High Strength Soft Magnetic Composites
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High Strength Soft Magnetic Composites Ronghai Yu1, Michelle M. Corte-Real1, Adrian Gorea1, Libo Ren3, Azar Parvizi-Majidi3, and John Q. Xiao1 1 Department of Physics and Astronomy, University of Delaware, Newark DE 19716, USA 3 Department of Mechanical Engineering, University of Delaware, Newark DE 19716, USA ABSTRACT There has been an increasing demand for high temperature soft magnetic materials with mechanical properties better than those of existing commercial materials such as FeCo alloys. We have designed new magnetic composites by reinforcing FeCo alloys with high strength tungsten fibers. The composite materials were fabricated by electrodeposition. In general, the asdeposited composites have a relatively high coercivity Hc and low magnetic permeability µ, because of induced strain during fabrication. After appropriate thermal annealing, the composites have good soft magnetic properties, comparable to commercial bulk alloys. However, the saturation induction is reduced due to the non-magnetic inclusions. The composites also show significant enhancements in yield strength and tensile strength that increases linearly with fiber volume fraction as seen in other common composite materials. In addition, near zero creep is observed at 600 oC under a stress of 600 Mpa. The mechanical properties can be further improved by co-depositing soft magnetic material and Al2O3 onto the fibers. An approximately linear relationship was observed between the coercivity and volume fraction of Al2O3 particles. The square-root relationship was observed between the hardness and the Al2O3 concentration. INTRODUCTION There is an increasing demand for high-performance soft magnetic materials in the design of high-speed motors and generators for high temperature operation [1-5]. The mechanical strength of these materials at high temperatures is a crucial parameter in such applications. Fe-Co alloys exhibit high saturation magnetization Ms and high Curie temperatures Tc (Tc ≈900 oC) that make them potential candidates. The ordered Fe-Co alloys are excellent soft magnetic materials with negligible magneto-crystalline anisotropy K1 [6]. However, equiatomic Fe-Co alloys are extremely brittle and other elements such as V are usually added to obtain workable materials. These additions however, significantly deteriorate the soft magnetic properties, due to the precipitation of a second phase [7-9]. Therefore, mechanical strength is achieved at the expense of magnetic performance. We have developed new magnetic composites by reinforcing Fe-Co materials with tungsten fibers and Al2O3 particles. In the case of FeCo-fiber composites, the contributions to magnetic and mechanical properties come from two nearly independent entities, i.e. the FeCo matrix and fiber network, respectively. One immediate advantage of such materials is that one can optimize magnetic and mechanical properties in each entity independently, and thus no comprise between the magnetic and mechanical properties is necessitated. For further improvement in mechanical properties,
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