L10 rare-earth-free permanent magnets: The effects of twinning versus dislocations in Mn-Al magnets

Microstructure and magnetic properties of Mn-Al-C permanent magnets produced by various techniques

“…But it is noticeable the increase of defect density, including polytwinned microstructure with high density of dislocations. The high density of defects, especially dislocations, can hinder the domain wall motion leading to a higher coercivity, in accordance with the magnetic results presented in Figure 8c, and as previous reported by [30,31,41].…”

“…The sample after VIM shows ferromagnetic behavior, as presented in Figure 2b with relatively high magnetization at 3 T applied field (M 3T ) of 100 A • m 2 /kg, in agreement with the microstructure shown in Figure 2a, in which only t-phase is observed. It is important to notice the small value of coercivity (H C < 0.03 T), which is directly related with the coarse microstructure with high density of twin boundaries and the absence of defects which pin the domain wall motion, as similarly reported by [9,31]. The resultant phase/microstructure arises from the casting process, where the cooling is not fast enough to stabilize the high temperature e-phase and not slow to promote the stable g 2 -and b-phases.…”

“…The influence of plastic deformation as well as of crystal defects related to crystals plasticity on the formation magnetic properties of Mn-Al based alloys was intensively studied through recent years [30,31,40,41]. The high pressuretorsion (HPT À Walter Klements GmbH) process was used to compact and deform t-phase Mn-Al-C powder (particle size below 80 mm) into disc shaped samples of 10 mm diameter and 1 mm height.…”

“…Different studies have shown the complex relation between the several types of microstructural defects of t-MnAl phase and the magnetic properties, in terms of remanence, coercivity and, consequently, the BH max value [10,21,[27][28][29][30][31][32]. Among the possible defects in this material system, the existence of twin boundaries is often related to one of the major difficulties to obtainhighly textured samples and improved remanence in Mn-Al magnets [31]. This is linked to the high density of twin variants created during the t-phase formation, which will prevent the achievement high degree of texture along the easy magnetization axis since randomly multi-variant grains will be formed.…”

“…This is linked to the high density of twin variants created during the t-phase formation, which will prevent the achievement high degree of texture along the easy magnetization axis since randomly multi-variant grains will be formed. Moreover, in addition to twin boundaries, other defects and metallurgical variables whichwere reported, namely: stacking faults, antiphase boundaries, dislocations, grain size and lattice strain; are related and affect coercivity, as reported in the literature [10,21,31,32]. The effect of each specific defect on the magnetic properties is still under investigation, as different characterization techniques are necessary to obtain qualitatively and/or quantitatively data about the densityand distribution of these defects.…”