Characterisation of crystallisation in amorphous soft magnetic Fe 74Si 11B 14Ni 1 powders by EBSD...

Materials Science and Engineering B 129 (2006) 31–38

Characterisation of crystallisation in amorphous soft magneticFe74Si11B14Ni1 powders by EBSD method

M. Godec a,∗, D. Nolan b, M. Jenko a

a Institute of Metals and Technology, Lepi Pot 11, 1000 Ljubljana, Sloveniab Faculty of Engineering, University of Wollongong, NSW, Australia 2522

Received 29 August 2005; accepted 8 December 2005

Abstract

Amorphous Fe74Si11B14Ni1 soft magnetic powder was produced by water atomisation. Crystallisation of amorphous powder was studied usingmulti-technique approach based on electron beams, including high resolution transmission electron microscopy (HRTEM) and electron backscatterdiffraction (EBSD). It has been found that annealing of the amorphous Fe–Si–B powder at a temperature of 500 ◦C leads to the formation of �-Fephase in the form of two, three and four-branch dendritic grains. The Fe2B boride is crystallised at temperatures of over 500 ◦C. In spite of thedrift problem associated with EBSD, which is not yet resolved in any commercial EBSD system, the technique is very powerful and shows greatp©

K

1

seaaYfn

dp[sactmdp

0d

romise as a high resolution characterisation method.2006 Elsevier B.V. All rights reserved.

eywords: Crystallisation; EBSD; Fe–Si–B based powder; Nanocrystalline structure

. Introduction

Nanostructured soft magnetic materials derived from rapidlyolidified Fe–Si–B alloys exhibit excellent soft magnetic prop-rties due to the crystallisation of nano-scale �-iron grains inn amorphous matrix [1]. These alloys may be doped with Ndnd Cu to increase the seeds for crystallisation, as shown byoshizava et al. [2] and they are usually produced in thin ribbon

orm using either melt-spinning or planar flow casting tech-iques.

More recently, a very promising composite material has beeneveloped, where soft magnetic powders are isolated and com-ressed together with an insulating binder material [3]. Herzer4] has shown that Fe–Si–B powder produced by water atomi-ation can be compressed with binding insulation materials tochieve a soft magnetic composite material. However, carefulontrol of the annealing treatment is required in order to achievehe nanocrystalline structure [4]. The advantages of using such

aterials include improved capability in processing of 3D motoresigns and the flexibility afforded by powder metallurgy (P/M)rocess. Recent advances in the powder metallurgy industry,

including the efficient production of powders, new dielectricmaterials and improved coating techniques have resulted in anincrease in interest in this technology over last years [5].

There has been previous work concerning the characteriza-tion of crystallisation behaviour in amorphous Fe–Si–B softmagnetic materials [6,7], most of which has applied TEM toanalyse the crystallisation process [8,9]. However, the charac-terization of the early stages of crystallisation is still far fromcomplete. For example, there has been some recent evidenceof simultaneous crystallisation of both �-iron and Fe2B phases[10].

A range of different high resolution characterisation methodscan be used to study the crystallisation of amorphous metals.High resolution transmission electron microscopy (HRTEM) isone of the most powerful methods, but one which has some prac-tical disadvantages. For instance, the preparation of the samplesis very complicated and time consuming, and it is usually notpossible to accurately locate fine features in the macrostruc-ture. The application of electron backscatter diffraction (EBSD)and field emission scanning electron microscopy (FESEM) tech-niques for studying the crystallisation effects have some distinct

∗ Corresponding author. Tel.: +386 1 4701952; fax: +386 1 4701939.E-mail address: [email protected] (M. Godec).

advantages in this regard. For example, the method is high reso-lution, it can be used to detect different crystallographic phases,and even to observe the orientation of the individual crystals. Foran in-depth discussion of the application of EBSD to the determi-

921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2005.12.016

32 M. Godec et al. / Materials Science and Engineering B 129 (2006) 31–38

nation of crystallographic orientation on a grain-by-grain scalein metals and minerals, the reader is referred to the review byDingle and Randle [11].

