Synthesis and Annealing of Nanocrystalline Fe-Si-Co
Alloy Powders
P. C. Shyni1*, A. Perumal1
1Department
*Corresponding
of Physics, Indian Institute of Technology, Guwahati, Guwahati, Assam- 783019, India
author’s e-mail:shyni.kannur@gmail.com, Tel.: +91-9678774771; Fax: +91-361-2690762
Abstract
We report the improvement in the soft magnetic
properties of the nanocrystalline Fe-Si-Co alloys powders
prepared by mechanical alloying (MA) technique followed
by systematic heat treatment at different temperatures.
MA resulted a non-equilibrium bcc Fe(Si,Co) solid solution
with nano-sized crystallites with high coercivity. Post
annealing process not only improves the saturation
magnetization, but reduces coercivity largely enabling
these materials for high temperature soft magnetic
applications.
Keywords: Mechanical alloying, Nanocrystalline material,
Dislocation Density, Non-equilibrium solid solution, Soft
magnetic material.
milling and bcc Fe peaks got broadened which confirms
the formation of non-equilibrium bcc Fe(Si,Co) solid
solution and development of highly refined as well as
internally strained grains. Average crystallite size (D)
and dislocation density () were determined using
modified Williamson-Hall plot (D~10 nm; ~1017 m-2).
Introduction
Among different Fe based systems, Fe-Si and Fe-Co
alloys have vital importance due to their major
applicability as a soft magnetic (SM) material in wide
variety of devices in the power and aerospace industries
[1]. A careful literature review reveals that appropriate
addition of Si and/or Co to Fe improves the SM
properties. However, the attempts to tailor the SM
properties by adding Si and Co together into Fe have not
been studied in detail. Hence, in this study, we report
the development of nanocrystalline (NC) Fe-Si-Co
alloys via mechanical alloying (MA) method and the
resulting properties after subsequent annealing process.
Experimental Methods
A mixture of high purity elemental Fe, Si and Co
powders corresponding to the compositions Fe75Si15Co10
and Fe70Si15Co15 were milled up to 40 hours in a
planetary ball mill at a speed of 10 rev/sec. As-milled
powders sealed in silica ampoules under high vacuum
were annealed in a furnace at 500 ºC for 2.5h and 900ºC
for 5h. Crystal structure and morphology were analyzed
by high power X-ray diffractometer (XRD), field
emission
scanning
electron
microscope
and
transmission electron microscope, respectively. Room
temperature (RT) hysteresis (M-H) loops were measured
using vibrating sample magnetometer (VSM).
Results and Discussions
Fig.1(a) depicts XRD patterns of Fe70Si15Co15 alloy
powders in different conditions. Si and Co peaks,
appeared in the un-milled powders, disappeared after
Fig. 1: (a) XRD patterns and (b) RT M-H loops for
Fe70Si15Co15 alloy powders
Post annealing leads to strain relaxation and grain
growth in the as-milled powder. Fig.1(b) shows RT MH loops of Fe70Si15Co15 alloy powders. Saturation
magnetization (MS) improved and coercivity (HC)
reduced remarkably as compared to as-milled powder.
Fe70Si15Co15 powder annealed at 900ºC for 5h results HC
of 39 Oe and MS of 208 emu/g. This shows the
improvement in the SM properties after annealing [2].
Similar results have also been obtained for Fe75Si15Co10
alloys. A systematic study carried out based on the
strain relaxation, ordering and grain growth mechanisms
in the NC Fe-Si-Co alloys would be presented in detail.
Acknowledgement
Infrastructure facilities provided by DST, New Delhi
[SR/S2/CMP-19/2006,
SR/FST/PII-020/2009]
are
gratefully acknowledged.
References
[1] H. Shokrollahi, K. Janghorban, “Soft magnetic
composite materials (SMCs)”, J. Mater. Proc. Tech.
189, 1-3, (2007), pp. 1-12.
[2] J. Crangle and G.M. Goodman, “The Magnetization
of Pure Iron and Nickel”, Proc. R. Soc. Lond. A, 321,
(1971), pp. 477-491.