Developments in composition and processing for MnAl - system alloys for permanent magnetic material applications

At present there is a growing demand for high performance permanent magnetic materials for electricity generation and electric propulsion. The scale of demand is such that some of the highest performing current materials, which depend on rare earth elements, cannot be produced in sufficient quantity. Not all combinations of required properties can be met with currently available systems but one candidate to occupy part of the materials capability gap is τ−MnAl , though barriers remain to industrial application. Current issues with the commercialisation of this material are the low thermal stability and low BHMax of the compositions and conditions explored thus far. This thesis explores a number of approaches to improve the performance in these aspects, involving compositional adjustment and thermomechanical processing.

The findings of this thesis can be expressed as: the alloying addition of Bi does not form a stable single τ−MnAl phase with the Bi dissolved in but instead a ternary system of τ−MnAl , unreacted Bi and aMnBi intermetallic, increasing HC at the expense of intrinsic magnetic properties; Ga as an alloying agent can be used in lower concentrations than previously investigated to improve the thermal stability of the τ−MnAl with improved intrinsic magnetic properties, which can be varied as a result of heat treatments to meet engineering specifications and that it is possible to plastically deform the precursor ϵ-phase such that, following heat treatments to precipitate transformation to the desired τ−MnAl phase, directional dependency can be observed in extrinsic magnetic properties, meaning that it is possible to indirectly influence the microstructure of the precursor state such that it affects the desired ferromagnetic state to improve material properties for generator and motor applications. Overall the findings of this thesis result in a targeted alloy composition with a proof of concept processing pathway that shows potential for optimisation as a low cost per kilo permanent magnet with thermal stability suitable for modern low requirement electric motor and generator applications