In the present work, high resolution microscopy methods,such as transmission electron microscopy (TEM), field emissionscanning electron microscopy (FESEM) and electron backscat-ter diffraction (EBSD), were used to characterize crystallisationeffects in amorphous F74Si11B14Ni1 powders (Fe–Si–B) sub-jected to a range of annealing treatments. The objective was pri-marily to investigate the application of high resolution methodssuch as EBSD to the characterization of nanoscale crystalliza-tion effects in amorphous materials.

2. Experimental procedure

2.1. Water atomisation and annealing

The experimental Fe–Si–B–Ni powders were prepared bywater atomisation using a Davy McKee-type D5/2 pilot wateratomiser. A charge of 10 kg was prepared, melted in an inductionheated furnace and rapidly cooled in the water spray producedby a nozzle of dimensions Ø1.2 dia. × 1.05 mm and a pres-sure of 20 MPa. After applying a sieve-separation technique,the finest fraction (<45 �m) was obtained with the followingchemical composition (in mass percent): Fe 88.1%, Si 6.84%,B 3.14%, Ni 1.45%, C 0.025% and Al 0.12%. These resultswta11

2

tfiETf3wwtoca1ieiabpwEw

2.3. X-ray diffraction and transmission electron microscopy

The powders were analysed with XRD and TEM to vali-date the structural data obtained by EBSD. A Siemens D5000with a Bragg-Brentano reflection geometry and Cu K� radia-tion (λ = 0.1540562 nm) was applied. The measurements wereperformed in the 2θ range from 10 to 80◦ with a step size of0.04◦ and a step time of 1 s. For the TEM analyses, the powderswere chilled in liquid nitrogen and mechanically crushed in acubic boron nitride mortar to obtain small particles that weretransparent to the electron beam. The analyses were carried outin a JEOL JEM-2010F TEM.

3. Results

3.1. Light microscopy

The water atomised powder particles are predominantlysphere-shaped, although some appear elongated as a result ofbonding of droplets during the solidification process. There isalso some minor porosity apparent, as well as some instances ofnon-typical grain structures, probably due to particle–particlecontact during water atomisation and subsequent localizedreduction of cooling rate (Fig. 1). Cross-sections of typical par-ticles (Fig. 2) showed a lack of obvious grain structure followinga ◦ ◦im

3

3igiit

Ft

ere obtained by wet chemical analyses, except for carbon con-ent, which was obtained by ELTRA CS 800 using the infraredbsorption method. Powder samples were annealed in air for5 min at temperatures between 300 and 900 ◦C in steps of00 ◦C.

.2. Scanning electron microscopy

The microstructures of the as-water atomised powders andhe annealed powders were analysed using a JEOL JSM 6500Feld emission electron microscope equipped with EDS INCANERGY 400, WDS INCA WAVE 700 and EBSP INCA CRYS-AL 300. The accelerating voltage of the primary electron beamor backscattered images was 8 kV, the aperture diameter was0 �m, the probe current was 0.06 nA and the working distanceas 6 mm. All the EDS and WDS analyses were conducted at aorking distance of 10 mm. For the ED/WD mapping, an aper-

ure of 50 �m, a current of 1 nA and an accelerating voltagef 5 kV were applied. Line analyses were performed at lowerurrent of 0.45 nA. EBSD mapping was performed on samplesnnealed at 500, 600, 700 and 800 ◦C, using a beam voltage of5 kV, probe current of 1.9 nA, sample tilt angle of 70.5◦, work-ng distance of 15 mm and an aperture of 50 �m. Secondarylectron images acquired during EBSD mapping were necessar-ly of reduced quality because the samples were tilted to 70.5◦nd the working distance was 15 mm. Also, high current muste used to increase the rate of mapping in order to avoid theroblems associated with stage drift. To date, there is no soft-are available which can take into account the sample drift forBSD analyses. In spite of this, some useful EBSD mappingas performed.

nnealing treatments of up to 400 C. At 500 C, a visible changen microstructure is apparent suggesting crystallisation has com-

enced.

