Microstructure and Phase Analysis in Mn-Al and Zr-Co Permanent Magnets

Authors

Michael J. Lucis, University of Nebraska-LincolnFollow

Date of this Version

Spring 4-27-2015

Comments

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: April, 2015

Copyright (c) 2015 Michael J. Lucis

Abstract

In America’s search for energy independence, the development of rare-earth free permanent magnets is one hurdle that still stands in the way. Permanent magnet motors provide a higher efficiency than induction motors in applications such as hybrid vehicles and wind turbines. This thesis investigates the ability of two materials, Mn-Al and Zr-Co, to fill this need for a permanent magnet material whose components are readily available within the U.S. and whose supply chain is more stable than that of the rare-earth materials. This thesis focuses on the creation and optimization of these two materials to later be used as the hard phase in nanocomposites with high energy products (greater than 10 MGOe).

Mn-Al is capable of forming the pure L1₀ structure at a composition of Mn₅₄Al₄₃C₃. When Mn is replaced by Fe or Cu using the formula Mn₄₈Al₄₃C₃T₆ the anisotropy constant is lowered from 1.3∙10⁷ ergs/cm³ to 1.0∙10⁷ ergs/cm³ and 0.8∙10⁷ ergs/cm³ respectively. Previous studies have reported a loss in magnetization in Mn-Al alloys during mechanical milling. The reason for this loss in magnetization was investigated and found to be due to the formation of the equilibrium β-Mn phase of the composition Mn₃Al₂ and not due to oxidation or site disorder. It was also shown that fully dense Mn-Al permanent magnets can be created at hot pressing temperatures at or above 700^oC and that the ε-phase to τ-phase transition and consolidation can be combined into a single processing step. The addition of small amounts of Cu to the alloy, 3% atomic, can increase the compaction density allowing high densities to be achieved at lower pressing temperatures.

While the structure is still under debate, alloys at the composition Zr₂Co₁₁ in the Zr-Co system have been shown to have hard magnetic properties. This thesis shows that multiple structures exist at this Zr₂Co₁₁ composition and that altering the cooling rate during solidification of the alloy affects the ratio of the phase composition and therefore affects the magnetic properties. Phase diagrams for the Zr-Co system show that the Zr₂Co₁₁ phase is stable to a temperature of 1272^oC, at which point the Zr₆Co₂₃ phase is the most favorable. However, this thesis shows that the Zr₆Co₂₃ phase forms at room temperature during high energy mechanical milling and at annealing temperatures as low as 600^oC. Since high energy mechanical milling was not a potential method to creating single crystalline particles, hydrogen embrittlement was investigated. Hydrochloric acid was used to induce hydrogen embrittlement in the Zr₂Co₁₁ alloy, modifying the fracture characteristics of the alloy causing it to occur primarily along grain boundaries resulting in single crystalline particles with remanent magnetization enhancement.

Adviser: Jeffrey E. Shield