TEMPERING OF HIGH-COERCIVITY ALNICO ALLOYS

G. P~ Vasin and E. Go Povolotskii UDC 669.25' 24' 771:621. 785.72

In manufacturing permanent magnets of Alnico alloys, thermomagnetie or isothermal thermomagnetie treatment (ITMT) is followed by tempering, which increases the coercive force and the maximum magnetic energy of the alloys by 30-100%. However, the tempering conditions used do not ensure obtaining the highest magnetic properties.

We studied the kinetics of the processes occurring during tempering of the most widely used Alnico alloys -Magnieo and Ticonal~

The composition of the alloys investigated is given in Table I (the first three are Tieonal alloys, which are anisotropic in the crystalline condition, and the fourth YuNDK24 is Magnico, which is isotropic in the crystalline condition). Castings with a section of 12 x 12 mm were cut into lengths of 50 mm, homog- enized, and given the standard thermomagnetic treatment -the Magnieo alloy was heated at 1280~ for 30 min, cooled at the rate of 30 dog/rain (with application of magnetic field) to 700~ and then cooled in air; the isothermal treatment for the Ticonal alloys consisted of heating at 1230~ for 15 min, transfer to a tin bath at 820~ for 15 min, and cooling in air. The castings were cut into pieces 5 x 5 x 20 ram, from which samples with approximately identical magnetic properties were selected for tempering (in order to exclude the effect of an initial difference in the properties).

The kinetics of tempering was studied at 750-550~ At higher temperatures there are irreversible changes in the structure and properties due to disruption in the texture of the dispersed/32 ~ fl + fi2

decomposition as a consequence of the growth and solution of

TABLE 1 t_ Composition, % * Alloy

Co NI ] At ~ - I TI

YuNDK42T81 42 YuNDK40T8 40 YuNDK38T7 38 YuNDK24 24 (Magnico)

t3,5 7,513,01 8,3 13,5i 7,5 3,01 7,85 13,5 7,5 3 0[ 7,0 13,5 7,5 3101 --

* T h e rest Fe in a l l a l loys .

Te

par t i c l e s , while below 550~ the p r o c e s s e s occur r ing during t emper ing cease .

1,4 1 ,4 1 ,4

Figure 1 shows the var ia t ion in the magnet ic p rope r t i e s of Magnico and Ticonal al loys with t ime at different t e m p e r a - t u re s . The re is a peak of coe rc ive fo rce at each t emper ing t e m p e r a t u r e , the s h a r p e r the peak and the s h o r t e r the t ime in which it is at tained. This is the r eason fo r mul t i s tage temper ing , with gradual reduction of the t e m p e r a t u r e and increas ing holding

Br, G

Z 13000 ~ : - e & 7 8

Joo ,,..z I T M T l Z 3 t~ 5 6 7

a 8h

Br, G ,gO00

6ago Hc, Oe

. o o l ~000 II

ITMT 2 ~ 5 8 I

10 ff 14 t5 2 f h

F i g . l ~ Var ia t ion of the magnet ic p rope r t i e s with t ime at different t e m p e r a t u r e s for Alnico al loys, a) YuNDK24; b) YuNDK40T8~ 1) T e m p e r e d at 730~ 2) 700~ 3)670~ 4)6500C; 5) 6400C; 6) 620~ 7) 600~ 8) 570~

Saratov Polytechnical Ins t i tu te . T rans la t ed f r o m Metal lovedenie i T e r m i c h e s k a y a Obrabotka Meta l - tov, No ~ 8, pp. 47-51, August, 1970.

�9 1971 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved�9 This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

679

Br, G YOOO

Br, G 7 0 0 0 ~ ' 7Juud

BHmax, G-Oe, 10 ~ ~

~oo!., , f ' - ] ' i I 6oo l

000 ~ G-Oe. 10 ~ ~ 1 " I ~0 ITMT 660~176 560TlOh53o~ TMTG,,~*IhGIO~ 6OOT 4 h 556710 h 706Yh Tempering stages

Tempering stages b

Fig. 2. Variat ion of magnetic propert ies with the conditions of mult istage tempering and the thermomagnetic t reatment preceding it. a) Magnico alloy; b) Ticonal al loys. 1) TMT with cooling rate of 30 deg /min ; 2) cool- ing rate 100 deg/min (at 840-760"C): 1) YuNDK42T8; 2) YuNDK40T8; 3) YuNDK38T7.

