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FIM-ATOM PROBE INVESTIGATIONS OF PERMANENT MAGNETIC MATERIALS
A. Hütten, P. Haasen
To cite this version:
A. Hütten, P. Haasen. FIM-ATOM PROBE INVESTIGATIONS OF PERMANENT MAG- NETIC MATERIALS. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-205-C7-209.
�10.1051/jphyscol:1986736�. �jpa-00225929�
JOURNAL DE PHYSIQUE
Colloque C7, supplhment au no 11, Tome 47, Novembre 1986
FIM-ATOM P R O B E I N V E S T I G A T I O N S O F P E R M A N E N T M A G N E T I C M A T E R I A L S
A. H ~ ~ T T E N and P. HAASEN
Institut fiir Metallphysik, Universitat Gdttingen, 0-3400 Gdttingen, F.R.G.
and Sonderforschungsbereich 126, 0-3392 Clausthal-Zellerfeld.
(Gottingen), F.R.G.
Abstract - The kinetics of the decomposition reaction of AlNiCo 5 has been in- vestigated using Field Ion Microscopy (FIM) and Atom Probe (AP) techniques.
At the aging temperature of 680 OC the decomposition reaction is spinodal in character. The observed c v r ~ n i n g data of the resulting isotropically inter- connected phases follows t ' kinetics. Furthermore the compositions of the two phases were obtained and are presented in this paper.
I - INTRODUCTION
In 1932 T-Mishima / I / discovered that FeNiAl alloys with compositions in the range of (55-63) wt% Fe, (25-30) wt% Ni and (12-15) wt% A1 have more than twice the coercivity of the magnet steels which were available before. Based on these
"Mishima alloys1* the magnetic values were improved later by major additions of Co and minor additions of Cu. Finally, a thermomagnetic treatment firstly used by Oliver and Shedden /2/ led to a significant increase in the energy product (B*H)max and to the group of anisotropic AlNiCo 5 magnets. Their magnetic properties are associated with a microstructure which results during thermomagnetic treatment from the decomposition of the high temperature bcc a-phase in a Fe-Co rich a and an A1-Ni rich a -phase. Both phases have also bcc structure. From recent aiim probe studies /3,4$ the exact compositions of both phases are known for the magnetic op- timum state. The influence of Co additions on the miscibility gap in Fe-NU1 pseudo-binary systems at various temperatures and hence the shape of the miscibili- ty gap in the resulting Fe-Ni-Al-Co system has been determined by S.M. Hao et al.
/5/ using a diffusion couple technique. As seen in fig.1 the peak of the miscibi- lity gap is lowered and shifted to the Fe-rich corner with an increase of Co con- tent. Therefore, by Co additions on the one hand the critical temperature is lowered and the two phase field is narrowed. On the other hand the Co atoms which dissolve in the &,-phase raise the Curie temperature to about 850 OC. If the decomposition occurs at a temperature just inside the m i ~ i b i l i t y gap the shape and orientation of the al-particles are controlled by the applied magnetic field be- cause they are already ferromagnetic. Relatively unknown is the type of decomposi- tion which leads to the microstructure and hence to the magnetic behaviour of AlNiCo 5 magnets.
It was the objective of the present study to characterize the decomposition by us- ing FIM and AP techniques.
I1 - EXPERIMENTAL
The AlNiCo 5 alloy was provided as cast plates, annealed for 30 min at 1285 OC, by Thyssen AG, Dortmund. The nominal composition of the material is given in the second column of table 1, whereas the results from the AP- analysis are given in the last column.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986736
C7-206 JOURNAL D E PHYSIQUE
Table - I
Components Nominal-
Composition in at%
A 15 h heat treatment at 900 OC in argon-filled capsules led to homogeneous speci- mens as was proved by the autocorrelation analysis of the field evaporated atoms.
The composition of homogenized AlNiCo 5 specimens which were given also in table 1 were determined by AP- investigations. FIM tips were made from spark eroded rods with dimensions of 0.3 x 0.3 x 15 d by electrolytic thinning in two stages:
first in 25% perchloric acid and 75% acetic acid, then in a solution of 1 g sodiumchromate in 10 ml acetic acid. The temperature for imaging and atom probing was about 80 K.
In contrast to specimens in the magnetic optimum state for AlNiCo 5 specimens an- nealed at 680 OC no FIM contrast was visible between the two phases. Therefore, composition data had to be determined from concentration profiles where the concen- tration of each component reached its maximum, as seen in fig.2. Hence the probe hole fully lays inside the correspondent particle. In order to eliminate inaccura- cies in the determination of the concentration amplitude of each component every AP-analysis was performed with a sufficient high pulse ratio Vp/Vdc which was ob- tained in a pilot study. For AlNiCo 5 a value of Vp/Vdc greater than 15.5% was necessary to prevent preferential evaporation of a component. Due to the lack of phase contrast the average particle diameters could not be measured from FIM images but had to be calculated from autocorrelation analysis. It should be pointed out that it was possible to study the morphology by variation of the imaging voltage Vdc for one aging time at 680 OC. From a relatively low value of Vdc corresponding to a state without field evaporation the imaging voltage was increased quickly cor- responding to a high rate of field evaporation. By lowering the voltage to the former level, contrast between the two phases was obtained (fig.3).
