Targeted hydrolysis and decrepitation of Mn₃AlC precipitates: a route to a novel precursor of rare earth free MnAl–C permanent magnets

Abstract

In this work, Mn-Al-C alloys consisting of ε-MnAl and carbides of the types Mn3AlC and Al4C3 were produced and exposed to water at 80°C for 35 days. This led to the hydrolysis and decrepitation of the carbides and ultimately, of the material. The hydrolysis behaviour was studied by combined mass spectrometry, thermogravimetry and differential scanning calorimetry. Microstructural studies with scanning electron microscopy and x-ray diffraction were carried out. X-ray computed tomography was used to study the evolution of cracks on a macroscopic scale. As the Mn3AlC precipitates were present as parallel platelets inside the ε-grains, the hydrolysis resulted in parallel voids separating regions of ε with the same orientation. This ε-skeleton could be easily milled into ε flakes, which could be mechanically aligned so that their <001> directions were parallel. The ε powder was transformed into the ferromagnetic τ phase via heat treatment and is therefore suitable as a novel precursor material for the production of anisotropic τ-MnAl-C-based permanent magnets. Employing such a textured, ε-phase, precursor powder in an extrusion process could result in increased texture quality of the τ-phase with correspondingly enhanced magnetic properties.