Structural and magnetic characterization of spark plasma sintered Fe-50Co alloys
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Structural and magnetic characterization of spark plasma sintered Fe-50Co alloys 1*
Mahesh Kumar Mani , Giuseppe Viola2,3, Mike J Reece2,3, Jeremy P Hall1, Sam L Evans4 1-Wolfson Centre for Magnetics, Cardiff School of Engineering, Cardiff University, UK 2- School of Engineering and Materials Science, Queen Mary University of London, London, UK 3-Nanoforce Technology Ltd., London, U.K 4-Institute of Medical Engineering and Physics, Cardiff University, UK
ABSTRACT Fe-50 wt% Co alloy powders with average particle size of 10 μm were compacted by spark plasma sintering (SPS) at 700, 800, 900 and 950oC by applying 40, 80, 100 MPa uniaxial pressures for 2, 5, 10 minutes. The densities of the samples were found to increase with temperature from 700 to 900oC for constant sintering pressure and time and to decrease for the material sintered at 950oC. The effects of sintering time on density were more significant in samples sintered at 700oC and 800oC than those densified at 900oC. The consequences of small increases in mechanical pressure during sintering on density values were significant for samples sintered at 700oC. The coercivity (Hc) of the compacts decreased significantly with increasing sintering temperature, and with increasing dwell time at sintering temperatures lower than 700oC. The sample sintered at 950oC, which contains the largest grains among the prepared samples and porous microstructure, exhibited the minimum coercivity. Unlike Hc, the remanence (Br) and saturation induction (Bsat) values were more strongly affected by the specimen density than by grain size. Br and Bsat values were found to vary linearly with sintering temperature and pressure owing to increasing density. An increase in soaking time at 800 and 900 oC, although enabling higher density, exhibited contradicting effects on Bsat values. The SPS parameters to obtain maximum density and optimum magnetic properties for Fe-50% Co alloy were found to be 900oC, 80 MPa and 2-5 minutes. INTRODUCTION There is continuing interest to replace hydraulic and pneumatic systems in aircraft with reliable, easy to maintain and efficient electrical subsystems [1,2]. The soft magnetic material selected for such electric subsystems should retain good magnetic characteristics at high operating temperatures (500-700oC) and should exhibit high saturation induction (> 2 T) to maximise power density in weight sensitive applications [3]. Fe-(30-50) Co alloys, exhibiting high saturation magnetisation (2.3–2.45T) and Curie temperature (920–985oC) are suitable commercial soft magnetic alloys for such applications. In addition, Fe-Co alloy with nearequiatomic composition displays the highest permeability and zero magnetocrystalline anisotropy in the FeCo system. However, the ordered intermetallic Fe-50% Co alloys are brittle and are difficult to deform without any ternary addition [4-6]. Powder metallurgy (PM) is the only available alternate fabrication route to shape the brittle alloy into useful parts. PM technologies like metal injection moulding (MIM), cold compaction
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