Effect of ball milling time and age hardening cycle on permanent magnetic properties of
Sm(Co,Fe,Cu,Zr)8.5 alloy
Mithun Palit, D M Raj Kumar, S Pandian and S V Kamat
Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India
Email: mithunpalit@gmail.com, mithun@dmrl.drdo.in
Abstract: Effect of milling parameters on permanent magnetic properties of a commercial Sm2(Co,Cu,Fe,Zr)17
type alloy has been investigated. The study indicates milling parameter controls the grain size of the sintered
compacts and thereby affects the precipitation of SmCo 5 and thus the coercivity.
1. INTRODUCTION
Amongst the high performance permanent magnet
materials, Sm2Co17 type alloys are preferred for
stability of magnetic properties at high temperature
(upto 250oC) owing to its high Curie temperature and
large coercivity. The intrinsic magnetic properties of
this material are intimately related to an optimised
combination of microstructure and alloy composition.
However, very few reports are available in the
literature [1,2] on the effect of ball milling parameters
on permanent magnetic properties of such complex
alloys. Therefore, the present study aims to understand
of effect of milling time and ageing treatment cycle on
magnetic properties of one such complex alloy of
Sm2(Co,Cu,Fe,Zr)17 type.
in Table 1 and Fig. 1. The sample aged at 760oC/10 h
followed by cooling at 0.3oC/min exhibited maximum
energy product of 21.5 MGOe and Hk of -10 kOe.
This is due to the optimum volume fraction of fine
SmCo5 precipitates formed during aging. The sample
with finer grain size exhibited lower coercivity than
the sample having larger grain size. This is attributed
to less SmCo5 precipitation in material with finer
grain. The more grain boundary area in finer grain
material offering less nucleation sites for precipitation
since defects close to grain boundary sinks into it.
4. CONCLUSIONS
The study shows that an optimum grain size is
Table1: Magnetic properties of samples aged at
different conditions
Magnetic Properties
Milling
2. EXPERIMENTAL DETAILS
Aging
Time
H
(BH)max Hk
Two starting alloys viz. Adder (richer in Sm) and
Cond.
Br (kG) i c
(hrs)
(kOe)
(MGOe)
(kOe)
Base (leaner in Sm) were blended in the ratio 60:40
3
7.4
>20.0
13.6
-7.8
and ball milled for different duration i.e. 3, 4 and 5
8000C/8h
4
6.2
12.5
9.3
-4.2
hrs. The milled powders were compacted under
5
7.2
9.9
12.2
-4.4
magnetic field and the green compacts thus obtained
3
9.2
16
21.5
-10
were sintered at 1215oC for 30 min and solutionized at
7600C/10h
4
8.4
8.3
17
-5.2
1195oC for 2.5 h. The sintered compacts with 96-98%
density were aged at various temperatures in the range
5
7.0
4.5
11.8
-3.5
for 760oC-800oC for different durations. The magnetic
properties of the magnets were evaluated using B-H
loop tracer and correlated with microstructural
Fig. 1: B/M-H
observations.
plots of sample
prepared
from
3. RESULTS AND DISCUSSION
powder
milled
for
The starting alloy exhibits presence of 1:5 type and
3h,
aged
at
2:17 type phases. The 5h milling of it led to formation
o
760
C/10h
of finest particle of 4 µm size. The particle size
decreases with increase in milling time. The
microstructure of sintered block showed equiaxed
grains of matrix phase with Sm2O3 distributed along
the grain boundary. The XRD of the sintered block
indicates presence of 2:17H and 1:7H phases with a
strong morphological texture in 2:17H phase. The
sample prepared from 3 h and 5 h milled powders
resulted in grain size of 14 µm and 6 µm respectively.
The B-H loops of the sintered samples indicate
maximum saturation magnetization (9.5 kG) for the 3
h milled sample and highest coercivity of 600 Oe for
the 5 h milled samples. The large coercivity in 5 h
milled sample is owing to finer grain size. The
sintered blocks aged at 760oC and 800oC for different
durations and the magnetic properties are summarized
required to obtain a balance between coercivity
offered by grain boundary and sufficient precipitation
of SmCo5.
ACKNOWLEDGEMENT
The authors thank DRDO for financial support and
Director DMRL for permitting to publish this work.
REFERENCES
[1]. Kaplesh Kumar, J Appl.Phys.63 (1988) R13.
[2]. Xiong et.al. Acta Mat. 52 (2004) 737