Study On Aging Treatment And Magnetic And Mechanical Properties Of Sm₂Co₁₇-type Rare-earth Permanent Magnets

Sm₂Co₁₇-type rare-earth permanent magnets are widely used in aerospace,instruments,and other industries due to their high Curie temperature,excellent thermal stability,and good magnetic properties.The solid solution state Sm(Co_balFe_0.25Cu_0.06Zr_0.02)_7.5 magnets were prepared by traditional powder metallurgy technique,and further subjected to microwave and traditional aging process.The correlation between magnetic,mechanical properties and microstructure were investigated via powder metallurgy with microwave and traditional aging heat treatment,combining with magnetic and mechanical properties test,microstructure characterization and first principles calculation.The chemical segregation mechanism of Fe and Cu during aging process and the reason of intrinsic brittle of the magnets were elucidated,which provided some guidance for the development of high-performance Sm₂Co₁₇-type rare-earth permanent magnets.The main conclusions of the thesis are as follows:（1）The Sm(Co_balFe_0.25Cu_0.06Zr_0.02)_7.5 magnets fabricated by powder metallurgy and microwave aging treatment exhibited good magnetic properties with remanence B_r=11.1 k Gs,intrinsic coercivity H_cj=17.4-23.1 k Oe,maximum energy product（BH）_max=27.1-30.0 MGOe and mechanical properties with bending strengthσ_b=168.2-177.5 MPa,fracture toughness K_IC=1.60-1.79 MPa·m^1/2.With the aging time increasing from 1 to 5 h,the microstructure transformed from 20-nm particles into 80-nm cellular structure with Fe-rich Sm₂Co₁₇R（2:17R）cell phase and Cu-rich Sm Co₅（1:5）cell boundary phase.The Sm(Co_balFe_0.25Cu_0.06Zr_0.02)_7.5 magnets subjected to optimal multistage microwave aging exhibited good magnetic properties with B_r=10.8-11.1k Gs,H_cj=23.1-24.4 k Oe,（BH）_max=20.4-27.4 MGOe and mechanical properties withσ_b=232.2-266.0 MPa,K_IC=1.79-2.01 MPa·m^1/2.Microwave multistage aging led to larger cellular structure,thicker 1:5phase and significantly increased Cu concentration at the center of the 1:5cell boundary phase and the center of triple junction of 2:17R/1:5 phase interface,resulting in an increased H_cj of the magnets.（2）With the aging temperature increasing from 845 to 860℃,the magnetic properties of conventional aged Sm(Co_balFe_0.25Cu_0.06Zr_0.02)_7.5magnets increased first and then decreased.The magnet aged at 855℃exhibited good magnetic properties with B_r=11.0 k Gs,H_cj=34.1 k Oe,（BH）_max=27.4 MGOe.The bending strength of the magnets decreased monotonously from 277.6 MPa at 845℃to 218.3 MPa at 860℃.By adjusting aging time at 855℃,the magnets exhibited excellent magnetic properities with B_r=11.5-11.7 k Gs,H_cj=30.5-32.9 k Oe,（BH）_max=31.1-33.0 MGOe and mechanical properties withσ_b=193.9-254.7 MPa.The magnet possessed uniform and complete cellular structure with Fe-rich2:17R phase（60-70 nm）and Fe-and Cu-rich 1:5 cell boundary phase（8-10 nm）.（3）The first principles calculations showed that Cu atom is prefer to occupy 2c site in the 1:5 phase.With increasing Cu content at 2c site,it weakened the exchange-coupling interactions between Sm 4f-Co 3d electrons and Co（2c）3d-Co（3g）3d electrons,resulting in low exchange constant of the 1:5 phase A^1:5 and enhancement in domain wall energy differenceΔγbetween 2:17R phase and 1:5 phase,and then led to an increase H_cj.The segregation mechanism of Cu and Fe atoms was found for the first time:with increasing Cu content in Sm Co_5–yCu_y,a pseudogap（Sm Co₃Cu₂）or a bandgap（Sm Cu₅）at Fermi level（E_F）was formed.It led to increased stability of the 1:5 phase and the segregation of Cu atoms to the 1:5 phase.The first principles calculations showed that Fe atom is prefer to occupy 6c and 18f sites in the 2:17R phase,resulting in reduced total density of states（DOS）at Fermi level（E_F）and introduction of an Fe-Co covalent-like bond.Then it led to increased stability of the 2:17R phase and the segregation of Fe atoms to the 2:17R phase.（4）The strengthening mechanism of Sm₂Co₁₇-type rare-earth permanent magnets was grain refinement,and the fracture behavior was brittle cleavage transgranular fracture.The calculated elastic constants showed that the 2:17R phase is brittle,1:5 phase possess the Possion’ratio（ν）of 0.32 and has trend from brittleness to toughness,resulting in plastic platform at small strain.The mechanical anisotropy of 2:17R phase was greater than that of 1:5 phase.The tensile deformation simulations along[001]direction showed that the energy of 2:17R main phase increases rapidly when the strain is less than 0.008,leading to structural instability.Since the 2:17R main phase was sensitive to the strain,the fracture behavior of Sm₂Co₁₇-type rare-earth permanent magnets was brittle fracture.The energy of the 1:5 phase remained small change until the strain was less than 0.04,indicating that the 1:5 phase was less sensitive to the strain.Then with raising applied loading,the loading-displacement curves got more curvier and produced plastic deformation platforms.