No abstract

In our search for an optimum soft magnet with excellent mechanical properties which can be used in applications centered around “electro mobility”, nanocrystalline CoCrFeNiGax (x = 0.5, 1.0) bulk high entropy alloys (HEA) were successfully produced by spark plasma sintering (SPS) at 1073 K of HEA powders produced by high energy ball milling (HEBM). SPS of non-equiatomic CoCrFeNiGa0.5 particles results in the formation of a single-phase fcc bulk HEA, while for the equiatomic CoCrFeNiGa composition a mixture of bcc and fcc phases was found. For both compositions SEM/EDX analysis showed a predominant uniform distribution of the elements with only a small number of Cr-rich precipitates. High pressure torsion (HPT) of the bulk samples led to an increased homogeneity and a grain refinement: i.e., the crystallite size of the single fcc phase of CoCrFeNiGa0.5 decreased by a factor of 3; the crystallite size of the bcc and fcc phases of CoCrFeNiGa—by a factor of 4 and 10, respectively. The lattice strains substantially increased by nearly the same extent. After HPT the saturation magnetization (Ms) of the fcc phase of CoCrFeNiGa0.5 and its Curie temperature increased by 17% (up to 35 Am2/kg) and 31.5% (from 95 K to 125 K), respectively, whereas the coercivity decreased by a factor of 6. The overall Ms of the equiatomic CoCrFeNiGa decreased by 34% and 55% at 10 K and 300 K, respectively. At the same time the coercivity of CoCrFeNiGa increased by 50%. The HPT treatment of SPS-consolidated HEAs increased the Vickers hardness (Hv) by a factor of two (up to 5.632 ± 0.188) only for the non-equiatomic CoCrFeNiGa0.5, while for the equiatomic composition, the Hv remained unchanged (6.343–6.425 GPa).

Powder bed fusion of metals using a laser beam (PBF-LB/M) is an established additive manufacturing (AM) method that can be used to fabricate geometrically complex NdFe-B magnets. However, the magnetic properties of Nd-Fe-B magnets manufactured by PBF-LB/M are typically inferior to conventionally produced magnets. To overcome this drawback, we modified the surface of the permanent magnet feedstock powder with 1 wt.% surfactant-free Ag nanoparticles (NPs) supporting the formation of relevant phases required for permanent magnetic performance to achieve a suitable micro- and nanostructure after AM. Our study is accompanied by finite element simulations, revealing the impact and dependency of process parameters during PBF-LB/M: a wide temperature field with a high-gradient profile in the front and on the bottom of an overheated region, implying a vast local heating/cooling rate and in-process high thermal stress. We found experimentally that the as-built part density can be affected by both the laser power and scan speed, causing a reduction in density as both parameters increase. The functionality and microstructural properties are also investigated via VSM, HR-SEM, EDX, EBSD, and exemplarily with HR-TEM-EDX and APT. Our study found that modifying MQP-S with Ag NPs increases the coercivity by approximately 20%, which we correlate to a decreased grain size. Additionally, we identified three distinct phases in the modified and unmodified samples, where Ag is primarily found in the intergranular and Nd-rich phases of the as-built parts. Overall, the study's findings contribute to the understanding of the factors that affect the quality and magnetic properties of Nd-Fe-B magnets fabricated through PBF-LB/M and provide valuable insights for further research in this area.