Lawrence Berkeley National Laboratory
Lawrence Berkeley National Laboratory
Title
MICROSTRUCTURE AND PROPERTIES OF STEP AGED RARE EARTH ALLOY
MAGNETS
Permalink
https://escholarship.org/uc/item/2994n24w
Author
Mishra, R.K.
Publication Date
1980-11-01
Peer reviewed
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University of California
tti&B
LBL-11680
P^ATF
- •8QHA4— 13
Lawrence Berkeley Laboratory
UNIVERSITY OF CALIFORNIA
Materials & Molecular
Research Division
Presented at the American Physical Society 26th
Annual Magnetism and Magnetic Materials Meeting,
Dallas,
TX, November
11-14,
1980
MICROSTRUCTURE AND PROPERTIES OF STEP AGED RARE
EARTH ALLOY MAGNETS
Raja K. Mishra, G, Thomas, T. Yoneyama, A. Fukuno,
and T. Ojitua
November 1980
Prepared for the U.S. Department of Energy under Contract W-7405-ENG-48
LEL-11680
Microsti"ucture and Properties of Step Aged Rare Earth
Alloy Magnets
Raja K. Mishra and G. Thomas
Materials and Molecular Research Division, Lawrence
Berkeley Laboratory, University of California,
Berkeley, California 94720 and
T. Yoneyama, A. Fukuno and T. Ojima
R
& D Laboratory, TDK Electronics Co., Ltd.
2-15-7
Higashi Owada, Ichikawa, Chibaken, 272-01 Japan
ABSTRACT
Alloys with Compositions Co-25.5wt/o Sm-Sw/o Cu-15 w/o
Fe-3w/o Zr and Co-Sra-Cu-Fe-1.5 w/o Zr have ibeen step aged to
produce magnets with coercive force (iHc) in the range of 10-
25k0e,
much higher than those reported so far in the litera-
ture for the Zr alloys. The high coercive force magnet
s
are typically aged at 800-850°C for 10-30 hours following
the solution treatment at 1150°C. Subsequently, these are
step aged to produce materials with high coercivity. The
microstructure in all these alloys has a 2 phase cellular
morphology with 2:17 phase surrounded by a 1:5 boundary
phase.
The long aging treatments at 800-850°C lead to
coarsening of the two phase structure. The subsequent step-
aging does not change the morphology, but only changes the
chemical composition of th:* two phases.
Best properties are obtained in materials with a cohe-
rent microstructure of optimum boundary phase thickness and
optimum chemical composition. The highest values of iHc ob-
tained so far are % 26k0e and
a.
16 kOe for the 3% Zr and 1.52
Zr alloys respectively. The (best hard magnetic properties of
(B?0 max = 33 MGOe and iHc = 13k0e are for a 25% Sm-20%Fe-4
Cu-2%Zr alloy.
INTRODUCTION
Efforts to design hard magnetic materials based on the
Sin
(Co ,Cu,Fe)
system have led to the
invest igat ion
of tho
effects of various heat-treatments and additions of other
alloying elements on the magnetic properties. (1»2,3) Alloys
containing small amounts of Zr have been shown to have energy
products ^ 30 MGOn, *• 'despite the low iHc CWkOe)
.
The
microstructures of these alleys consist of a cellular mor-
phology with 1:5 type phase surrounding the 2:17
"cell
s"'^-'The
This manuscript was printed from originals provided by the authors.
1
The magnetisation mechanisms ii these materials has been
shown to be the domain wall oinning mechanism (4). Al-
though the role of Zr in these alloys is not yet
well-
understood, it is believed that the step aging treatments
lead to chemical partitioning between the 1:5 and 2:17
phases,
and thus harden the material magnetically, with-
out anv morphological changes occuring during step aging
In this paper, the effects of the chemical composi-
tions and heat treatments on the magnetic properties and
the microsrructure are studied systematically to estab-
lish the relationships between the microstructure and
the properties on one hand and the microstructure and
the chemistry, processing history, etc. on the othei.. It
has been possible to prepare material with (BH) of
33MGOe and iHc of 16k0e for certain chemical
max
compositions and heat treatments discussed below. The
microstructural features responsible for high coercivi-
ty in this class of materials are also identified in
the paper.
