Effect of combined metal-carbon additions on the microstructure and structure of Sm 2 Fe 17
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J.E. Shield Department of Mechanical Engineering, University of Nebraska, Lincoln, Nebraska 68588 (Received 15 July 2002; accepted 18 October 2002)
The effect of combined alloying additions on the structure and scale of rapidly solidified Sm–Fe alloys was investigated. Transition metal additions tend to promote the formation of the disordered TbCu7-type structure in Sm2Fe17 alloys, as determined by monitoring the long-range order parameter. Essentially no order was observed for M ⳱ Ti, Zr, V, or Nb. Thus, the structure was close to the prototypical TbCu7-type structure. With M ⳱ Si, a large amount of order was observed (S ⳱ 0.62), resulting in a structure closer to the well-ordered Th2Zn17-type. The microstructural scale was also affected by alloying. In this case, refinement depended on the substituent and also on carbon for microstructural refinement. The scale of the as-solidified grain structures ranged from 100 nm for SiC-modified alloys to 13 nm for NbC-modified alloys. The degree of refinement was directly related to the atomic size of the M addition. The refinement was the result of solute partitioning to grain boundaries, resulting in a solute drag effect that lowered the growth rates. I. INTRODUCTION
The microstructure and atomic structure combine to critically influence the magnetic behavior of materials. The atomic structure—the size, shape, and atomic configuration of the unit cell—is associated with intrinsic properties such as the saturation magnetization and magnetocrystalline anisotropy. The microstructure—the grain size, morphology, and uniformity—often affects the demagnetization process. It is important, then, to carefully control the microstructural evolution and structural state during processing. Routinely, this is done through alloy design. In the interstitial Sm–Fe-based permanent magnets, the structural state is critically dependent on the processing conditions and alloy composition. While the equilibrium compound is the Th2Zn17-type Sm2Fe17 structure, the TbCu7-type SmFe7 has also been observed after rapid solidification or upon a ternary alloying.1–9 The SmFe7 and Sm2Fe17 structures are related by an order/disorder transformation involving the transition metal dumbbell sites; in the Sm2Fe17 structure, the Fe–Fe dumbbells are ordered, while in the SmFe7 structure they are placed randomly on the lattice. The amount of long-range order can be controlled through processing, allowing the structural state to be carefully controlled.10–12 Specific site substitution preference also can
alter the intrinsic magnetic properties,13 and this is also affected by specific alloying additions and synergistic effects. The microstructural evolution in Sm–Fe-based alloys is also affected by the processing and alloying characteristics. Combined TiC additions were found to effectively refine the grain size14,15 and improved the glass formability.16 Other additions, including Ga17 and Ga with Nb, Cu or Zr, all in combination with C,9 have also effectively refined the grain size. Because of the overarching i
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