Magnetically soft, high temperature cobalt-iron allo

Barranger, J.

English

NASA Technical Reports Server

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INVESTIGATION OF MAGNETICALLY SOFT, 
HIGH-TEMPERATURE COBALT-IRON ALLOY 
by John Burrunger 
Lewis Reseurch Center 
CZeveZund, Ohio 
N A T I O N A L  A E R O N A U T I C S  A N D  SPACE A D M I N I S T R A T I O N  W A S H I N G T O N ,  0. C .  OCTOBER 1966 
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TECH LIBRARY KAFB, NM 
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NASA T N  D-3693 
INVESTIGATION O F  MAGNETICALLY SOFT, HIGH-TEMPERATURE 
COBALT-IRON ALLOY 
By John B a r r a n g e r  
Lewis  R e s e a r c h  Cen te r  
C leveland, Ohio 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
For sale by the Clearinghouse for Federal Scientific and Technical Information 
Springfield, Virginia 22151 - Price $1.00 
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INVESTIGATION OF MAGNETICALLY SOFT, HIGH-TEMPERATURE 
COBA LT-1RON ALLOY 
by John Barranger 
Lewis Research Center 
SUMMARY 
Considerable improvement in maximum permeability is obtained in certain high-
Curie-point binary cobalt-iron alloys by using ultrapure materials. The iron content of 
the alloys described is in the range 6 to 10 weight percent. The magnetic properties of 
the 9.3-percent-iron alloy are given from room temperature to 1000° C. A table of co­
ercive force and residual induction is also included for the 9.3-percent-iron material. 
The results indicate that the alloys in the range 7 to %1percent iron promise to be impor­
tant magnetic materials for high-temperature applications such as power transformers, 
electrical machinery stators, and square loop devices. 
INTRODUCTION 
The space requirements for very-high-temperature magnetic materials for power 
conversion and conditioning equipment (refs. 1 and 2) have revived interest in the lesser 
known high-Curie-point alloys of cobalt-iron. The binary alloys in the range 5 to 
2 1  percent iron have not only a high Curie point but also a high permeability at 25' C 
(refs. 3 and 4). The maximum permeability according to Bozorth (ref. 5) occurs at ap­
proximately 8 percent iron. The alloy also has a high saturation magnetization, and 
there are no phase changes or  order-disorder transformations. The purposes of this 
investigation are (1) to determine the maximum permeability of the alloy in the vicinity 
of 8 percent iron when ultrapure materials are used and (2) to determine the effect of 
temperature on the maximum permeability and the hysteresis loop. 
The iron content of the alloys described in this report is in the range 6 to 10weight 
percent, the balance being cobalt. The magnetic properties of the 9.3-percent-iron 
alloy are given from room temperature to 1000° C. A table of coercive force and resid­
ual induction is also included for the 9.3-percent-iron material. The results indicate 
that the alloys in the range 7 to 31percent iron promise to be important magnetic mate­
rials for high-temperature applications such as power transformers, electrical machin­
ery stators, and square loop devices. 
The alloys were refined and prepared by the Materials Research Corporation, 
Orangeburg, New York. 
TEST SAMPLES AND PROCEDURE 
A very-high-purity alloy was obtained by selection of high-quality starting materials 
and purification by electron-beam float-zone techniques. The refinement was obtained by 
subjecting the starting materials to three zone passes in a vacuum of torr.  This re­
duced the impurities to a total of less  than 90 parts per million. The materials were 
then alloyed by a r c  melting the beam-refined metals in purified argon. Five 0.5-inch 
(1.27-cm) thick rods were prepared with iron contents of 6.4, 7.0, 8.0, 9.3, and 
10.0 percent. Each rod was then cold rolled to a 0.012-inch (0.0305-cm) thick strip. 
During the cold rolling operation only the 9.3- and 10.0-percent-iron strips required an 
intermediate anneal. All specimens received a final stress-relieving anneal at 1020' C, 
a temperature which is approximately 20' C under the Curie point. In general, all the 
alloys had good ductility and malleability. Total impurities of the final product were less 
than 250 parts per million. 
The alloy strip was spirally wound into a toroid with an inside diameter of 2 inches 
(5.08 cm). A test transformer was made by winding coils around the toroid. Nickel 
wire was used for the electrical winding, and boron nitride and aluminum oxide insulators 
were used throughout. The sample was tested in an argon atmosphere furnace from room 
temperature to I O O O ~C. 
Magnetization curves and hysteresis loops were measured for all compositions by 
the usual ASTM ballistic methods (ref. 6). The field applied to the specimen was in the 
direction of rolling and was determined from the current in the windings. A fluxmeter 
was used to measure the induction. The accuracy of the current measuring device and 
the fluxmeter was 52 percent for each instrument. 
RESULTS AND DISCUSSION 
The direct-current data were taken at 25", 250°, 650°, 800°, 900°, and 1000° C. 
Figure 1 shows the maximum permeability as a function of the percentage of iron at vari­
ous temperatures and Bozorth's maximum value of permeability at 25' C (ref. 5). The 
maximum permeability increases with temperature for most compositions. Although the 
2 
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I I I I I I 	 maximum permeability at any given temperature 
usually occurs in the specimen that contain 9.3 per­
cent iron, all the curves exhibit high values of per­
meability from 7 to 51 percent iron. 
Examination of the 25' C curve in figure 1 
shows that the maximum permeability decreases 
at both ends of the range. This decrease is prob­
ably due to the change to lower permeability crys­
talline phases at 6 and 10 percent iron. The equi­
librium diagram (ref. 7) of the cobalt-iron system 
is illustrated in figure 2. It indicates that at 25' C 
there is a phase transformation from y to y .t a! 
at approximately 10 percent iron. The y phase 
is the face-centered cubic structure, while the CY 
phase is the body-centered cubic structure. At 
approximately 6 percent iron, there is a trans­
formation from y to E ,  where E is the close­
9 a 7 packed hexagonal structure. Thus, the high perme-I r on  content, wt % 1I I I ability y region of 7 to % percent iron is bounded 
91 92 93 94 
Cobalt content, wt 70 
Figure 1. - Maximum permeability as function 
of weight percent i ron at various temperatures. 
1200 
1000 
800 
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c 
m
al 

