It’s well known that MnAl(C) material consists of a metastable phase (τ) with promising ferromagnetic properties, produced either by controlled cooling from the high-temperature hexagonal ε-phase or rapid cooling that freezes the ε-phase followed by low-temperature annealing. Due to the high cooling rates involved, additive manufacturing (AM) especially selective laser melting (SLM), has been identified as a possible method to retain the high-temperature ε-phase, hence containing a potential capacity to produce permanent magnets upon low-temperature annealing. Moreover, the competency of additive manufacturing to address manufacturing design complexity, material scarcity and tailored properties, yields a great opportunity to produce permanent magnets with suitable magnetic properties for complex applications. This work provides a systematic study on three main aspects; development of printing parameters for improved relative density of as-printed MnAl(C) samples; investigation of the influence of scanning strategies on the crystallographic texture of as-printed and annealed samples; investigation of the influence of annealing time and temperature on τ-phase purity and magnetic properties. It was found that laser remelting (multiple laser exposure) combined with specific scanning strategies is a promising path to enhance the relative density of as-printed samples. Some specific scanning strategies were found to be capable of retaining relatively strong crystallographic textured ε-phase in as-printed samples. Following the annealing process for ε→τ transformation, only a partial transformation of crystallographic texture was observed. Characterization of annealed samples through XRD (x-ray diffraction) and phase fractions calculations through Rietveld refinement reveals that relatively short annealing times and low temperatures result in incomplete ε→τ transformation. In addition, longer annealing times and higher temperatures surpass the complete ε→τ transformation and lead to the formation of equilibrium phases subsequently reducing the magnetic performance. Furthermore, the experimental findings demonstrated a pronounced influence of higher carbon content in the powder, resulting in improved magnetic properties.

Detailed microstructural characterisation has been conducted in both ‘as-transformed’ and ‘hot deformed’ samples of 𝜏-MnAl-C using transmission electron microscopy. After hot deformation, true twins, dislocations, intrinsic stacking faults and precipitates of Mn3AlC are the main defects in the recrystallised grains. A significant fraction of non-recrystallised grains existed, which had microstructures based on combinations of high densities of true twins, dislocations, and deformation bands. The formation of the Mn3AlC precipitates was confirmed and related to the reduction of saturation magnetization and the increase in the Curie temperature of 𝜏-MnAl-C after hot deformation. Antiphase boundaries, which are believed to act as nucleation sites for reverse domains, were not observed in the hot deformed sample. Increasing structural disorder is expected for the tetragonal L10 𝜏-MnAl-C (c:a = 0.91) going from coherent true twin boundary to incoherent true twin boundary to the order twin boundary. This was demonstrated from the interface structure in the HAADF-HRSTEM images. The disorder at different types of twin boundaries is also associated with the degree of segregation of the constituent elements. By using STEM-EELS, higher Mn enrichment and Al deficiency was observed at the order twin boundary with a thickness about 6 nm, slightly Mn segregation was observed at incoherent true twin boundary with reduced thickness about 1.5-2 nm and no segregation was found at the coherent true twin boundary. In addition, the distribution of carbon is inhomogeneous across the twin boundary and carbon cluster was found at the twin boundary. Micromagnetic simulations and machine learning were conducted in an international collaboration with Danube University Krems, Austria, which enabled the quantification of the effect of twins on the magnetic properties of 𝜏-MnAl-C.

MnAl-C alloy is a very promising candidate as a rare-earth free magnet. Additions may affect the microstructure, stability of the τ-MnAl-C and magnetic properties. MnAl-C alloys with various additions (Ni, Ti, Zr or Zn) and powder extruded MnAl-C-Ni alloys have therefore been investigated. With 0.6 or 1.5 at.% Ni addition, the τ-phase has greater stability than that in the ternary alloy: after heat treating the homogenised alloys for 7 days at 700°C, the estimated remaining volume fractions of the τ-phase are 0.35, 0.85 and 0.91 for 0, 0.6 and 1.5 at.% Ni addition, respectively. Rare-earth-free MnAl-C-Ni permanent magnets have been produced for the first time by extruding powders milled from bulk. The best performance obtained for a whole, transverse section of the extruded material was (BH)max = 46 kJm-3, and was (BH)max = 49 kJm-3 for a sample taken from the edge of this section, which was comparable to the long-established benchmark. The microstructure of the materials produced here consisted of fine, recrystallised grains, which exhibited an <001> fibre texture with intermediate texture quality and of larger, non-recrystallised regions, which contained hierarchical twinning and a high density of defects. The volume fraction and size of the non-recrystallised regions were greatly reduced by decreasing the size of the initial powder particles. This led to a large increase in the squareness factor of the demagnetisation curve and consequently to the high (BH)max values observed.