TOPIC:
Consolidation of ferromagnetic Fe-Co-V alloy by Spark Plasma Sintering (SPS).
AIM:
The main objective of this project is to optimize sintering conditions of Fe-Co-V magnetic
alloy using Spark Plasma Sintering.
INTRODUCTION
Iron, Cobalt and Nickel are the three transition elements that show ferromagnetism at ambient
temperature. The bearers of this magnetic property on these elements are the 3d electrons,
which tend to form band structures together with the 4s electrons (Chritsazan, et al. 2010).
Hence, most alloys designed for magnetic properties in engineering for use contain either of
these elements. Alloys containing these metals are applicable in a wide range of applications
whereby magnetic materials are required [1],[2].
Fe-Co based alloys have attracted great attention due to their excellent soft magnetic properties,
high saturation induction, low magnetic core loss and high Curie temperature. Such magnetic
materials which not only possess the abovementioned magnetic but also, good mechanical
properties such as high strength and ductility are considerably required for high performance
applications in many fields which may include rotor parts of synchronous hysteresis motors,
electrical generators in aviation and other special power applications [3],[4]. The discovery of
FeCo-2V system by White and Wahl in 1932 became a critical step towards achieving
industrial relevance of the alloy. Hence, great amount of work has been put forward towards
new compositions emerged due to the fact that solute additions results to reduced saturation of
the alloy and generally an increase of the alloy coercivity which further translates to degrading
of magnetic performances [5].
Regardless of the exceptional magnetic properties including high saturation magnetization, low
magnetic anisotropies and low magnetic losses offered by these alloys due to the nearequiatomic among Fe-Co alloy however, the alloy remained without industrial application,
reason being its extreme brittleness. The revived interest in Fe-Co alloys due to increased area
specific demands such as demands of modern electrical power generation and power
distribution equipment, to name a few, have ignited demands of these alloys especially in
electrical applications where hydraulic, pneumatic and mechanical components are to be
replaced by their electric and magnetic counterparts [6],[7],[8].
Researchers over the years have found that the stoichiometric Fe50Co50 limitation of this alloy
which yields lower electrical resistivity and poor ductility can basically be alleviated by the
addition of Vanadium, and many commercial alloys exhibiting magnetic properties have
developed based on Fe-Co-V alloys. Fe 49 wt% Co-2% wt V is the commercially popular alloy
and is known to exhibit a relatively good blend of mechanical properties and soft-magnetics
performance [6],[9].
Most studies have covered different manufacturing techniques to produce Fe-Co-V alloy such
as Laser Engineered Net Shaping (Lens) metal additive manufacturing technique. Despite the
fact that these techniques produce nano-porous structure because of their longer sintering time,
temperature tend to weaken the structures mechanical and electro-chemical properties, which
results in material failure upon working conditions and affect long term performances
(Schneider et al. 1996) [10]. However, there is very less literature available on Spark Plasma
Sintering of Fe-Co-V ferromagnetic alloys. Hence, this study focuses more on optimizing
sintering conditions of Fe-Co-V employing Spark Plasma Sintering method. Spark plasma
sintering technique as a novel powder sintering process with distinct characteristics compared
conventional sintering techniques enables alloys with good properties to be fabricated with
shorter holding time and high pressures at a relatively rapid heating and cooling rates (> 1000C/
min) and also relatively lower sintering temperatures (Izui & Kikuchi, 2012) [11].
METHODOLOGY
2.1. Powder Characterization
The starting powder utilized for this study is pre-alloyed Fe-49 wt% Co-49 wt% V-2 wt%
produced by Tosoh Co. (Tokyo, Japan). The powder had an average particle size of 100 μm.
Powder analysis was performed on the powder using scanning electron microscopy (Quanta
450 FEG).
2.2. SPS Process
The pre-alloyed Fe-Co-V powder were sintered using a Dr. Sinter Lab Spark Plasma Sintering
System. Graphite sheets were used to shield the powders from the die, upper and lower punches
to ensure easy removal. All the alloys were sintered in vacuum and the sintering regimes
covered temperatures from 700 ⁰C to 950 ⁰C, pressures from 40 MPa to 100 MPa, and holding
times from 0 min to 5 min. The temperature was increased at a rate of 100 ⁰C/min, and the
specimen thickness ranged from 6 mm to 7 mm depending on the relative density reached.