.2. Scanning electron microscopy

No microstructural features are apparent after annealing at00 or 400 ◦C. After annealing at 500 ◦C, the microstructures characterised by the amorphous matrix containing isolatedrains with a dendritic morphology (Fig. 3a) of around 500 nmn size. Annealing at 600 ◦C increases the number of these grainsn the microstructure, although the scale of the grains is reducedo <200 nm (Fig. 3b). After annealing at 700 ◦C, the microstruc-

ig. 1. Amorphous water atomised Fe–Si–B particles with crystallised areas athe surface.

M. Godec et al. / Materials Science and Engineering B 129 (2006) 31–38 33

Fig. 2. Optical images of Fe–Si–B powder microstructure (a) in as-water atomised state, and after annealing for 15 min at the different temperatures: (b) 300 ◦C, (c)400 ◦C, (d) 500 ◦C, (e) 600 ◦C, (f) 700 ◦C, (g) 800 ◦C and (h) 900 ◦C.

ture consists of very fine equiaxed grains (Fig. 3c) with a size ofapproximately 100–200 nm. The higher annealing temperatureof 800 ◦C results in considerable grain growth (Fig. 3d), withsome of the lighter grains exhibiting twinning effects. As this isnot possible for �-Fe phase, it is assumed that these grains areiron boride phase expected in such alloys. Higher grain growthof iron phase after 900 ◦C annealing is restricted by boride phasepositioned at grain boundaries usually between two or threegrains.

3.3. XRD measurements

The X-ray diffraction patterns of water atomised andannealed powders are shown in Fig. 4. The water atomised pow-der shows a mostly amorphous structure, although the sharp

peak at 2Θ 44.35◦ is most likely due to localised grain growthas a result of particle–particle contact, as previously discussed.Annealing at 500 ◦C causes the strengthening of the �-Fe (1 1 0)and (2 0 0) peaks, although there is still a broad peak around the2Θ position of 45◦, indicating a significant amorphous content.After 600 ◦C annealing, both iron boride and �-iron peaks areclearly apparent, suggesting that the crystallisation of a two-phase structure occurs at an annealing temperature between 500and 600 ◦C.

3.4. TEM measurements

Most of the water atomised particles observed under thetransmission electron microscope are amorphous. In Fig. 5, thediffuse rings typical for amorphous state are visible. HRTEM


Fig. 3. Backscattered electron images of Fe–Si–B annealed at different temperatures for 15 min: (a) annealed at 500 ◦C, showing isolated grains with dendriticmorphology; (b) annealed at 600 ◦C, showing among well defined dendrites uniformly arranged over the whole area; (c) annealed at 700 ◦C, showing very finenano-scale equiaxed grains and (d) annealed at 800 ◦C, showing a two-phase grain structure, where the brighter is Fe2B and exhibits a twinning effect.

image shown in Fig. 6 also illustrates the amorphous appear-ance of water atomised powder particles. However, in this casevery small areas of <5 nm are visible with already crystallineatomic order. When the sample is annealed at 600 ◦C (Fig. 7),then the sharp rings relating to �-iron and Fe2B phases appearin the diffracted image, and the HRTEM image shows a fullycrystalline structure with no amorphous areas.

