Franc
Institute of Metals and Technology, Ljubljana, Slovenia
LATE PAPER
22"^ International Conference on Microelectronics, MIEL'94
iL	'
30 Symposium on Devices and Materials, SD'94 September 28. - September 30., 1994, Rogla, Slovenia
Key words: permanent magnets, FeCrCo alloys, FeCrCo magnets, magnetic properties, spinodal decomposition, cold deformation, material ductivity, heat treatment, material microstructure, magnetic remanence, coercitivity field strenght
Summary: In technical iron-chromium cobalt alloys the microstructure of ferromagnetic phase a is obtained with addition of suitable alloying elements preventing the formation of phases 7 and a. Alloys have pour ductility by ambient temperature. Magnetic properties depend upon the proper combination of spinodal decomposition, deformation and aging. All magnetic properties are improved by cold deformation. The greatest remanence is obtained by appr. 60% of deformation. The coercivity grows proportionally to the deformation and to the decrease of the distance between particles of phase ai, while the remanence grows proportionally to the allongement of particles of this phase.
Magnetne lastnosti, spinodaino razmešanje in hladna deformacija v zlitinah FeCrCo
Ključne besede: magneti trajni, FeCrCo zlitine, FeCrCo magneti, lastnosti magnetne, razmešanje spinodaino, deformacija hladna, raztegljivost materiala, obdelava toplotna, mikrostruktura materiala, remanenca magnetna, poljska jakost koercitivna
Povzetek: V tehničnih zlitinah železa, kroma in kobalta je potrebno z dodatkom sekundarnih legirnih elementov preprečiti nastanek faz y in 0 in doseči mikrostrukturo iz feromagnetne faze a. Zlitine imajo zelo majhno duktilnost pri temperaturi ambienta. Magnetne lastnosti so pri pravi sestavi odvisne od kombinacije temperature spinodalnega razmešanja, stopnje deformacije in procesa staranja. Vse magnetne lastnosti se izboljšujejo z naraščanjem stopnje deformacije. Največja remanenca je dosežena pri ca. 60% deformaciji. Koercitivna sila raste proporcionalno z zmanjšanjem razdalje med delci faze ai, remanenca pa proporcionalno s podaljškom zrnate faze.
1, Introduction
The property of permanent magnetism is obtained in iron-chromium-cobalt alloys through the spinodal decomposition of the solid solution of both alloying elements in the ferromagnetic phase a Fe in two spinodal components. During this decomposition the matrix as is enriched in chromium and particles ai are enriched in cobalt. Both components have the same a ferromagnetic lattice, however a different lattice parameter because of the difference in composition. Both phases accommodate with elastic stresses which increase the hardness and stabilise the externally imposed uniform orientation of Weiss domains the more, the greater is the difference in composition, which is increased through a proper aging. Better magnetic properties are obtained by a combination of heat treatment and cold deformation by wire drawing, which produces a spinodal structure aligned and allonged in the deformation axis /1 -18/. On principle, good magnetic properties are obtained also by a very slow cooling in magnetic field. By the technically acceptable cooling in magnetic field, which gives the required properties to AINiCo alloys, several times smaller coercivity is obtained in a FessCrieCo alloy than combining heat treatment and cold deforma-
tion. The initial microstructure consists of coarse grains of phase a (fig. 1) obtained by annealing the alloy at 1200°C and quenching. The microstructure should be free of phase 0, which makes the alloys unductile and of the non ferromagnetic phase y. Already the thin grain boundary layer of phase y in fig. 2, decreases the magnetic properties by appr. 20%. The proper micro-structure is obtained in technical alloys, containing elements stabilisers of the phase a , f.i. carbon, nitrogen and manganese through a proper addition of aluminium or titanium, which prevent also the formation of phase a. Twinning makes the monophase coarse grained microstructure virtually undeformable at room temperature, therefore the wire drawing deformation is performed by increased temperature, when deformation by sliding occurs.
In this paper a short and simplified presentation of the relationship between the spinodal decomposition, the deformation and the magnetic properties will be given. The microstructure and the ductility were presented earlier /26/. Unpublished findings will be discussed as well as already published data /19-25/. In the paper the denomination phase will be used for the spinodal components although physically both components are not
Fig. 1: mag. 50x, Fe28Cri6Co alloy. MIcrostructure after 30 min. of annealing at 120CPC and quenching.
Fig. 3:
600 620 Temperatura , °C
640
Fe3i CoioCo alloy. Influence of the 30 min. annealing for spinodal decomposition on coercitivity and remanence. Homogenisation temperatures 1200 and 125(fC.
Fig. 2: mag. SOOx. /4 thin layer of phase y at the boundaries of a grains.
real phases, since they are separated through a chemical gradient and not by a phase boundary.
2. Spinodal Decomposition and Magnetic
Properties
700 615	605 595
Temp, spinodolne premene, °C
0 ~"300
Fig. 4: The same alloy as In fig. 3. Influence of the 30 min. annealing for spinodal decomposition on hardness and ductility.
The size and the number of particles of phase ai as well as their composition depend upon the spinodal decomposition temperature and time. Fig. 3 shows that very similar coercivity and remanence are obtained by the alloy FesoCrisCo by 30 min. of annealing in temperature range from 615 °C to 595°C. By higher temperature the magnetic properties decrease very fast. By low spinodal temperature the hardness is increased and the ductility diminished (fig. 4). Experience shows that a sufficient ductility is obtained if the spinodal temperature is above 620°C. A similar effect of spinodal temperature on the ductility was found also for the alloy FezsCneCo. By short annealing time the spinodal structure is stable
below appr. 620°C. By the temperature of 630°C, which was found as optimal for ductility and magnetic properties after wire drawing deformation, the best properties are obtained by a 30 min. annealing (fig. 5). After the wire drawing deformation the alloys are submitted to a 12 hr. aging in temperature range from 600 in 500°C. During the aging the difference in chemistry between both phases, the accommodating stresses, coercivity and energy product are increased, while remanence shows a slight decrease at initial aging temperature (fig. 6).
lr25
B V vzdolžni in prečni smeri
OCT
Trajanje žarjenja, ure
500
400
300
200
100
Fe Cr 630	28 Co gas«	16 /94 ;no	, 1200="(	3 , gas	eno
1	30 zakljL obdelavi	X—y-- ični terr	--- nični		
					