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c 

200 
by phase structures that have characteristically 
low permeabilities. Figure 1 also shows that the 
high permeability developed at 25' C is carried 
through to the high temperatures. 
Since the maximum permeability at most tem­
peratures occurs in the sample having 9.3 percent 
iron, all the following curves and data are for that 
alloy. The general behavior of this alloy is also 
representative of the whole range from 6 . 4  to 
10 percent iron. 
The magnetization curves for 9.3 percent iron 
a re  shown in figure 3. The permeability at low 
inductions increases with increasing temperature, 
while the magnetization at high inductions de­
creases with temperature. The general behavior 
of the magnetization curves is similar to that of 
pure iron or nickel. In pure iron or nickel there 
0 40 6 0 8 0  100 
are no phase changes or order-disorder transfor-
Cobalt content, wt 70 mations from room temperature to the Curie point. 
Figure 2. - Equilibrium diagram of iron-cobalt alloys. Examination of the equilbrium diagram (fig. 2) 
3 
0 . 8  1.6 2.4 3.2 
Magnetizing force, H, Oe 
I 1 
0 80 160 240 
Magnetizing force, H, A h  
Figure 3. -Magnetization curves for alloy of 
9.3 weight percent i ron  (balance cobalt) at 
various temperatures. 
TABLE I. - COERCIVE FORCE AND RATIO OF RESIDUAL TO 
MAXIMUM INDUCTION FOR ALLOY OF 9.3 PERCENT 
IRON (BALANCE COBALT) 
Temper- Maximum induction, Coercive force, Ratio of 
ature, Bm HC residual 
O C  to  maximum 
Oe A/m induction, 
kG I B P m  
25 12 1.2 0.50 39.8 0.94 
250 12 1.2 .3a 30.2 .96 
650 8 .8 .20 15.9 .98 
800 8 .8 .15 11.9 .95 
900 8 .8 .08 6.4 .94 
1000 4 .4 .05 4.0 .85 
shows that the 9.3-percent-iron alloy also falls into this category; hence, it behaves in 
a similar way. 
The alloys exhibited good temperature cycling stability. During three successive 
temperature cycles from 25' to 1000° C the material essentially retraced the previous 
magnetization curves. 
The hysteresis loops for selected values of maximum induction of the 9.3-percent­
iron alloy are illustrated in figure 4. Numerical values obtained from these curves are 
included in table I. The hysteresis loss and coercive force continuously decrease with 
4 
1. 

