Fine-grain Strengthening And Nano-indentation Size Effect Of Sm(Co,Fe,Cu,-Zr)_7.5 Permanent Magnets

As a kind of functional materials,Sm₂Co₁₇-type sintered permanent magnetic material has the characteristics of excellent magnetic property,strong corrosion resistance and good temperature stability.However,Sm₂Co₁₇-type permanent magnet has insufficient internal slip system and high brittleness at room temperature,which not only brings difficulties to the machining process,but also adversely affects the stability of permanent magnet devices,and also limits the application of Sm₂Co₁₇-type permanent magnet in strong impact and vibration occasions.The main factors that affect the mechanical properties of materials are the chemical composition,grain size,microstructure and so on.This paper mainly explores the influence of grain size,microstructure and other factors on its mechanical properties.The main research conclusions are as follows:(1)The as-solution-treated Sm₂Co₁₇-type magnets exhibiting a single 1:7H phase with different average grain sizes are designed.Anisotropy of bending strength(R_bb)and compressive strength(R_mc)of the magnets was investigated.Moreover,the R_bbincreases from 86 MPa to 173 MPa with grain sizes decreasing from?52?m to?18?m for group c//h samples.For the compressive strength,when the orientation is perpendicular to the loading direction,the bending strength and compressive strength increase from 1155 MPa to 1311 MPa,indicating that grain refinement is one of the effective way to enhance the mechanical properties of Sm₂Co₁₇-type sintered magnets.(2)The Hall-Petch correlation was employed to reveal the effect of grain size on mechanical properties of the magnets,giving deep understand the mechanical anisotropy characteristics.For the bending strength of c//l,the Hall-Petch coefficient K^Rbbvalue is only 0.24 MPa.m^1/2,for c//b,the K^Rbb value is 0.59 m MPa.m^1/2,which is more than 2 times of c//l,and the maximum K^Rbb value is 0.79 MPa.m^1/2 for c//h,which indicates that the anisotropy of bending strength is due to the difference in the strength of between the cleavage plane of crystal structure and strengthening of the grain boundary.(3)The mechanical properties of magnets in solid solution state and aged state are compared.The results show that for the condition of c//h,the bending strength of the magnet increases from 115 MPa in the solution state to 145 MPa after aged treatment for 15h.It can be seen that the appearance of the cellular structure of the magnet after aged treatment is helpful to improve the bending strength of the magnet.(4)Nanoindentation experiments are carried out on samples with different orientations cut from Sm₂Co₁₇-type permanent magnets in as-solution-treated state and final by quasi-static method,respectively.The results show that the hardness and elastic modulus of the two magnets on different orientations surface decrease with the increase of the loading depth,showing a typical indentation size effect.The fitting results of hardness and loading depth based on the Nix-Gao model show that the hardness H₀ and characteristic length h^*of the two types of magnets have obvious anisotropy.The larger h^*value indicates that the indentation size effect can be observed in the larger range of indentation depth of the final magnet.Further,the indentation size effect factor i is calculated.The i values of the samples with different orientations of the final state magnet are all larger than those of the as-solution-treated magnet,especially when the orientation is parallel to the loading direction,which is 58%higher.This means that the nanostructure cellular structures in the final Sm₂Co₁₇-type magnet enhance the indentation size effect factor,which improves the mechanical properties of the Sm₂Co₁₇-type magnet in the elastic-plastic deformation stage.