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Mössbauer effect of 57Fe in nanostructured transition metal — iron alloys obtained by mechanical alloying
NamS~ctud
Mataids. Vol. 6, pp. 629-633,1995 CopyrigM Q 1995 Eik-vic? sci~e Ltd F’rinted in the USA. All rights resavcd 0965-9773195$9.50 + .oo
09699773(95)00137-9
MOSSBAUER EFFECT OF S7FE IN NANOSTRUCTURED TRANSITION METAL - IRON ALLOYS OBTAINED BY MECHANICAL ALLOYING G . Rixecker, R. Birringer, A. Hartenberger,A. Himbert’, A. Ries’and H. G leiter UniversitatdesSaarlandes, FRlS. 1 Werkstoffwissenschaften Gebaude43, Postfach151150,D-66041Saarbrticken,Germany 1 Institut fur neueMaterialienges.GmbH, Im Stadtwald43, D-66123Saarbrticken AbstracHWssbauer spectroscopy can yield spectral information from highly disordered regions of a solid and is therefore a promising method for the characterization of nanocrystalline and amorphous materials. In this work, we shall present results obtained by MCssbauer spectroscopy and x-ray diffraction on the microstructure of several mechanically alloyed M-Fe alloys, M= Y, Hf, Nb, MO, Pd, with “7Fe contents of about 1 at.%. Special emphasis will be put on the chemical analysis of impurities originating porn abrasive wear of the milling tools.
Mechanicalalloying has provedto be a versatilemethodfor the synthesisof materials with novel propertiesand microstructures.The structureand properties’ofsuch materialsmay be significantly influencedby contaminationdue to abrasivewear of the milling tools and to light elementimpurities.Especiallyif spectroscopic methodsareusedto investigatethe atomic structureof new materials,the influenceof impurities on the experimentaldata needscareful analysis.In this paper,we shall presentresultsobtainedon mechanicallyalloyed M-Fe (M= Y,Hf, Nb, MO, Pd) alloys with low iron content. Standardsteel and tungstencarbidemilling tools wereused.If 57Feis to be usedas a dilute probefor Mossbauerspectroscopy, an iron content I 1 at.% is required.At higheriron concentrations, iron-ironnearestneighbourinteractions becomeincreasinglyimportant.The sameis true for impuritiesotherthan iron. Mechanicalalloying (MA) was performedwith a SPEX 8000vibratoryball mill using either a hardenedsteelvial togetherwith stainlesssteelballs, or a WC/Co vial set. Milling was carriedout underargonatmosphere, the ball-to-powderweight ratio beingabout5: 1 in the steel vial and 2.5:1 with the WC tool. Consequently,the milling intensitywas lower in the WC vial. Although the hardnessof tungstencarbidemilling tools is superiorto hardenedsteel,which is more frequentlyused as a vial material,WC as well as Co contaminationis usually presentin materialsmilled in WC vials. Due to the brittlenessof WC, the amountof wear debrismay vary from run to run. This complicationis not found for hardenedsteel vials. Elementalpowders with a purity of 99.9% or betterwereusedas raw materials.Iron enrichedin 57Feto about95% was usedin form of a foil which was cut into piecesof about0.5 x0.5 mm2. The sampleswere characterized by x-ray diffractometry(XRD) using CuKar and CoKar radiation.The spectrawere analyzedfor crystallite sizeusing a standardmethod(1). MSBbauer spectrawere acquiredin transmissiongeometry(TMS), exceptfor PdFe wherebackscattered x-rayswere detected(CXMS). 57Coin a Rh-matrixwith activitiesbetween150...950M.Bq was 629
630
G
RIXECKER
ET AL
