MECHANICAL MILLING OF CO-RICH MELT-SPUN SM-CO ALLOYS

Authors

Farhad Reza Golkar-Fard, University of Nebraska - LincolnFollow

Date of this Version

Spring 5-2010

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master Science Major: Engineering Mechanics Under the Supervision of Professor Jeffrey E. Shield Lincoln, Nebraska May, 2010 Copyright [2010] [Farhad Reza Golkar-Fard]

Abstract

MECHANICAL MILLING OF CO-RICH MELT-SPUN SM-CO ALLOYS Farhad Reza Golkar-Fard, M.S UNIVERSITY OF NEBRASKA, 2010 Advisor: Jeffrey E. Shield Rare-earth, high-energy permanent magnets are currently the best performing permanent magnets used today. The discovery of single domain magnetism in 1950’s ultimately led to the development of nanocomposite magnets which had superior magnetic properties. Previous work has shown that mechanical milling (MM) effectively generates nanoscale structures in Sm-Co-based alloys. MM of more Co-rich, melt-spun Sm-Co alloys (up to the eutectic composition) and the role of initial structure on the milling behavior were investigated.

Sm-Co alloys with compositions of Sm_10.5Co_89.5 and Sm₈Co₉₂ were produced by arc melting, melt-spinning and mechanical milling. X-ray diffraction results showed that the as-melt spun materials formed in the disordered TbCu₇-type variant for both compositions, and that FCC-Co is also present in Sm₈Co₉₂. At long milling times, FCC-Co developed in the Sm_10.5Co_89.5 alloy. MM resulted in a decrease of grain size, indicated by the broadening of the x-ray diffraction peaks. However, the samples remained crystalline, as peaks characteristic of the TbCu₇-type structure were always present. The grain sizes obtained in both compositions ranged between 10-20 nm. Scanning electron microscopy images showed that the powder particles contain agglomerates and that a broad micron-scale particle size exists in the samples. The coercivity of both samples initially increased with increasing milling time, reaching plateau values of ~4.6 kOe and 1.6 kOe for Sm_10.5Co_89.5 and Sm₈Co₉₂, respectively. The lower coercivity values of the Sm₈Co₉₂ alloy were due to its more Co-rich composition.