Publication Date

8-2023

Date of Final Oral Examination (Defense)

1-20-2023

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Peter Mullner, Ph.D.

Advisor

Brian J. Jaques, Ph.D.

Advisor

Alexander H. King, D.Phil.

Abstract

Ubiquitous rare-earth, (i.e. lanthanide) permanent magnets (PMs) composed of Nd-Fe-B are the strongest PMs on the market. However, limited mineral sources, complex elemental separation, and supply chain volatility have shifted the lanthanides to critical material status. Inexpensive, readily available materials are sought to fill an energy product performance gap to reduce dependency on lanthanide PMs. Frequently, lanthanide PMs are used in applications where their energy products are excessively high. Mn-Al based permanent magnets offer a potential substitute for PM applications which do not require the high energy product of a lanthanide PM. MnAl alloys form a metastable ferromagnetic tetragonal L10-ordered τ-phase. Previous work has shown that adding Ga to the Mn-Al alloy stabilizes the τ-phase. This study investigates the impact of alloying Mn-Al-Ga with 3d transition metals Fe, Co, and Ni on the stability and formation mechanisms of the τ-phase and the resulting magnetic properties. The stabilizing effect of Ga on τ-phase was verified and the ternary alloy’s magnetization was measured up to M2T = 482 kA m−1. The phase transformation from γ2 to τ was observed microscopically. The Co-added alloy did not stabilize the τphase. Two transformation mechanisms were demonstrated for the Fe-added samples, namely a displacive and a diffusional transformation. The magnetization measured higher after the displacive transformation than after the diffusional transformation M2T = 272 kA m−1 and M2T = 200 kA m−1, respectively. The solubility limit of the Ni into τ-phase was exceeded at 3 at.-% and a primitive cubic κ-phase formed. The Ni addition stabilized the τ-phase. The highest magnetization measured for the Ni-added samples was M2T = 416 kA m−1. A new transformation pathway was demonstrated by first annealing a Ni-added alloy at 800 °C for 24 hours which forms nearly single κ-phase, followed by a second anneal at 500 °C for 24 hours at which the τ-phase formed with some remaining κ-phase. This is a new transformation mechanism since it involves a phase reaction from κ to τ. The energy product of Ni-added Mn-Al-Ga alloys exceeded that of the ternary Mn-Al-Ga alloy by a factor of 3. The κ-phase in the Ni-added alloy functions to hinder magnetic domain boundary motion, thus providing a method for magnetic hardening (i.e. increasing the coercivity), and, thus increasing the energy product.

DOI

https://doi.org/10.18122/td.2123.boisestate

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