Mechanical & Materials Engineering, Department of

 

Date of this Version

Fall 12-2013

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 of Science, Major: Mechanical Engineering & Applied Mechanics, Under the Supervision of Professor Jeffrey E. Shield. Lincoln, Nebraska: December, 2013.

Copyright 2013 Jacob A. Lewis

Abstract

Magnetic materials are a critical component of the modern technological society. The market of high-energy product hard magnets is dominated by the rare-earth element containing neodymium-iron-boron (NdFeB) magnets. As a result of worldwide demand coupled with trade restrictions on rare-earth minerals, new magnet compositions are explored to reduce pressure on the already strained market. The hafnium-cobalt alloy system has shown promise as a candidate for filling the gap in saturation magnetization between rare-earth containing alloys and those made with abundant elements. This study explores the magnetic effects of silicon, titanium, iron, manganese and boron substitutions in the hafnium-cobalt-7 (1:7 atomic ratio) alloy. The compositions investigated followed the HfCo7-x(Si, Ti, Fe, Mn, B)x (x = 0, 0.5, 1 for Si, Fe, Mn, B | x=0, 0.25, 0.5 for Ti) atomic formula. All specimens were produced by arc melting followed by melt spinning (crucible orifice diameter of 0.5mm and wheel speed of 10 m/s). Structural information was obtained with an x-ray diffractometer (XRD) and atom probe tomography (APT). Magnetic measurements were performed in a super-conducting quantum interference device (SQUID). The silicon and titanium substitutions caused the disappearance of the orthorhombic phase and a large reduction in magnetization. Substitution of cobalt for iron atoms did not affect the coercivity but increases magnetic susceptibility. In the case of the manganese, the original phase was not present. There is an increase of about 7.5% magnetization at 50 kOe for the HfCo6Mn composition (101 emu/g at 50 kOe) and higher magnetic susceptibility. However, these alloys have very little coercivity. The boron samples were found to retain the structure of the base alloy with additional regions of a boron-rich phase. This second phase is responsible for separation of magnetic domains, increasing coercivity from 0.42 kOe to 4.49 kOe for HfCo6B0.75 (an increase of nearly 10x). However, the non-magnetic phase reduces the magnetization at 50 kOe of the bulk material by approximately 30%.

Adviser: Jeffrey E. Shield