influence of various heat exposures on alnico v. magnets

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Influence of Various Heat Exposures on Alnico V. Magnets Rudolf K. Tenzer Citation: Journal of Applied Physics 30, S115 (1959); doi: 10.1063/1.2185848 View online: http://dx.doi.org/10.1063/1.2185848 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/30/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crystal Lattice Investigation of the Permanent Magnet Precipitate in Various Alnico Materials J. Appl. Phys. 32, S196 (1961); 10.1063/1.2000399 Structure of Alnico V J. Appl. Phys. 32, S192 (1961); 10.1063/1.2000397 Transmission Electron Microscopy of Alnico V J. Appl. Phys. 32, 323 (1961); 10.1063/1.1735999 Transmission Electron Diffraction of Alnico V J. Appl. Phys. 31, S80 (1960); 10.1063/1.1984612 Relation between Colloid Pattern and Permanent Magnet Precipitate during the Magnetization Reversal in Alnico V J. Appl. Phys. 29, 299 (1958); 10.1063/1.1723108 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 128.192.114.19 On: Thu, 18 Dec 2014 14:09:49

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Page 1: Influence of Various Heat Exposures on Alnico V. Magnets

Influence of Various Heat Exposures on Alnico V. MagnetsRudolf K. Tenzer Citation: Journal of Applied Physics 30, S115 (1959); doi: 10.1063/1.2185848 View online: http://dx.doi.org/10.1063/1.2185848 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/30/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Crystal Lattice Investigation of the Permanent Magnet Precipitate in Various Alnico Materials J. Appl. Phys. 32, S196 (1961); 10.1063/1.2000399 Structure of Alnico V J. Appl. Phys. 32, S192 (1961); 10.1063/1.2000397 Transmission Electron Microscopy of Alnico V J. Appl. Phys. 32, 323 (1961); 10.1063/1.1735999 Transmission Electron Diffraction of Alnico V J. Appl. Phys. 31, S80 (1960); 10.1063/1.1984612 Relation between Colloid Pattern and Permanent Magnet Precipitate during the Magnetization Reversal in AlnicoV J. Appl. Phys. 29, 299 (1958); 10.1063/1.1723108

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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Page 2: Influence of Various Heat Exposures on Alnico V. Magnets

ACTIVITY OF MAG NETOSTR IC T I VE MATERIALS 115S

EXPERIMENTAL RESULTS

A comparison of the properties of various materials hown in Table I. The maximum values of k acc listed

• i the second column. Large values were obtained with ~ckel, 2V-Permendur, a cobalt nickel alloy, 37.5% to ~% nickel-iron, II. 7% to 15% ordered Alfenol, and nl;el cobaltous ferrites containing from 2% to 7% divalent iron . When Alfenol was disordered by quench­"n&: from above 600°C, the value of kmax was reduced I proximately an order of magnitude. The values of

ihown in the thi rd column, remained relatively B, . . . Oitan t for a given compositIOn . Ai, can be seen in the fourth column , the values of JJ.r

e largest in alloys. The maximum value was exhibited ¥ 11.7% ordered Alfenol. The fifth column contains e values of A and the sixth the product of AI",. The

imum va lues of hllr were exhibited by 2V-ermendur, 42.5% nickel-iron, and 11. 7% ordered

enol. When Alienol was disordered, A}J.r decreased much as 85%. ]n the case of ferri les the value of AJ.lr

exhibits a maximum for 6% divalentiron. This maximum only occurs when the samples are cooled by air quenching from the sintering temperature to room temperature. Various processing techniques were employed on the Ni-Fe alloys : slow cooling, quenching, magnetic annealing, and variation of annealing temperature between 900°C and 1220°C. Such variations fai led to produce more than a 10% change in the value of AI",.

CONCLUSIONS

For the material.s i.nvestigated the strain sensitivity due to magnetostnctIOn was found to be a maximum in the neighborhood of 8 oersteds for alloys and 16 oersteds for ferrites. The strain sensit ivity of both the ferri tes and the Allenols decreased when the samples were quenched. Th~ decrease amounted to as much as 40% for ferrites and·"85% for Alfenol. Variations in the processing techniques used have very little effect on the strain sensit ivity of nickel-i ron alloys.

OU R NAL OF APPLIED PHY S I CS SUPPLEM ENT TO VOL. 30. NO . APR I I~. 1959

Influence of Various Heat Exposures on Alnico V.Magnets*

RUDOLF K. TENZER

TIM Indiana Sted Products Company, Valparaiso, Indiana

Alnico V magnets were e:'l:posed to temperatures between 350 and 550°C for as long as 1000 hours. R ema­nence was determined at room temperature intermi tte ntly during the exposures. The changes in remanence appeared to be caused by metallurgical as well as magnetic processes . The two effecls were separated and it was found that the Alnico V material responds to temperatures as low as 350"C.

IN several recent publicationsl .2 Alnico V magnets are judged to be well suited for use up to 500°C. We

ound that the temperature range can be extended to °C when the remanence is allowed to change within

5% . In appl ications that requi re tolerances of ± 1% r less metallurgical changes may cause difficulties , as e following experiments show . We studied the changes in the remanence of Alnico V

ll1a.&nets that were exposed to various temperatures up 550°C. The materia l under investigation differed slightly

om the standard Alnico V composition in that it con~ ·ned a small addit ion (fraction of 1%) of titanium .

· e test magnets were prepared as follows: rods of i -in. ciJameter were cast in sand molds. The rods were re-

eated, cooled in a dc field of about 2000 oe between and 650°C, aged for 1 hour at 625°C and for 5 hours

t 555°C. Then the rods were cut and ground to different e"&th: diameter ratios (L/ D).

