Optimizing Mn-Al permanent magnet performance through control of the phase transformation, ternary element addition, and advanced processing

Author

Thomas R. Keller, Dartmouth CollegeFollow

Author ORCID Identifier

https://orcid.org/0000-0003-0231-8971

Date of Award

9-2023

Document Type

Thesis (Ph.D.)

Department or Program

Engineering Sciences

First Advisor

Ian Baker

Second Advisor

Geoffroy Hautier

Third Advisor

Charles Sullivan

Abstract

The growing need for electrical power in machines and vehicles brings with it a growing need for critical materials. The current high-performance permanent magnets (PMs) based on rare-earth (RE) elements Nd and Sm cannot escape the problem of raw material cost, geographic scarcity in the earth’s crust, and lack of circular global supply chains. This poses a problem of industrial ecology: can PMs be made from inexpensive, more abundant materials while still meeting sufficient performance criteria? PMs based on the magnetic τ phase of Mn-Al offer a possible alternative to REPMs. However, years of innovation have not yet achieved real-world performance on par with the theoretical maximums needed to compete with REPMs. This thesis sets forth a pathway for understanding how controlling the phase transformation, alloying with ternary elements, and processing using additive manufacturing and plastic straining can improve the intrinsic and microstructural characteristics of a Mn-Al PM, bringing it closer to commercialization.

First, the τ phase transformation thermodynamics and kinetics were studied to better understand the role of the ordered ε’ phase at low temperatures. Ab-initio modeling and advanced characterization techniques demonstrated that ε’ is ferromagnetic and its ordering provides a kinetic advantage for the displacive mode of the transformation to τ at low-temperatures.

Second, the addition of ternary alloyants were used to suppress anti-phase boundary (APB) defects in the τ phase. The addition of 1 at.% Ti to Mn₅₄Al₄₆ was shown through ab-initio modeling and experiment to suppress APB formation and prevent antiferromagnetic behavior in the τ phase, improving (BH)_max by 33% over the base alloy.

Third, the Mn₅₄Al₄₆ τ phase was processed using additive manufacturing and equal-channel angular extrusion (ECAE) to improve its performance over the τ phase produced by annealing alone. Gas atomized powder was successfully consolidated into a PM using laser powder-bed fusion (LPBF) and bulk material was processed via ECAE. These two methods resulting in PMs with a refined grain size, improved dislocation density, and magnetic anisotropy along the print and extrusion directions, respectively. LPBF improved (BH)_max by 70% and ECAE improved (BH)_max by 220% over the τ phase produced by annealing.

Recommended Citation

Keller, Thomas R., "Optimizing Mn-Al permanent magnet performance through control of the phase transformation, ternary element addition, and advanced processing" (2023). Dartmouth College Ph.D Dissertations. 163.
https://digitalcommons.dartmouth.edu/dissertations/163