3.5. SEM ED/WD analysis

3.5.1. MappingIn order to provide a baseline differentiation of Si and B

distribution, typical of that to be expected during crystallization,

F1

microchemical analyses were performed on powders annealedat 700 ◦C by using the EDS and WDS methods. Fig. 8a is asecondary electron image showing a clear two-phase structure.The boron distribution was observed by WDS (Fig. 8b) andfor silicon EDS (Fig. 8c) was applied using Si K�1 line. Sincesilicon is not soluble in boride phase, but it is soluble in �-iron phase up to 11.9% mass fraction, it is clearly demonstratedthat the brighter grains in the SE image correspond to the Fe2Bphase and the darker grains correspond to �-iron. The boride

Fmt

ig. 4. X-ray diffraction patterns of water atomised and annealed powders for5 min at different temperatures.

ig. 5. Image of crumbled water atomised powder particle obtained by trans-ission electron microscopy with diffracted pattern image showing diffuse ring

ypical for amorphous matter.


Fig. 6. HRTEM image shows very small, <5 nm, areas of crystalline areasembedded in amorphous matrix.

grains are brighter in the secondary electron image due to surfacerelief effects, occurring during polishing as a result of differentialhardness of the two phases.

3.5.2. Line analysesED/WD line analyses were performed on samples annealed

at 500, 600, 700 and 800 ◦C. For the 500 and 600 ◦C annealedsamples, the line analysis was performed in such a way that theline passed through the observed dendritic grains. The distribu-tion of boron and silicon were assumed to indicate the boride and�-iron phases, respectively, as discussed in the previous section.In the 500 ◦C annealed sample, the distribution of silicon is uni-form more or less through the whole line. However, the boronconcentration drops significantly in the region of the dendriticgrain (Fig. 9a). Since boron is insoluble in �-Fe, but silicon issoluble to more than 10% mass fraction, it is confirmed thatthe first-formed grains are �-Fe with a dendritic morphology.Annealing at 600 ◦C leads to the appearance of a greater num-ber of smaller dendritic grains, which are shown by line analysisto also be �-Fe phase (Fig. 9b). After 700 ◦C annealing, the line

Fnd

Fig. 8. (a) Secondary electron image of nanocrystalline structured powderannealed at 700 ◦C with corresponding (b) boron and (c) silicon maps. Theboron map was obtained by WD spectroscopy using K�1 signal, and silicon wasobtained by ED spectroscopy, also using the K�1 signal.

analysis shows that more equiaxed, nanocrystalline (∼200 nm)grains of both phases are formed (Fig. 9c), since where boronconcentration is increased, the silicon concentration is reduced,and vice versa.

3.6. EBSD

Results for EBSD mapping are shown in the order of decreas-ing annealing temperature to provide a clear illustration ofthe characterization of �-iron and Fe2B phases by the EBSDmethod. Fig. 10a shows the results of EBSD mapping for thesample annealed at 500 ◦C. White marked areas correspond to�-iron grains, grey areas to boride grains and black areas repre-sent unsolved areas. It is obvious that a two-phase structure is

ig. 7. HRTEM image of Fe–Si–B powder annealed at 600 ◦C for 15 min showsanocrystalline structure of grain size around 10–20 nm. The correspondingiffraction pattern confirms the formation of �-Fe and Fe2B phase.


Fig. 9. ED/WD line scan analysis shows boron and silica concentrations forsample: (a) annealed at 500 ◦C, with the line crossing the crystal with dendriticmorphology (the arrow point to the position of the dendrite); (b) annealed at600 ◦C, with the line crossing a few crystals with dendritic morphology (thearrows point to the positions of the dendrites) and (c) annealed at 700 ◦C, withthe line crossing a few iron and boride grains (the arrows point to the brighterboride grains). The boron line analysis was obtained by WD spectroscopy usingK�1 signal, and silicon was measured by ED spectroscopy, also using K�1

signal.

present. Unsolved areas in positions of grain boundaries are typ-ical of such analyses, since the Kikuchi pattern quality is poorin such regions. The quality of the Kikuchi pattern also dependson the orientation of the grain and, in addition, different phasesshow different pattern quality. For example, the boride phasewas shown to exhibit much poorer pattern quality. Thereforemore unresolved data points are found to be associated with theboride phase.