used as the Mossbauer source. All isomer shifts are given relative to o-Fe. The samples were analyzed spectrochemically by inductively coupled plasma atomic-emission spec~o~opy (AES-ICP). Some were also analyzed for light impurities (HZ, Nz, and 02) by hot-extraction gas chromatography. Finally, the carbon content was measured by IR absorption spectroscopy. The chemical compositions of the mechanically alloyed samples ate listed in table 1. In addition to these alloys, TaFe, WFe, and IrFe with the starting composition Mgg2?Fe075 were also studied. However, it was found that they exhibit extensive contamination if alloyed in tungsten carbide. Using EDX microprobe analysis and carbon analysis as described above, the approximate compositions of the latter three samples after 20h of milling were found to be TawW#eiColCs, W93FeCozC4,and IrssWloFeCoG. The refractory metals Ta, W and Ir cannot be alloyed in a steel vial either: it was shown, for example, that WFe alloys with arbitrary starting compositions contain about 70 at.% Fe after 24h of milling time (2). Figure 1 shows the Mossbauer spectra of two XFe alloys having similar compositions. They were milled for 1Oh and 20h, respectively, under otherwise identical conditions in a steel vial. The Mossbauer parameters are given in Table 2. Comparing Figs. la and lb, we see that after 10h the alloying process has not yet terminated. The six-line subspectrum is due to residual iron. The magnetic hyperfme splitting of the a-Fe component is significantly reduced (HR= 10.625 mm/s for pure iron), indicating that Y atoms occupy substitutional sites in the Fe lattice. This effect is also visible in the h&Fe and M&e samples containing a-Fe. After 20h, aFe is no longer present. The hyperfine parameters of the two-line subspectrum coincide with those of amorphous Y-Fe alloys (3) and differ significantly from those of the crystalline phase YFez (4). It is known that iron is practically insoluble in crystalline yttrium (5). In the XRD spectrum, two components can be discerned: first, crystalline yttrium with lattice parameters (a= 3.65 lk7 A, c=5.744*7 A) close to the literature values (6), and second, an enhanced background. The absence of Fe at substitutional sites in the Y lattice does not necessarily follow from the unchanged lattice constants because light impurities (Table 1) would have the reverse effect of iron (i.e., lattice parameter increase) if dissolved interstitially. The Mossbauer and xray phase analyses suggest the Y-Fe microstructures specified in Table 3. Whether or not the structurally disordered Y-Fe phase is located at the grain boundaries of adjacent yttrium crystallites cannot be inferred from our results. TABLE 1 Ball Milling Parameters and Results of the Chemical Analyses
Pdss.ZFeo.7s
] 20 h 1 steel
Pdgdk.9
] 0.9k.3 HZ, ~0.1 N2, co.1 02 1
631
M~SSBAUEREFFECTIN TRANSITION METAL-IRONALLOYS
1 .oo
1.00
-
0 98
-
0 97 z GO.96
c t; $0 ,z
94
y1
00
; > ii
-
E 5
-----CT t i
b
0 96
0 92
I -6
I -3 VELOCITY
I 0
/
I
3
6
VELOCITY
rmm/sl
[mm/s]
FIGURE 1 Miissbauer Spectra of De: (a) Y99 zsFeo75,IOh, Steel Milling Vial/Balls. (b) Yqa7Fel3,20h, Steel Milling Vial/Balls
FIGURE 2 HfFe Mechanically Alloyed for 20h: (a) Hfw 2sFe~175, WC Milling Vial/Balls. (b) HfggFel, Steel Milling Vial/Balls.
TABLE 2 Mijssbauer Results: Isomer Shift (IS), Quadrupole Splitting (QS), Magnetic Hyperfine Splitting (HfS), Full Linewidth at Half Maximum (LW), and Relative Spectral Area (RA).
Mosd%2 I Pd96.IFe3.9
(steel) (steel)
sextet single line single line
IS= +0.02, HfS= 10.59, LW= 0.31 IS= -0.05, LW= 0.50 IS= +0.17, w= 0.30
40
632
G RIXECKER
ET AL
TABLE 3 Overview of M icrostructuresas Suggestedby TMS andXRD. (The XRD CrystalliteSizesarefor the Main CrystallineComponents.)
