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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.
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.
Available for download on Wednesday, September 18, 2024