TABLE 2 Tempering se- ]Hc, ~r,

Alloy quenee iOe

YuNDK40T8

After ITMT 700~ 1 h 680~ 1 h 660~ 3 h 640~ 4 h 620~ 3 h 600~ 3 h

After tTMT 700~ 2 h 660~ 5 h 640~ 7 h 600~ 6 h

1080 1580 1690 1860 1930 1950 1960 1090 1600 1760 1880 1910

7150 6300 6350 6450 6450 6450 6450 7150 6800 6900 6900 6800

time. Alnico alloys have an initial two-phase structure, f i 2 ~ f l + f i ~

decomposition, af ter thermomagnet ic or isothermal thermomagnet ic treatment, with fair ly high coercive force. The changes in Alnico alloys during tempering evidently resul t f rom the decomposition of the f12 matrix, which must be accompanied by morphological and compositional changes in the successive fi and fi2 phases and cause changes in the magnetic proper t ies . The increase of the coercive force after tempering with controlled holding times indicates that such changes do occur - the anisotropy of the shape of the par t ic les increases, their s izes become equal, and the concentration gradi - ent between them increases .

YuNDK24 (Magnico)

After TMT 700~ 0,bh 650~ 0,5 h 620~ 1,5 h 600~ 2,5 h After TMT 600~C 5 h

403 470 520 640 670 403 650

13000 12900 12700 12300 12300 13150 12400

Figure 2 shows the variat ion in the propert ies of the alloys with the conditions of multistage tempering. The propert ies of the Magnico alloy (Fig. 2a) are given af ter two thermomagnetic t reatments with different cooling ra tes . The holding time at each stage was sufficient to obtain the maximum coerc ive force under those conditions. With increasing tempera tures , mult istage tern-

pering leads to an increase of coercive force, the convexity of the demagnetization curve, and the maximum magnetic energy. It cannot be asser ted that these increases in the magnetic charac te r i s t i c s exhaust the possibilit ies, since the set of fac tors is a rb i t ra ry . Fur thermore , the values of the magnetic proper t ies obtained depend on the conditions of the preceding thermomagnet ic t reatment (Fig. 2a), i o e . , on the original propert ies of the alloys before tempering. The lower the values af ter thermomagnet ic t reatment, the lower the values af ter tempering. Evidently the propert ies result ing f rom thermomagnet ic t reatment under con- ditions that are not optimal can be almost completely compensated by tempering with a fair ly large number of stages at temperatures above 600~ In this case the main increase of the coerc ive force and the maxi- mum magnetic energy will be attained in tempering by stages at an elevated temperature . This is confirmed by the data in Table 2, where the propert ies of the alloys are given after multistage tempering with various numbers of stages and different holding t imes. The coerc ive force is always higher as the number of tem- pering stages is increased at tempera tures above 600~ Thus, the a rb i t ra r i ly chosen multistage tempering conditions are not optimal for Alnico alloys. It was desirable that in the region of elevated tempering tem- peratures the tempering stages be replaced with controlled cooling at different ra tes . The data obtained on the kinetics of tempering at different tempera tures were combined in a single diagram of temperature vs time. Figure 3 shows such a diagram for the Magnico and Ticonal alloys. Each point on the curves corresponds to the time of attaining the maximum coercive force at each tempering temperature . The curves are approximated by parabolic relationships y2=A% where y is the cur rent temperature in the units

680

~ z 7~0 0 , e se r6 z0 z* ~,eu

71o ' 1 . . . . i : . . . . . . . .

I I \ i , , ' ! !

z . . . . . + - - - - - . . . . . . 530 r i "

6 1 0 -15 i 5 s o . I \ :,, i i ~ ~ " i ,

Fig~ 3. T e m p e r a t u r e - t i m e diagram of tempering for Mag- nico (1) and Ticonal (2) alloys.

p rocesses occur more slowly.

given (each division corresponds to 10~ counting f rom the maximum tempering temperature for each alloy to; r is the tempering time (h). The significant charac te r i s t ic is the coefficient A, which de- termines the steepness of the curves and charac te r i zes the rate of the aging p rocesses during tempering. Fo r the Magnico and Tieonal alleys investigated this coefficient is approximately equal to 30 and 15 respect ively . At each rate the values of coefficient A change, but the relationship between them, reflecting the relative rate of aging during tempering, remains the same. Consequenly, this pa ramete r is a function of the composit ion of the alloy. As can be seen f rom Fig. 3, it is lower for Alnico alloys containing titanium. Thus, as compared with the Magnico alloy the Ticonal alloys age in a wider t empera ture range, and at a given tempering tempera ture the aging

The absolute increase of the coercive force of these alloys is inversely proport ional to the rate of aging.