The following interpretationsate obvious in the case of HlFe, NbFe and MoFe.In the Mossbauerspectraof HlFe, m illed for 20h in tungstencarbide (Fig. 2a) as well as steel (Fig. 2b), no a-Fe is detectable,althoughthe solubility of Fe in Hf is as small as 0.01 at.%. Both spectracan be fitted with asymmetricdoubletswith a linear interdependence betweenisomer shift andquadrupolesplitting (Table2). This is characteristicof non-magneticmetallic glasses. In contrast,a magneticallysplit a-iron subspectrumis presentin the Mossbauerspectra of NhFe (Fig. 3a) and M&e m illed for 20h in WC. It is absentin the spectraof NbFe (Fig. 3b) andb&Fe m illed in steeldespitethe fact that thesesampleswere supersaturated in iron by 11.6 and6.6 at.%, respectively.The broadenedsinglelines arein eachcasedue to the solid solutions M(Fe). The failure to obtain chemicallyhomogeneous NbFe and M&e sampleswith the WC vial may be due to the lower m illing intensity or to a detrimentalinfluenceof the impuritiesincorporatedduring m illing. The x-ray spectraof HlFe, NbFe (Fig. 4a) andMoFe consistof the lines characteristicfor the crystalline elementsplus an enhancedbackgroundin the case of hafnium and traces of tungstencarbidein the samplesm illed in the WC vial. The crystallite sizesof the HfFe andI&Z Fe samplesproducedwith the WC vial are smallerthan thoseproducedwith the steelvial in spite of the fact that both the m illing intensity and the contentof alloying elementsare lower with WC. The crystallite size of M&e, on the other hand, is smallerfor the ‘steel’sample,as expectedconsideringthe higherm illing intensity. This supportsthe conceptionthat the incorporatedWC particlesare not necessarilyinert but may partly reactwith the samplematerial.The reactivity towardscarbonincreasesin the sequenceM O - Nb - Hf. Furtherevidencefor this idea comesfrom the observedlattice parameters, which are reducedin M&e (WC + steel) and NhFe (steel) due to the formation of substitutionalsolid solutionswith iron. A comparisonwith data from (6) suggestssoluteiron contentsof about 2%, 5.5%, and 3%, respectively.The lattice constantsof hafnium in HfFe (WC) (a= 3.206*7 A, c= 5.0829 A) andniobium in I&Fe (WC) (a= 3.314A) are slightly larger than that of the pure elements,indicating that light elementssuch as carbonmay be dissolvedinterstitially. The lattice constantsof hafnium in HfFe (steel)(a= 3.198 A, c= 5.066 A) correspondto the literaturevalues.
M~SSIWUER
EFFECTINTRANSI~ON
633
METAL-IRON ALLOYS
r 1.00
0.98
!$l.OO ; 5 u
1.02 r F7 B !i - 1.01
0 96
L
1 , 1 , ,
-6
-3 0 3 VELOCITY rmmjsl
6
FIGURE 3 h&Fe MechanicallyAlloyed for 20h: (a) Nbsg.zFeox, W C M illing Vial/Balls, (b) Nb99Fel, SteelM illing Vial/Balls.
? z 1: ~1.00 -0.5
0 VELOCITY
05
1
rdsi
FIGURE 4 (a) NbgszsFeoa, 2Oh/WC: XRD Spectrum. (b) Pdgg.zsFeox, 2Oh/Steel: CXM Spectrum, Small DopplerVelocity.
F igure 4b showsa M iissbauerspectrumrecordedwith small Doppler velocity of ballm illed PdFe.The singleline is dueto the solid solutionof Fe in Pd. It is remarkablethat more than 3 at.% Fe are incorporated in Pd during2Ohof m illing. Palladiumis a relatively soft material, comparablein hardnessto Cu andAg; in the latter metals,no iron contaminationat all can be foundby EDX m icroprobeanalysisafter20h of m illing in the steelvial. Evidently, with metallic m illing tools, too, hibochemicaleffectshaveanimpacton the wearingbehaviour. In conclusion,mechanicalalloying,especiallyin systemswherethe effects underinvestigation dependon m inor fractionsof alloying elements,requirescareful considerationof the contaminationintroducedby abrasivewearof the m illing tools.The wearingcharacteristicsare determinedby the hardnessof the materialsto be alloyedandon their chemicalreactivity with respectto the m illing tools.For spectroscopic studiesof nanostructured materials,it is advisable to fabricatededicatedm illing toolsfrom the samematerialthat is to be m illed. References
1. A. Kochendorfer,2. Kristallogr.,M ineral.u. Petrogr..l.Q&393(1944) 2. U. Herr andK. Samwer,Nanostruct.Materials1,5 15(1992) 3. J. Chappert,J. M . D. Coey,A. LienardandJ. P. Rebouillat,J. Phys.F LL, 2727(1981) 4. A. M . van der KraanandK. H. J. Buschow,PhysicaB 138,55 (1986) 5.0. Kubaschewski,Iron Binary PhaseDiagrams. SpringerVerlag,Berlin, 1982. 6. P. Eckerlin andH. Kandler,eds.,vol. 6 of LundoZt-Bdmstein,Zuhlenwerte und Funktionen uus Nuturwissenschaft und Technik, Neue Serie. SpringerVerlag,Berlin, 1971.