.. gork has been performed under the sponsorship of the Wright 'Wevelopment Center of the U. S. Air Force. • . H. Roberts, J. Appl. Phys. 29, 405 (1958).

A. G. Clegg and M. McCaig, Brit.}. Appl. Phys. 9,194 (1958).

In order to fi nd out how the different portions of the demagnetizing cu rve arc influenced by the heat ex­posures, a set of th ree magnets of diffe rent length : diameter ratios was used in each experiment. One mag~ net of each set operated in the flat part of the demag_ netization curve (LI D= 8), the second one operated close to the (BH )max value (L/ D= 4) and the thi rd one in the steep part of the demagnetization curve (L/ D=3 ).

The heat exposures were done in a box furnace that contained a big copper block. The temperature varia­tions of this block were kep t within ±2Q C. One set of magnets was inserted into the th ree holes of the block. The distance between the holes was great enough to pre­vent mutual influence of the magnets. Also the heating elements exerted no measurable effect on the remanence.

Remanence was measured by the usual ballistic method. The usc of a mechanical device enabled us to duplicate remanence values within ±O.5%. All magnetic measurements were done at room temperature. For this purpose the heat exposu res were interrupted. At the beginning of each exposu re this was done at hourly intervals, later the intervals were increased up to 100 hours.

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Page 3: Influence of Various Heat Exposures on Alnico V. Magnets

1165 RUDOLF K. TENZER

/~-.~.=:= - ' Q

oF=====~~~~====~~

~~~/~. ' ~~. ~:~dl 400 4SO 500 550

TEMPERATURE - t

FIG. 1. Changes in the remanence of Alnico V magnets caused by metallurgical processes. (a) Changes with increasing tempera· lure after incomplete heat treatment. (b) Changes with decreasing temperature after complete heat treatment.

I n the first experiment a set of three Alnico V magnets was exposed to 548°C fo r 1000 hours. The total losses of re~anencc after the exposure were 0.7% for the long, 3.2% for the medium, and 4.5% for the short sample. These figures comprise both metallurgical and magnetic changes. I n order to separate these two effects the samples were remagnetized and the remanence values were determined thereafter. T hey agreed within meas­uring tole rances with the original values obtained prior to the heat exposure. We take this as an indication that the metallurgical changes are negligibly small and ascribe the losses in remanence to magnetic aging at 548°C. This interpretation has another support : A re­peated exposure of the test magnets to 548°C for two hours resulted in closely the same losses that each sample had experienced during the same period of the fi rst exposure.

Based on these results we may assume that the Alnico V mate rial had reached metallurgical equ ilibrium al­ready during the 555°C treatment preceding the ex­posure to 548°C.

In the second experiment another set of three magnets was exposed to 517°C for 500 hours. Remagnetization after this treatment resulted in a gain of remanence over the original values. The increase was 1.5% for the long, 2.2% for the medium, and 1.8% for the short sample.

If we assume that these samples had also reached metallurgical equilibrium at 555°C, then we must con­clude that a different equilibrium was established during the exposure to 517°C. In other words, when the Alnico V material has reached metallurgical equilibrium at about 550°C, it is not necessarily stable at lower temperatures.

This result was supported by another experiment. The set of magnets that had been exposed to 548°C for 1000 hours was treated at 526°C. After an exposure of 72 hours, remagnetization at room temperature resulted in slight changes of remanence. At the end of 233 hours

the remagnetized samples showed gains in remanen up to O.i%.

Due to th is experience we became interested in th temperature that limits metallurgical changes whie permanently effect the hysteresis loop of Alnico V.

A set of th ree magnets was prepared to be particularl sensitive to precipitation. For this purpose the magne Y; were cooled in a magnetic field from about 900°C a thereafter t reated at 630'C for 1 hour. The magne were then exposed to lower temperatures from 3500 up. The exposures were interrupted at intervals of to 50 hours to determine remanence at room temper ture. The samples were always remagnetized before th remanence measuremen ts. The resul ts are plo~ted in Fig. l(a) .

To our surprise the remanence of the short and medium sized sample showed decreases of about 8% at 3S0'C and 10% at 418°C. Remanence of the Ion; sample increased about 1%. These changes occurred after a 20 hour exposure to each temperature. The changes of remanence after exposures to 488 518 558°C resemble those of the heat treatme~ts i~ the manufacturi ng process of Alnico V magnets.

The changes of remanence at 350 and 418°C are un. expected for two reasons.

(1) T hey show that the incompletely heat treated Alnico V material responds to temperatures as low .as 350°C in a rather short t ime.

(2) The changes of the magnetic properties at tbese temperatures are opposite in sign to the changes oc· curring at higher temperatures.

An interpretation of this effect cannot be given as yet. In a last experiment we exposed a set of fully heat­

treated magnets to the same temperatures as in the preceding experiment. This time we lowered the temper. ature starting from 558°C. The magnets were held for about 30 hours at each temperature. The results are also contained in Fig. 1 (b) (different scale!). All sampl .. showed a gain in remanence after remagnetization. The increase is small (max 1.6%) due to the stabilizing effect of the heat t reatments and the exposures above 488°C. Here again the remanence values of the medium and short sample were lowered due to the exposures to 418 and 350°C.

"Ve summarize our results as follows: Alnico V mag­nets can be used up to 550°C. ~ I agnets that have been heat t reated at this temperature still show metallurgical changes of a few percent at lower temperatures. These changes can be reduced by an extended heat treatment. In order to minimize metallurgical changes in applica.· tions below 550°C, the magnets should be heat treated at the highest operating temperature.

The author wishes to thank ~Ir. R. E. Que.r who did the magnetic measurements.

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