4 
- , 
c 

m' -1. -m'= -12I A 
(a) Temperature, 25" C. (b) Temperature, 250" C. (el Temperature, 650" C. 
8x103 
-1.6 -.8 0 .8 1.6 -1.6 -.8 0 .a 1.6 -.8 0 .8 
Magnetizing force, H, Oe Magnetizing force, H, Oe Magnetizing force, H, Oe 
I I l l I I I I I I I I I I I I I 
-120 -io -40 0 40 80 120 -120 -80 -40 0 40 80 120 - 4 0 0 4 0  
Magnetizing force, H, Nm Magnetizing force, H, Nm Magnetizing force, 
H, A h  
(d) Temperature, 800" C. (e) Temperature, 900' C. (f) Temperature,looo" c. 
Figure 4. - Hysteresis loops for alloy of 9.3 percent i r on  (balance cobalt) at various temperatures. 
5 

IIIII II II I I I 

temperature for a given induction. The high values of the ratio of residual to maximum 
induction suggest that the material is suitable for square loop applications. 
These alloys have the highest Curie point of all known magnetically soft materials. 
They have many desirable magnetic properties such as high permeability, low hysteresis 
loss, high saturation magnetization (19 000 G or 1.9 T, ref. 5), and good temperature 
stability. They are also easy to fabricate into thin sheets. As a result of these charac­
teristics, the 7 to 9ij1percent iron - balance cobalt alloys promise to be important basic 
materials for high-temperature applications such as power transformers, electrical ma­
chinery stators, and squake loop devices. 
Lewis Research Center, 
National Aeronautics and Space Administration, 
Cleveland, Ohio, July 22, 1966, 
120-27-04-18-22. 
REFERENCES 
1. English, Robert E. ; Slone, Henry 0. ;Bernatowicz, Daniel T. ;Davison, Elmer H. ; 
and Lieblein, Seymour: A 20,000-Kilowatt Nuclear Turboelectric Power Supply for 
Manned Space Vehicles. NASA Memo 2-20-593, 1959. 
2. 	 Glassman, Arthur J. : Summary of Brayton Cycle Analytical Studies for Space-Power 
System Applications. NASA TN D-2487, 1964. 
3. 	 Masumoto, Hakar: On the Intensity of Magnetization in Iron-Nickel-Cobalt Alloys. 
Science Repts., Tohoku Imp. Univ., vol. 18, 1929, pp. 195-229. 
4. 	 Wenny, D. H., Jr. ; and Gould, H. L. B. : The Polyform Hysteresis Loops of Thin-
Gage High Cobalt-Iron Alloys. AIME Trans., vol. 233, no. 4, Apr. 1965, pp. 836­
838. 
5. 	 Bozorth, Richard M. : Ferromagnetism. D. Van Nostrand Co., Inc., 1951, pp. 194, 
197. 
6. 	 Anon: Normal Induction and Hysteresis of Magnetic Materials. Magnetic Properties; 
Metallic Materials for Thermostats and for Electrical Resistance, Heating, and Con­
tacts; Materials for Electron Devices and Microelectronics. Part 8 of ASTM Stan­
dards, 1965, ASTM Designation: A341-64, pp. 33-35. 
7. 	 Hansen, Max: Constitution of Binary Alloys. 2nd ed., McGraw-Hill Book Co., Inc., 
1958, p. 472. 
6 NASA-Langley, 1966 E-3438 
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Investigation of magnetically soft, high-temperature cobalt-iron alloy

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