Mataids. Vol. 6, pp. 629-633,1995 CopyrigM Q 1995 Eik-vic? sci~e Ltd F’rinted in the USA. All rights resavcd 0965-9773195$9.50 + .oo
09699773(95)00137-9
MOSSBAUER EFFECT OF S7FE IN NANOSTRUCTURED TRANSITION METAL - IRON ALLOYS OBTAINED BY MECHANICAL ALLOYING G . Rixecker, R. Birringer, A. Hartenberger,A. Himbert’, A. Ries’and H. G leiter UniversitatdesSaarlandes, FRlS. 1 Werkstoffwissenschaften Gebaude43, Postfach151150,D-66041Saarbrticken,Germany 1 Institut fur neueMaterialienges.GmbH, Im Stadtwald43, D-66123Saarbrticken AbstracHWssbauer spectroscopy can yield spectral information from highly disordered regions of a solid and is therefore a promising method for the characterization of nanocrystalline and amorphous materials. In this work, we shall present results obtained by MCssbauer spectroscopy and x-ray diffraction on the microstructure of several mechanically alloyed M-Fe alloys, M= Y, Hf, Nb, MO, Pd, with “7Fe contents of about 1 at.%. Special emphasis will be put on the chemical analysis of impurities originating porn abrasive wear of the milling tools.
Mechanicalalloying has provedto be a versatilemethodfor the synthesisof materials with novel propertiesand microstructures.The structureand properties’ofsuch materialsmay be significantly influencedby contaminationdue to abrasivewear of the milling tools and to light elementimpurities.Especiallyif spectroscopic methodsareusedto investigatethe atomic structureof new materials,the influenceof impurities on the experimentaldata needscareful analysis.In this paper,we shall presentresultsobtainedon mechanicallyalloyed M-Fe (M= Y,Hf, Nb, MO, Pd) alloys with low iron content. Standardsteel and tungstencarbidemilling tools wereused.If 57Feis to be usedas a dilute probefor Mossbauerspectroscopy, an iron content I 1 at.% is required.At higheriron concentrations, iron-ironnearestneighbourinteractions becomeincreasinglyimportant.The sameis true for impuritiesotherthan iron. Mechanicalalloying (MA) was performedwith a SPEX 8000vibratoryball mill using either a hardenedsteelvial togetherwith stainlesssteelballs, or a WC/Co vial set. Milling was carriedout underargonatmosphere, the ball-to-powderweight ratio beingabout5: 1 in the steel vial and 2.5:1 with the WC tool. Consequently,the milling intensitywas lower in the WC vial. Although the hardnessof tungstencarbidemilling tools is superiorto hardenedsteel,which is more frequentlyused as a vial material,WC as well as Co contaminationis usually presentin materialsmilled in WC vials. Due to the brittlenessof WC, the amountof wear debrismay vary from run to run. This complicationis not found for hardenedsteel vials. Elementalpowders with a purity of 99.9% or betterwereusedas raw materials.Iron enrichedin 57Feto about95% was usedin form of a foil which was cut into piecesof about0.5 x0.5 mm2. The sampleswere characterized by x-ray diffractometry(XRD) using CuKar and CoKar radiation.The spectrawere analyzedfor crystallite sizeusing a standardmethod(1). MSBbauer spectrawere acquiredin transmissiongeometry(TMS), exceptfor PdFe wherebackscattered x-rayswere detected(CXMS). 57Coin a Rh-matrixwith activitiesbetween150...950M.Bq was 629
630
G
RIXECKER
ET AL