	F	>0 viečer	iju		
					
					
10 20 30 40 -»»- % deformacije
50
60
Fig. 5:
700
o
0
650
S
1	600
<u
o.
S 550 tS
/6

3: 03
FesoCr-ioCo alloy. Effect of annealing for spinodal decomposition at 63CPC on coercitivity, remanence and hardness. The alloy was homogenised at 120CPC and quenched.
,670°C/30min ■ m°C/h 6l5°C/30min ■i5°C/h
■:^540°C/3h 525°C/4-]2h
—^	D
/		
~ / / / / /	— ^	
- /	/ / (BH)K--/ / / /	10,il<J/m^
/ ABH),s _1_J_L		-
		-1-UU—
10 -	32
	S
	X
c	
CQ	
0,5 -	mi
4 6 8 10 12 U 22 Trajanje toplotne obdelave, (h}
O -J
Fig. 7: FezsCr^eCo alloy. Effect of deformation by wire drawing on hardness before and after aging. The alloy was initially annealed at 120CPC, quenched, reannealed 30 min. at 63CPC and quenched.
0,5
indeksi al< ra 615°C,	(aksialno) (radialno) 1 30 min ^^	Br ak	
	Bfak.ei	-	—cr- ) min _^__ -	1 H Br ra He al	____—s <
	(BH)m ak (BH)m ra JA	He ra	
40
e
o
<
20
x:
CD
20
40
Deformacija , %
60
80
Fig. 6: FesoCnoCo alloy. Evolution of magnetic
properties and hardness by controlled slow cooling (aging). The alloy was initially annealed at 120CrC, quenched, annealed for 30 min. at 62(fC and quenched.
3. Deformation and Magnetic Properties
The wire drawing deformation is carried out at increased temperature. This should be, however, below that which could affect the spinodal decomposition and the dynamic as well as static softening processes. By wire drawing deformation a low strain hardening is obtained (fig. 7), appr. proportional to the degree of deformation. It is interesting that after aging, which is started at a temperature appr. 200°C above the wire drawing temperature, the strain hardening is conserved. The deformation increases the magnetic properties in axial direction and
Fig. 8: Fe32CrioCo alloy. Influence of wire drawing deformation on magnetic properties in axial (ak) and radial (ra) directions. Initially the alloy was annealed at 120CPC, quenched, reannealed 30 min. at 61^C and quenched.
diminishes these properties in radial direction (fig. 8). Correspondingly, the shape of the demagnetisation curve becomes more rectangular (fig. 9). A careful evaluation of experimental findings showed that the remanence grows proportially to the square of the ratio between the initial (di) and the final (da) diameter of the deformed rod (fig. 10), while the coercivity is increased proportionally to this ratio (fig. 11) and it is appr. proportional to the strain hardening. The relationships in fig. 10 and 11 can be explained supposing that the change in magnetic properties in dependence of the deformation is connected to the modification of the shape of the particles of
Ora
J/ak
—faSU-Ira 4ra ^ 'lOak
lak
7,9ra 10 ra
o
o <j
x:
	/ /A	
3( A // /n /	// y^y^ ^Qmn r/ yAsmin	
/i^ / 'd / A p		
15
72 64 56 48 «) 32 24 16
0 Ho!kA/m]
Fig. 9:
Alloy form fig. 8. Influence of the deformation degree on the shape of the demagnetisation cun/e in axial and radial directions.
03
a 03
		\ . An min	
615 °C	/	- -—----ti / / /	30 min 15 min
	// ^/A / / v /		
	/		
o/j			
Fig. 10:
do /
Alloy from fig. 8. Relationship allongement-remanence.
phase ai. These particles are initially spherical and are deformed in the same way as the matrix of phase «2, because the change in chemistry has no significant effect on the hardness of both phases through solid solution hardening. Shematically, the effect of the shape change of particles of phase cci is shown in fig. 12. The coercivity is the higher the smaller is the distance between particles of phase ai, it increases thus with the gradient of chemistry between both spinodal phases. The effect of deformation on coercitlvity is similar to the effect of the lowering of the spinodal temperature. The
Fig. 11: Alloy from fig. 8. Relationship decrease of rod thickness-coercitivity.
Pogled

