Mechanical & Materials Engineering, Department of


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A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Engineering (Materials Engineering), Under the Supervision of Professor Jeffrey E. Shield. Lincoln, Nebraska: May, 2014

Copyright (c) 2014 Xiujuan Jiang


There is an increasing interest in developing desirable microstructures in hard magnetic materials. Sm-Co-based magnets, bearing superior intrinsic magnetic properties, are good candidates for further development. Two Sm-Co-based alloys, (Sm12Co88)100-x-yCryCx (taking SmCo7 phase) and SmCo4-xFexB (a derivative of SmCo5 phase), were produced using melt-spinning technique. The magnetic properties are correlated to the structural and microstructural properties.

Within the SmCo7 stoichiometry, cumulative effects of Cr and C additions on the structural and magnetic properties have been investigated. Experimental results have shown that these additions along with nanostructuring stabilized the 1:7 phase, refined the grain size and triggered promising modifications in the magnetic properties. Annealing was also performed to further optimize the magnetic properties. For both the as-spun and as-annealed samples, structural, magnetic and microstructural results will be shown and correlated among each other to explain observed behavior. Specifically, the maximum coercivity obtained was 10.1 kOe at 3 at.% C and 4.5 at.% Cr conditions annealed at 600 °C.

Within the SmCo4B stoichiometry, efforts were made to explore the possibility of a potential exchange coupled nanocomposite based on Fe additions into 1:4:1 structure. As-spun Fe-containing samples were amorphous. For the crystallized samples, different Fe content brought in significant changes in the phase evolution and magnetic behavior. A secondary phase Sm2(Co,Fe)17By was also observed at higher Fe content. We find that Fe additions increase the coercivity for up to x = 1 (SmCo4-xFexB) and increase magnetization for up to x = 2, the latter of which is highly desired for permanent magnet applications. In order to further optimize the magnetic properties of the composition with both high coercivity and magnetization (SmCo2Fe2B), secondary annealing at a lower temperature was performed. Pronounced enhancement in both magnetization and coercivity was observed. Microstructural analysis and microchemistry information obtained from TEM/HRTEM and 3D atom probe revealed the possible reasons behind the improvement.

Adviser: Jeffrey E. Shield