I11 - RESULTS AND DISCUSSION
Figure 3 shows the microstructure of AlNiCo 5 after aging for 20 min at 680 OC.
Due to the heat treatment without an applied magnetic field both phases formed an isotropic interconnected network. It consists of a brightly imaging al- and a dark a2-phase. Furthermore the visible poles are found to be continuous through the in- terface. This means that the two phases are coherent. The morphology is in fair agreement with the simulated one of an isotropic spinodally decomposed alloy given in fig.4. The microstructure can be considered as a hint for the spinodal type of decomposition.
In order to characterize the decomposition process with respect to the distribution of Fe and A1 between both phases, AP-analysis was performed. In fig.5 the meas- ured Fe- and Al-concentrations of the ul-, a2-phase respectively are shown as a function of aging time at 680 OC. The A1 concentration of the a phase as we11 as the Fe concentration of the a -phase increases with aging time. G t e r about 25 min in both phases these concentrations are constant. 1 This behaviour is typical for spinodal decomposition in contrast to a nucleation and growth mechanism, where the particles reach their equilibrium composition already for the shortest aging times and smallest sizes.
The composition of both phases which were formed during the spinodal reaction were determined from the concentration profiles (fig.2) as given in table 2.
Table - I1
Composition of both phases in at%
These results are in fair agreement with those measured by Hetherington et al. /4/
using a higher resolution atom probe. Due to the content of A1 and Ni the ci phase appears to be paramagnetic at room temperature whereas the ci -phase is ferromag- 2- netic according to the high Fe and Co concentration. The time dependence of the 1 mean diameters of the areas enriched in A1 ( cr phase ) or Fe ( crl-phase ) is shown in f ig.6. The measured coarsening data were found to follow a D 2- a tX time law with an exponent of 0.23 for both phases. A similar value was obtained by Zhu et al.
161 for Fe-28 wt% Cr-15 wt% Co-1 wt% A1-0.25 wt% Zr system, which decomposed also by a spinodal reaction with comparable morphology. In both systems the time ex- ponents agree with Langerts 171 of about 0.2 obtained from non-linear spinodal decomposition calculations. However, it has to be considered that the time law observed here is applicable over a far longer time span than that considered by the theory.
Acknowledgements
The authors wish to thank Dr.K.Kuntze (Thyssen AG, Dortmund) for the provision of the material and Dr.R.Wagner, L.v.Alvensleben, R-Griine and M-Oehring for helpful discussions.
References
/I/ Mishima, T., 1932, Ohm 19, 353.
/ 2 / Oliver, D.A. and J.W. Shedden, 1938, Nature 142, 209.
131 Zhu, F., L.v.Alvensleben and P.Haasen, 1984, Scripta metall. 18, 337.
/ 4 / Hetherington, M.G., A.Cerezo, J.P.Jakubovics and G.W.D. Smith, 1984,
J. de Physique 45, C9-329.
/5/ Hao, S.M., T.Takayama, K-Ishida and T.Nishizawa, 1984, Met. Trans.
A15, 1819.
/6/ Zhu, F., P-Haasen and R-Wagner, 1986, Acta metall. 34, 457.
/7/ Langer, J.S., 1979, Acta metall. 21, 173.
/8/ Cahn, J.W., 1965, J. Chem. Phys. 42, 93.
C7-208
Fig.l - Miscibility gap of the pseudo- binary Fe-NiAl system influenced by a 20 at% Co addition /5/.
Fig.2 - Concentration profiles of AINiCo 5 components along a <002> axis into the depth of a specimen aged for 80 min at 680°C.
Fig.3 - Neon field-ion image of AINiCo 5 aged for '20 min at 680 "C.
Fig.4 - Computer-simulation of the micro- structure of an isotropic spinodally de- composed material with an assumed volume fraction of 50% for the second phase /8/.
Fig.1 Fig.2
Temperature / ° C
F i g . 5 - Time e v o l u t i o n of t h e A 1 concen- t r a t i o n i n t h e a2-phase and Fe concen- t r a t i o n i n t h e al-phase d u r i n g aging a t 680°C.
Fig.6 - V a r i a t i o n of t h e mean diameter D of a r e a s e n r i c h e d i n A1,Fe r e s . p e c t i v e l y a s a f u n c t i o n of a g i n g time a t 680 "C.