The aging process of Alnico alloys during tempering depends also on the completeness of the dispersed ~2 ~ fl + /?2 decomposit ion during the preceding TMT or ITMT. This is par t icular ly noticeable for Magnico alloys without titanium treated with controlled continuous cooling in a magnetic field. Changes in the con- ditions of TMT may lead to small changes in the upper limit of the tempering tempera ture t o and coefficient A.

The t e m p e r a t u r e - t i m e diagram for different Alnico alloys can be used to p rogram tempering in o rde r to obtain the optimal magnetic proper t ies . These p rograms must be carefully followed in the high- tempera ture region (700-600~ for Magnico alloys and 730-640~ for Ticonal alloys). The replacement of p rogrammed tempering by cooling at some average rate (calculated for all tempering temperatures) can- not be allowed, since it may lead to overaging of the alloy at the cur rent tempera ture and thus to i r r e v e r s - ible losses of the magnetic proper t ies .

Of great importance is the ra te of heating magnets to the initial tempering temperature , which depends on var ious fac tors - t h e dimensions and number of magnets , the heating capacity of the furnace, etc. Ob- viously, the initial t empera ture of the beginning of p rogrammed tempering can never be equal to t o . The slower the initial heating of the magnets , the lower the initial temperature must be, and the l a rge r the in- evitable losses of magnetic charac te r i s t i c s , and therefore in selecting the optimal initial t empera ture one must use the t e m p e r a t u r e - t i m e d iagram if the approximate time of heating to different tempera tures is known.

Plotting of the complete t e m p e r a t u r e - t i m e d iagram does not require investigating the kinetics of aging at many tempera tu res . It is necessa ry to know only the time of attaining the maximum coercive force at two, preferably elevated, tempering tempera tures . In this case the time of attaining the maximum coerc ive force is c l e a r e r because overaging occurs rapidly, and the testing time is short . The following equations hold true for any tempera tures t t, t 2 selected:

where Yl and Y2 are the tempering tempera tures in units of the tempera ture scale; r I and r 2 are the r e spec - tive t imes of attaining the maximum coerc ive force , It is well known that with Y2 > Yi ( i . e . , t 2 < ti) Y2 = Yl + oz, where o~ is the difference between the tempera tures selected (in units of the tempera ture scale)~ Then, solving the sys tem of equations

- - , y 2 = y ~ a , 9

where Ti, ~'20 and ~ have known values, we obtain the values of Yl o r Y2 in units of the tempera ture scale. Reverting to the tempera ture scale, we obtain the upper tempering temperature t o for the given alloy. By any of the equations (1) we find the second pa rame te r needed to plot the complete t e m p e r a t u r e - t i m e diagram - t h e coefficient of the parabolic function A. Thus, the optimal conditions of p rogrammed tempering (tem- p e r a t u r e - t i m e diagram) are determined by pa ramete r s t o and A, the values of which can easi ly be de te r - mined by exper imental -analyt ical means for any alloy.

681

Unlike the upper limit t0, the lower temperature limit for the end of programmed tempering is un- determined. It is well known that in the case of multistage tempering the effect of holding is very small in the lower temperature stages. As can be seen from Fig. 2b, this treatment of the Ticonal alloy results in a decrease of the convexity of the demagnetization cux-ce and stabilization of the maximum magnetic energy. Thus, to shorten the tempering time and obtain the maximum fullness factor the programmed cool- ing should be stopped at a given temperature.

CONCLUSIONS

1o The standard ,and multistage tempering of Alnico alloys (Magnico and Ticonal) ordinarily used is not the optimal treatment for obtaining the highest magnetic properties of permanent magnets. The magnetic properties (coercive force and maximum magnetic energy) are inversely proportional to the tem- perature of stepped tempering with optimal holding times (particularly at elevated tempering temperatures).

2. From the time of attaining the maximum coercive force at different tempering temperatures it is possible to plot a temperature- t ime diagram that gives the tempering conditions to ensure the highest magnetic properties for any Alnico alloy.

682