Magnetic Properties And Microstructures Characteristics Of High-performance Iron-rich Sm₂Co₁₇-type Permanent Magnets

Sm₂Co₁₇-type sintered permanent magnet with precipitation hardening,has attracted much attention due to its distinguished high temperature magnetic properties,extremely low temperature coefficient,excellent corrosion resistance and oxidation resistance.The typical cellular microstructure of Sm₂Co₁₇-type sintered permanent magnet is critical to its magnetic performances,whose formation after a series of heat treatments including sintering,solution treatment,isothermal aging and step cooling.In this paper,the high performance Sm₂Co₁₇-type sintered permanent magnets are prepared by means of process control.The phase structures,crystalline and magnetic microstructures of Sm₂Co₁₇-type permanent magnets at the sintering,isothermal aging and step cooling stages are investigated and discussed systematically.The conclusions are as follows:?1?Results show that smaller cellular structure size,larger Zr rich lamellar density and bigger gradient of Cu element at the cell wall are the keys to obtain high performance Sm₂Co₁₇-type permanent magnets.The magnets with?BH?_max=32.08MGO,B_r=11.62 kGs,H_cj=31.48 kOe are prepared successfully.?2?This is the first time to systematically reveal that the average surface width and the main domain width firstly decreases and then increases.It is impressive that the domain walls form in straight line through cellular structures as isothermal aging time is less than 5 h,and in a zig-zag shape when aging time is over 5 h.Results show that the phase is single phase with 1:7H structure in the solid solution treatment magnet without cellular and lamellae microstructures.With increasing the isothermal aging time increasing,the average cellular size and the density of the lamellae phase gradually increase.It is found that 1:7H phase completely transforms to be 2:7R and5:19H when isothermal aging time is over 5 h.The transformation of phase structure and the size change of the cellular structure change the material parameters?K₁?of phases in the sample,and then increase the difference of domain wall energy between the main phase and the cellualr wall phase,thus optimizing the magnetic properties of the magnets.?3?It is found the average cellular size in magnets decreased and the density of the lamella phase increased during the step-cooling procedure.Interestingly,it is found that the phase transformation has completed after the step cooling to 500 ^oC.And the phase constitution in specimens is stable in the final state magnet.It is observed that the Cu enriched in the cell boundaries in magnet quenched from 500 ^oC compared to that of the magnet quenched from isothermal aging temperature.The increase of the Cu concentration in the cell boundaries phase should lead to decrease of K₁,resulting in the increase of the difference of domain wall energy density.Nevertheless,the fact that the average cell sizes become smaller is also one of the factors giving rise to the increasing of the H_cj.This discovery has great significance guiding for optimizing the preparation process of high performance magnets.?4?The results show that the density of the lamellae phase gradually increases,the average cellular size and the width of cell wall phase decrease,while the Cu content in the cell wall phase trends to increases with the sintering temperature decreasing from 1215 ^oC to 1200 ^oC.The average domain width is smaller and more uniform for the magnet sintered at 1200 ^oC than that of the magnet sintered at 1215 ^oC.During the densification process of Sm₂Co₁₇-type sintered permanent magnets,it is found that the average size of the cavity in the magnet is gradually increasing firstly and then decreasing rapidly.The volume ratio is gradually decreasing.And the decreasing of void ratio is the reason for the gradual increasing of the density.The smaller density results in poor mechanical property of the magnet.It is found that the rich-rare earth phase is gradually gathered together,and the rich-rare earth phase can effectively improve the mechanical properties of the magnet.