used as the Mossbauer source. All isomer shifts are given relative to o-Fe. The samples were analyzed spectrochemically by inductively coupled plasma atomic-emission spec~o~opy (AES-ICP). Some were also analyzed for light impurities (HZ, Nz, and 02) by hot-extraction gas chromatography. Finally, the carbon content was measured by IR absorption spectroscopy. The chemical compositions of the mechanically alloyed samples ate listed in table 1. In addition to these alloys, TaFe, WFe, and IrFe with the starting composition Mgg2?Fe075 were also studied. However, it was found that they exhibit extensive contamination if alloyed in tungsten carbide. Using EDX microprobe analysis and carbon analysis as described above, the approximate compositions of the latter three samples after 20h of milling were found to be TawW#eiColCs, W93FeCozC4,and IrssWloFeCoG. The refractory metals Ta, W and Ir cannot be alloyed in a steel vial either: it was shown, for example, that WFe alloys with arbitrary starting compositions contain about 70 at.% Fe after 24h of milling time (2). Figure 1 shows the Mossbauer spectra of two XFe alloys having similar compositions. They were milled for 1Oh and 20h, respectively, under otherwise identical conditions in a steel vial. The Mossbauer parameters are given in Table 2. Comparing Figs. la and lb, we see that after 10h the alloying process has not yet terminated. The six-line subspectrum is due to residual iron. The magnetic hyperfme splitting of the a-Fe component is significantly reduced (HR= 10.625 mm/s for pure iron), indicating that Y atoms occupy substitutional sites in the Fe lattice. This effect is also visible in the h&Fe and M&e samples containing a-Fe. After 20h, aFe is no longer present. The hyperfine parameters of the two-line subspectrum coincide with those of amorphous Y-Fe alloys (3) and differ significantly from those of the crystalline phase YFez (4). It is known that iron is practically insoluble in crystalline yttrium (5). In the XRD spectrum, two components can be discerned: first, crystalline yttrium with lattice parameters (a= 3.65 lk7 A, c=5.744*7 A) close to the literature values (6), and second, an enhanced background. The absence of Fe at substitutional sites in the Y lattice does not necessarily follow from the unchanged lattice constants because light impurities (Table 1) would have the reverse effect of iron (i.e., lattice parameter increase) if dissolved interstitially. The Mossbauer and xray phase analyses suggest the Y-Fe microstructures specified in Table 3. Whether or not the structurally disordered Y-Fe phase is located at the grain boundaries of adjacent yttrium crystallites cannot be inferred from our results. TABLE 1 Ball Milling Parameters and Results of the Chemical Analyses
Pdss.ZFeo.7s
] 20 h 1 steel
Pdgdk.9
] 0.9k.3 HZ, ~0.1 N2, co.1 02 1
631
M~SSBAUEREFFECTIN TRANSITION METAL-IRONALLOYS
1 .oo
1.00
-
0 98
-
0 97 z GO.96
c t; $0 ,z
94
y1
00
; > ii
-
E 5
-----CT t i
b
0 96
0 92
I -6
I -3 VELOCITY
I 0
/
I
3
6
VELOCITY
rmm/sl
[mm/s]
FIGURE 1 Miissbauer Spectra of De: (a) Y99 zsFeo75,IOh, Steel Milling Vial/Balls. (b) Yqa7Fel3,20h, Steel Milling Vial/Balls
FIGURE 2 HfFe Mechanically Alloyed for 20h: (a) Hfw 2sFe~175, WC Milling Vial/Balls. (b) HfggFel, Steel Milling Vial/Balls.
TABLE 2 Mijssbauer Results: Isomer Shift (IS), Quadrupole Splitting (QS), Magnetic Hyperfine Splitting (HfS), Full Linewidth at Half Maximum (LW), and Relative Spectral Area (RA).
Mosd%2 I Pd96.IFe3.9
(steel) (steel)
sextet single line single line
IS= +0.02, HfS= 10.59, LW= 0.31 IS= -0.05, LW= 0.50 IS= +0.17, w= 0.30
40
632
G RIXECKER
ET AL
TABLE 3 Overview of M icrostructuresas Suggestedby TMS andXRD. (The XRD CrystalliteSizesarefor the Main CrystallineComponents.)
