Figure 2 (a), which shows grains with two differ-ent contrasts, it would be expected that the lighter grainswould correspond to the phase containing elements witha higher atomic number; this is because it is a backscat-ter image that is carrying information about the chemi-cal composition. However, this is not so in this case. AThermo-calc simulation 9 demonstrated that nearly allthe silicon is soluble in bcc -Fe phase, and therefore theatomic numbers are to a large degree equalised. The re-maining very high contrast in the backscatter image itseems results from the light-grey grains being harder,and so during the metallographic preparation a surfacetopography is generated. The backscatter detector usedfor these analyses is extremely sensitive to surface to-pography, because the backscatter image is derivedfrom secondary electrons as well.On closer observation, in Figure 4 (b) we can see thatsome of the grains of the Fe2B phase have a central re-METALURGIJA 47 (2008) 1, 9-12 11M.GODEC et al.: A HIGH-MAGNIFICATION EBSD MAPPING ANALYSIS OF Fe–Si–B POWDER PARTICLESFigure 3. The Kikuchi pattern of (a) -Fe phase and (b) Fe2BphaseFigure 4. Normal direction map is shown with colour keycode inverse pole figure of. Fe-Si-B powder par-ticle cross-section annealed at 800 °C of (a) -Feand (b) Fe2B phasegion that is oriented very differently to the rest of thegrain. This is due to a software error that occurred as aresult of changing the most intense lines in the Kikuchipattern. This most probably happened due to a poorerboride pattern. Longer OPS polishing would lead to ahigher topography effect and a higher accelerating volt-age, which would certainly improve the pattern quality,but would lead again to charging problems.CONCLUSIONSHigh-magnification EBSD mapping was performedon powder particles of the Fe–Si–B phase. We foundthat the annealed particles consisted of two phases, -Feand Fe2B, both detected using the EBSD analytical toolin combination with a field-emission-gun scanning elec-tron microscope.This technique provides us with superior lateral reso-lution for the identification of different phases. To beable to obtain good results on powder materials themetallographic preparation prior to the EBSD is crucial.An additional problem is the need to deal with the driftof the EBSD sample image map.REFERENCES1 G. Herzer, Journal of Magnetism and Magnetic Materials,112 (1992) 258-262.2 Y. Yoshizawa, S. Oguma and K, Yaauchi, Journal ofApplied Physics, 64 (1988) 6044-6046.3 M. Persson, P. Jansson, A.G. Jack and B.C. Mecrow, IEEConference, Publication No. 412, Durham, UK. Sept. 1995,242-246.4 G. Herzer, Journal of Magnetism and Magnetic Materials,112 (1992) 258-262.5 J.C.Swartz, R.Kossowsky, J.J.Haugh and R.F.Krause, Jour-nal of Applied Physics, 52 (1981) 3324-3330.6 Yong Ik Jang, Jongryoul Kim and Dong Hyuk Shin, Mate-rial Science and Engineering B, 78, (2000) 2-3, 113-118.7 D.J.Dingley and V.Randle, Journal of Materials Science,27, (1992) 4545-4566.8 V. Randle, Microtexture Determination and its Applica-tions, The Institute of Materials, 1992, London9 Thermo-calc, Thermo Calc Software (TCW) for calculationthe thermodynamic data, Stockholm Technology Park,Björnnäsvägen 21, SE-113 47 Stockholm, Sweden.Note: The responsible language lecturer for English language dr. PaulMcGuines, Ljubljana, Slovenia12 METALURGIJA 47 (2008) 1, 9-12M.GODEC et al.: A HIGH-MAGNIFICATION EBSD MAPPING ANALYSIS OF Fe–Si–B POWDER PARTICLES

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