B. Cui
Ames National Laboratory,
United States
Keywords: Sm-Co sintered magnets, microstructure engineering, mechanical strengthening, micromechanics modeling, mechanical and magnetic properties
Summary:
Sm-Co sintered permanent magnets (including SmCo5- and Sm2Co17- type magnets) have high magnetic energy densities, great demagnetization resistance, good corrosion resistance, as well as excellent thermal stability in a wide temperature range (e.g., –50 °C to 550 °C). They are widely used in energy conversion, telecommunication, medical devices, and magnetic sensors, especially for high-temperature operations (e.g., 200 - 550 oC). However, they are intrinsically brittle because of an intragranular cleavage fracture mechanism. Accordingly, they are prone to chipping, cracking, and fracturing during block magnet manufacturing, part machining, assembly, and operation. The brittleness of Sm-Co magnets makes machining processes high-risk, costly, and labor-intensive. The brittleness also leads to a high magnet production loss and imposes limitations on magnet shapes, sizes (both production of large magnet blocks and miniaturization of magnet parts), applications, and service durations. This ultimately hinders the development of the Sm-Co magnet market. It is known that there is a trade-off between high mechanical and high magnetic properties in conventional Sm-Co sintered magnets with a homogenous, unimodal grain-size, and highly textured microstructure . Developing mechanically robust, high-magnetic-performance Sm-Co sintered magnets is of great scientific and technical significance. In this presentation, we report a novel heterogeneous grain-microstructure engineering approach that allows for the nearly independent control of the excellent magnetic properties and mechanical properties of Sm–Co-sintered magnets [3]. The heterogeneous grain-refined microstructures were formed by either the strategically designed architectural assemblies of coarse and fine Sm–Co feedstock powder mixtures for sintered magnets or doping of the small amount (e.g., 0.5–3 wt%) of Sm2O3 submicron fine particulates into the magnet matrix. Typical novel heterogeneous microstructures comprise, but are not limited to, multi-modal, laminated, core/shell, or gradient fine-coarse-grained microstructures. These heterogeneous-grain microstructural architectures can significantly enhance (by approximately 62–73%) the flexural strengths of Sm-Co sintered magnets. Consequently, high mechanical strengthening and resilience were obtained. The micromechanical simulation indicates the fracture is dominated by intragranular mode while the grain refinement-induced strengthening effect is the main reason to improve the resulting material’s flexural strength, with the fine grains increasing the energy barrier for crack growth and acting as mechanical strengthening sites. Whereas the texture and the sub-microstructure of the micron-sized grains that determine the hard magnetic properties were only marginally affected, therefore, the excellent magnetic properties were retained in the heterogeneous grain-refined magnets. The heterogeneous grain-microstructure engineering approach for Sm-Co sintered magnets is an economical and effective method to increase their mechanical properties while maintaining their magnetic properties. This technology is compatible with existing magnet manufacturing processes and thus can be adopted readily by the magnet industry. These results are of scientific and technical importance for manufacturing mechanically robust, high-magnetic-performance Sm-Co sintered magnets. These magnets will be more cost-effective and efficient for magnet production, machining, and energy-related applications, survive more demanding service conditions, have a longer service life, and save expensive critical materials while reducing pressure on the critical material supply chain.