Izotropno (nedeformirano)
xir

J C
J C
—- o o-""!
3 o o
Anizotropno (deformirano)
P
An • Br = K {-iif = K An-Hc=K-^ = K-l^
K
Ce je X < Xi > o Če ie xi =0
Fig. 12:
Br in ( BH)m X Br in (BH)m\
Schematically representation of the dependence of the shape of particles of phase a 1 and the remanence and the coercitivity.
remanence depends upon the ratio length over diameter of particles of phase ai, which is proportional to the wire drawing allongement and to the ratio of the initial over final diameter of the deformed rod.
Let us suppose that one Weiss domain occupies a volume of one particle of phase ai with the corresponding part of the matrix of phase a2 /15/. If the allonged particles of phase ai approach below a critical distance or even a mutual contact is established, the shape and
the size of Weiss domains is changed, and the rema-nence, which depends upon their size, is diminished also. Indirectly this explanation is confirmed by the fact that the greatest remanence is obtained by a 45-50% deformation after 30 min. of spinodal decomposition at 630°C, while after 60 min. of spinodal annealing at the same temperature the highest remanence is obtained by appr. 65% of deformation. By isothermal annealing the number of particles of phase ai (N) is diminished accordingly to the parabolic law N = (t - annealing time) and parallelly their size is increased also.
4. Conclusion
In the technical iron-chromium-cobalt alloys is necessary through the addition of secondary alloying elements obtain a micrestructure of the ferromagnetic phase a. This microstructure gives, however, after homogenisa-tion and quenching a very poor ductility at ambient temperature. By a selected chemistry of the alloy the magnetic properties depend strongly upon the combination of the temperature of spinodal decomposition, the wire drawing deformation and the aging process. All magnetic properties are improved by the deformation. The highest remanence is found by appr. 60% of wire drawing deformation. The coercivity grows proportionally to the decrease of the distance between ai particles, while remanence increases proportionally to the allon-gement of these particles.
The support of the Ministry of Science and Technology of Slovenia is gratefully a knowledged.
5. References
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/26. F. Vodopivec: 45. Posvetovanje o metalurgiji in kovinskih gradivih in 2. Posvetovanje o materialih, Portorož, oktober 1994
Prof. dr. Franc Vodopivec, Institute of metals and Technology, Lepi pot 11, 61000 Ljubljana, Slovenia tel. + 386 61 125 11 61 fax. + 386 61 21 780
Prispelo (Arrived): 18.11.1994 Sprejeto (Accepted): 22.11.1994