Fig. 10b shows the EBSD mapping of the sample annealedat 600 ◦C. As expected, the mapping is not as clear as for the

sample with the larger grain structure. The problem is not somuch one of reaching the limit of lateral resolution of the EBSDtechnique, but rather the problem of getting reasonably goodpatterns from both phases.

In the sample annealed at 700 ◦C, a region containing adendritic �-iron was analysed (Fig. 10c). It is evident that sur-rounding the larger �-iron crystalline areas exist very small areasof the same crystalline phase. It is not possible to determine crys-tallisation of boride phase on a scale of ∼50 nm, possibly due tothe problem of the poor Kikuchi pattern quality associated withthe boride phase.

Analysis of a group of dendritic grains and surrounding mate-rial in the sample annealed at 800 ◦C showed that it is possible toresolve the dendritic �-iron phase by EBSD mapping (Fig. 10d).There is a greater proportion of unresolved regions, indicatingthat the regions surrounding the crystalline iron phase are mostlyamorphous. There are a few data points identified as boride or�-iron phases, but these single data points are most likely errorsdue to poor Kikuchi pattern quality.

4. Discussion

Water atomisation produces a wide range of particle sizes,from the smallest ones of a few �m with amorphous struc-ture, to the larger ones of mm size with a progressively greaterproportion of crystalline structure. After annealing at 500 ◦C,sdiafdotdt

twataiiseipc[

pfOtfiw

econdary electron images show formation of different shapedendrites. Yang et al. [8] have also reported the formation of sim-lar dendrite types following similar annealing treatments withsimilar material, but produced in thin ribbon form. We have

ound the mixture of two, three and four branch types of den-rites all forming in the same annealing conditions. The smallestnes have been found to be two or three branch dendrite type, andhe larger ones have tended to be four branch type. However, trueetermination of three-dimensional shape is not possible usinghe current analytical techniques.

Analysis by SEM technique did not show any nanocrys-alline phases in the as-water atomised powder. This structureas reveal using HRTEM, which shows that there are some

reas of crystalline form. XRD diffraction graphs show forma-ion of �-Fe phase at 500 ◦C. But it is not clear if Fe2B haslready crystallised at that annealing condition because all highntensity Fe2B peaks overlap with �-Fe peaks. From the lowntensity boride peaks it is not possible to be confident aboutome low concentration of boride crystalline phase. Bedenaskat al. [12] reported the stabilisation of Fe3B phase with increas-ng Ni content. However, no evidence of any residual of Fe3Bhase was found in the present work, despite the fact that theoncentration of Ni content was higher than in the previous work12].

The major problem in obtaining a good Kikuchi pattern is thereparation of the sample. There are different methods available,or instance, polishing by colloidal silica in slightly basic mediaPS, ion milling, electrolitical polishing, etc. However, due to

he fact that the powders were embedded in bakelite resin therst procedure was selected in the current work. Some attemptsere made to use a low melting point alloy and cast it together


Fig. 10. EBSD maps of Fe–Si–B cross-sections show different crystalline phases obtained by solving the Kikuchi patterns in each pixel: (a) annealed at 500 ◦C, (b)annealed at 600 ◦C, (c) annealed at 700 ◦C and (d) annealed at 800 ◦C.

with powders, but polishing was difficult due to the soft matrix.During the OPS polishing stage, a residual layer of polishingdebri was occasionally observed on the surface of the dendritegrains. After proper cleaning, this residue was removed. Theprevious work of Godec et al. [10] reported two different dendritetypes, with different contrast in the SEM images, light grey anddark grey. However, it is now clear that the light grey dendritesreported by Godec et al. were a polishing artefact, produced by avery thin oxide layer remaining in the polishing relief associated

F5c

with the �-iron phase. By applying proper polished procedure,it has been shown that the first phase to crystallize is �-Fe, andit forms at annealing temperatures of as low as 500 ◦C in theabsence of the Fe2B phase.