Mainly alloys of Co with
24-27wt%Sm,
4-Swt%,
1J-20
vt%Fe and
l-3wt%Zr
were studied. All alloys were
pre-
pared by melting together the component metals of sam-
arium, cc'alt, copper and iron, all with purities ex-
ceeding 99.5wt%, and ferrozirconium, in an induction
furnace under argon atmosphere. The cast ingots were
cru;hed and pulverized to 3-5'™ particles with a jet
mill.
The powder thus obtained was pressed ac a
pressure of
1.5ton/cm~
in a magnetic ^ield of lOkOe.
The green bodies were sintered for 1 hour at varies
temperatures between 1150 and 125Q°C. These specimens
were solution treated for 1 hour at a temperature be-
tween 1100 and 1220°C, then quenched in argon atmosphere.
Following the solution treatment, they uvre aged by
using isothermal aging and step aging. In this study,
step aging process is as follows \ following the
iso-
thermal aging, specimens were continuously cooled to
400°C at cooling rate of 1/2°C. /min and subsequently
aged at 400°C for 10 hours. The- magnetic properties
of these specimens were measured with an automatic
recording flux meter- Part of them were magnetized by
pulse maijnetizer. Electron transparent specimens were
prepared from thin platelets containing the C
-
axis
in the plane of the disc, via ion milling. These
foils were examined in a Philips EM 301 microscope
operating at lOOkV.
2
RESULTS AND INTERPRETATION
The effects of varying amounts of Sra, Fe, Cu and
Zr on iHc are shown in
Figs.
1,2 and 3 for both iso-
thermally aged and step aged alloys. iHc after
iso-
thermal aging decreases with incrnasing Sm content
from 20k0e at 23 wt/o Sm to 2kCe at 25.5 wt/o Sm,
However, step aging leads to an increase of iHc for
alloys with higher Sm content (fig. 1). Increasing
amounts of Sm also improves the squareness of the
demagne
t
isa
t
ion curve.
While isothermal aging at 850°C for 1 hour does
not have any effect on the iHc for alloys containing
varying amounts of Zr, step aging improves the iHc markedly
for these alloys; specifically for the alloys containing
2-3 wt/o Zr. For these alloys, the squareness of the
demagnetisation curve also improves (fig. 2).
iHc does not change with changing Fe concentration
for isothermally aged alloys. However, step aging
'rnproves the iHc and as can be seen in Fig. 3, depend-
ing on "-he Cu content in the alloy, iHc attains a
maximum after step aging.
The effect of Cu on iHc for given Sm or Fe content
in FigF. 1 or 3 is to have large iHc for alloys with
higher Cu concentration. In all these cases, the
amount of improvement in iHc after step aging depends on
the specific composition of the .-alloy.
Fig.
4 shows the effect of isothermal aging
t
ime
on iHc for 1.5
<-
-
t/o
Zr and 3 wt/o Zr alloys. Also,
iHc for alloys, which are aged isothermally for longer
times (> 8 hours for 1.5 wt/o Zr alloys and >3 hours
for 3 wt/o Zr alloys) increases rapidly after step
aging
(fij?.
4)up to a maximum value of '^26kOe.
The microst ruetures of all the.s™
.ill
oys consist
of a cellular morphology as bus been reported by
Livingstone & .
M
artin --
1
' and also by Misiira and Thomas
(4).
Alloys containing 1.5 w/o for 1 hour show a
microstruct ure with 2:17 phase
o
(^-300 A in diameter)
surrounded by 1:5 phase 0- 50 A thick) in cellular
configuration. The two phases are coherent and a
typical image is shown in fig ire 5
-
This micro-
structure coarsens very slowly with aging for aging
times of up to 10 hours or more. For aging times gf
-J
30 hours, the 1:5 boundary phase is coarse
(^500A
'•'ir'o) and semicoherent and the 2:17 phase is larger
in size ('v, 3000 A) as in fig. 6. Thus the increase in
iHc for isoLliermally aged alloys in fig. 4 is due to
the fact that the 1:5 phase bceumes a more effective
harr ier to the domain wail motion for thicker (but
continuous)'
cell walls.
Step aging of the 1.5 wt/o Zr alloys must affect
the chemical composition difference
litLw.
;»n
the 1:5
and 2:17 phases, since the morphologips of the
3