The following interpretationsate obvious in the case of HlFe, NbFe and MoFe.In the Mossbauerspectraof HlFe, m illed for 20h in tungstencarbide (Fig. 2a) as well as steel (Fig. 2b), no a-Fe is detectable,althoughthe solubility of Fe in Hf is as small as 0.01 at.%. Both spectracan be fitted with asymmetricdoubletswith a linear interdependence betweenisomer shift andquadrupolesplitting (Table2). This is characteristicof non-magneticmetallic glasses. In contrast,a magneticallysplit a-iron subspectrumis presentin the Mossbauerspectra of NhFe (Fig. 3a) and M&e m illed for 20h in WC. It is absentin the spectraof NbFe (Fig. 3b) andb&Fe m illed in steeldespitethe fact that thesesampleswere supersaturated in iron by 11.6 and6.6 at.%, respectively.The broadenedsinglelines arein eachcasedue to the solid solutions M(Fe). The failure to obtain chemicallyhomogeneous NbFe and M&e sampleswith the WC vial may be due to the lower m illing intensity or to a detrimentalinfluenceof the impuritiesincorporatedduring m illing. The x-ray spectraof HlFe, NbFe (Fig. 4a) andMoFe consistof the lines characteristicfor the crystalline elementsplus an enhancedbackgroundin the case of hafnium and traces of tungstencarbidein the samplesm illed in the WC vial. The crystallite sizesof the HfFe andI&Z Fe samplesproducedwith the WC vial are smallerthan thoseproducedwith the steelvial in spite of the fact that both the m illing intensity and the contentof alloying elementsare lower with WC. The crystallite size of M&e, on the other hand, is smallerfor the ‘steel’sample,as expectedconsideringthe higherm illing intensity. This supportsthe conceptionthat the incorporatedWC particlesare not necessarilyinert but may partly reactwith the samplematerial.The reactivity towardscarbonincreasesin the sequenceM O - Nb - Hf. Furtherevidencefor this idea comesfrom the observedlattice parameters, which are reducedin M&e (WC + steel) and NhFe (steel) due to the formation of substitutionalsolid solutionswith iron. A comparisonwith data from (6) suggestssoluteiron contentsof about 2%, 5.5%, and 3%, respectively.The lattice constantsof hafnium in HfFe (WC) (a= 3.206*7 A, c= 5.0829 A) andniobium in I&Fe (WC) (a= 3.314A) are slightly larger than that of the pure elements,indicating that light elementssuch as carbonmay be dissolvedinterstitially. The lattice constantsof hafnium in HfFe (steel)(a= 3.198 A, c= 5.066 A) correspondto the literaturevalues.
M~SSIWUER
EFFECTINTRANSI~ON
633
METAL-IRON ALLOYS
r 1.00
0.98
!$l.OO ; 5 u
1.02 r F7 B !i - 1.01
0 96
L
1 , 1 , ,
-6
-3 0 3 VELOCITY rmmjsl
6
FIGURE 3 h&Fe MechanicallyAlloyed for 20h: (a) Nbsg.zFeox, W C M illing Vial/Balls, (b) Nb99Fel, SteelM illing Vial/Balls.
? z 1: ~1.00 -0.5
0 VELOCITY
05
1
rdsi
FIGURE 4 (a) NbgszsFeoa, 2Oh/WC: XRD Spectrum. (b) Pdgg.zsFeox, 2Oh/Steel: CXM Spectrum, Small DopplerVelocity.
F igure 4b showsa M iissbauerspectrumrecordedwith small Doppler velocity of ballm illed PdFe.The singleline is dueto the solid solutionof Fe in Pd. It is remarkablethat more than 3 at.% Fe are incorporated in Pd during2Ohof m illing. Palladiumis a relatively soft material, comparablein hardnessto Cu andAg; in the latter metals,no iron contaminationat all can be foundby EDX m icroprobeanalysisafter20h of m illing in the steelvial. Evidently, with metallic m illing tools, too, hibochemicaleffectshaveanimpacton the wearingbehaviour. In conclusion,mechanicalalloying,especiallyin systemswherethe effects underinvestigation dependon m inor fractionsof alloying elements,requirescareful considerationof the contaminationintroducedby abrasivewearof the m illing tools.The wearingcharacteristicsare determinedby the hardnessof the materialsto be alloyedandon their chemicalreactivity with respectto the m illing tools.For spectroscopic studiesof nanostructured materials,it is advisable to fabricatededicatedm illing toolsfrom the samematerialthat is to be m illed. References
1. A. Kochendorfer,2. Kristallogr.,M ineral.u. Petrogr..l.Q&393(1944) 2. U. Herr andK. Samwer,Nanostruct.Materials1,5 15(1992) 3. J. Chappert,J. M . D. Coey,A. LienardandJ. P. Rebouillat,J. Phys.F LL, 2727(1981) 4. A. M . van der KraanandK. H. J. Buschow,PhysicaB 138,55 (1986) 5.0. Kubaschewski,Iron Binary PhaseDiagrams. SpringerVerlag,Berlin, 1982. 6. P. Eckerlin andH. Kandler,eds.,vol. 6 of LundoZt-Bdmstein,Zuhlenwerte und Funktionen uus Nuturwissenschaft und Technik, Neue Serie. SpringerVerlag,Berlin, 1971.