The EBSD method might also be useful for detailed analy-sis of the growth orientation of the phases. For example, it isthought that the preferred growth orientation of the �-iron den-drite branches might be associated with boron diffusion out of�-Fe [8]. This could be explored by the EBSD technique, sinceit is shown to be possible to accurately determine fine scale grainorientation effects in such materials. Fig. 11 shows the orienta-tion of the dendrite plane cross-section and the direction of thedendrite branches.

5. Conclusions

Amorphous Fe–Si–B powder of <40 �m in diameter wasmanufactured by water atomisation and then annealed at varioustemperatures in order to study the crystallisation behaviour inthe Fe–Si–B system. A range of characterisation methods wereapplied, among them the HRTEM and EBSD techniques. It hasbeen shown that EBSD could be a very powerful technique fordifferentiation of phases at scales of ∼50 nm, due to very highlateral resolution. In spite of the drift problem associated withEBSD, which is not yet resolved in any commercial EBSD sys-tem, the technique is very powerful and shows great promise asa

dp

ig. 11. The EBSD texture of a Fe–Si–B powder cross-section annealed at00 ◦C. The dark grey represents the plane {9 7 11} parallel to the cross-sectionut, and the direction 〈23,−39,6〉 is the direction of the vertical dendrite branch.

high resolution characterisation method.It has been found that annealing of amorphous Fe–Si–B pow-

er at temperature of 500 ◦C lead to formation of �-Fe crystallinehase in form of two, three and four branch dendrites. The boride


phase which is formed only in Fe2B form is crystallised at tem-peratures of 600 ◦C and over.

Acknowledgements

The authors sincerely express their gratitude to Borivoj Sus-tarsic for his diligent work preparing the powders. The authorswould also like to acknowledge Maja Remskar for her invaluableHRTEM work.

References

[1] G. Herzer, J. Magnet. Magnet. Mater. 112 (1992) 258–262.[2] Y. Yoshizawa, S. Oguma, K. Yamauchi, J. Appl. Phys. 64 (1988)

6044–6046.

[3] M. Persson, P. Jansson, A.G. Jack, B.C. Mecrow, IEE Publ. 412 (1995).[4] G. Herzer, J. Magnet. Magnet. Mater. 112 (1–3) (1992) 258–262.[5] C. Gelinas, F. Chagnon, S. Pelletier, Adv. PM Particul. Mater. 6 (1996)

20/85–20/97.[6] J.C. Swartz, R. Kossowsky, J.J. Haugh, R.F. Krause, J. Appl. Phys. 52

(5) (1981) 3324–3330.[7] T.V. Larionova, O.V. Tolochko, A.S. Zhuravley, Glass Phys. Chem. 21

(4) (1995) 297–299.[8] Y.I. Jang, J. Kim, D.H. Shin, Mater. Sci. Eng. B 78 (2–3) (2000)

113–118.[9] G.A. Stergioudis, G. Vourlias, H. Morawiec, D. Stroz, E.K. Polychro-

niadis, Mater. Res. Bull. 39 (2004) 236–321.[10] M. Godec, D.J. Mandrino, B. Sustarsic, M. Jenko, Vacuum 80 (2005)

(2005) 24–28.[11] D.J. Dingley, V. Randle, J. Mater. Sci. 27 (17) (1992) 4545–4566.[12] L. Bednarska, Y. Galadzhun, Y. Gorelenko, M. Kovbuz, B. Kotur, J.

Alloy Compd. 367 (1–2) (2004) 270–273.

Characterisation of crystallisation in amorphous soft magnetic Fe 74Si 11B 14Ni 1 powders by EBSD...

Documents

Transcript of Characterisation of crystallisation in amorphous soft magnetic Fe 74Si 11B 14Ni 1 powders by EBSD...