2.3. Microstructure Characterization
For microstructural evaluation, alloy samples were metallographically prepared by grinding
with silicon carbide (SiC) papers followed by polishing with colloidal silica suspension to
remove carbon on the surface of the samples and were then sonicated in ethanol for 15min to
remove residual carbon on the surface and in the pores of the samples. The samples were then
dried and the pertinent relative density measurements needed to use the Archimedes method
were taken. The polished specimens were then etched using 100 ml distilled water and 25 ml
of 69% Nitric acid (for magnetic based alloys) reagent. The microstructures of the alloys were
then evaluated using an Optical Microscopy (ZEISS, AxioCam ICc 5) and JEOL Scanning
Electron Microscopy (FESEM, JSM-7600F).
2.4. Evaluation of Mechanical Properties
2.4.1. Nanoindentation
Nanoindentation tests were performed with Anton Paar ultra-Nanoindenter (UNHT) fitted with
a diamond Berkovich indenter. The indentation experiments were run under constant load of
10mN to reach possible maximum depth. The UNHT nanoindenter was used to perform an
array of 5×5 indents with the separation distance of 25μm between indents covering the area
containing the all the phases and the grain boundary of the material. The loading cycle consisted
of loading up to the peak force and a pause time followed by unloading time, the parameters
are dependent upon applied force and rate.
2.4.2 Micro Hardness Testing
Micro hardness measurements of the materials were performed using a digital micro hardness
tester (INNOVATEST Falcon 500). A profile of a minimum of 10 indents at different locations
was made on the bulk sample. The applied load was kept at 10g for all the tests for a peak-load
time of 10s. The indent’s diagonals were measured optically to give Vickers hardness (HV)
values of the materials with an accuracy of ±0.1μm. The obtained values were then averaged
to determine the HV of each sample.
Literature Review:
Fe-Co alloys
Phase transformation in Fe-Co alloys
Upon the addition of Iron (Fe) atoms in the cobalt (Co) system, an orderly arrangement of
atoms with CsCl configuration as a result of the thermodynamically favourable B2 structure
(α2) is observed within the compositional range of 30 to 75 atomic % of Fe as illustrated by
figure….. The above-mentioned compositional range as illustrated in figure…. Shows the the
degree of order within the composition of the alloy decreases with an increase in temperature
below the order to disorder temperature.
The near-equiatomic Fe-Co alloys at relatively low temperatures are B.C.C crystal structure
whereas at temperatures above approximately 983 0C resembles F.C.C crystal structure.
However, the b.c.c crystal structure phase orders to a B2 at temperatures approximately below
730 0C. According to this work and in relation to other literatures has been reviewed, high
temperature fcc phase is referred to gamma (γ); disordered bcc phase is referred to α1 and
whereas the ordered phase is referred to α’1. Fe-Co-V and Fe-Co alloys in a quenched state
from a specific temperature that within α1 and γ region tend to go martensitic transformation
due to saturated γ phase that precipitated out from the specified region. The final phase that is
formed post to subsequent treating has at times referred to α2. The transformation temperature
of α to γ overlaps with the Curie temperature of Fe-Co alloy.
REFERENCES
1. Shokrallahi, H., (2009) ‘The magnetic and structural properties of the most important
alloys of iron produced by mechanical alloying’, Mater. Des. Vol. 30, pp. 3374-3387.
2. Chritsazan, B., Shokrallahi, H., Behvandi, A., & Ghaffari, M., (2010) ‘Magnetic,
structural and microstructural properties of mechanically alloyed nana-structured
Fe48Co48V4 powder containing inter-metallic Co3V’, Journal of Magnetism and
Magnetic materials. Vol. 323: pp. 1128-1133.
3. Mostaan. H., Shamanian. M., Hasani. S., Monirvaghefi. S.M., Behjati. P.,
Moghaddam. M.F., Amiri. M., & Szpunar. J.A., (2015) ‘Analysis and characterization
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