Study On The Microstructure And Mechanical Properties Of Fe-Si-B-Cu Nanocrystalline Soft Magnetic Alloys With High Fe Concentrations

Fe-based nanocrystalline alloys exhibit excellent soft magnetic properties including low coercivity?H_c?,high permeability,low magnetostriction coefficient,and low core loss,especially in high frequency.They are widely applied in high frequency transformer,high frequency switching power supply,sensor,current transformer and other power electronic equipment.Finemet(Fe_73.5Si_13.5B₉Nb₃Cu₁ in at.%)nanocrystalline alloy is currently the most widely used in industry.However,the saturation flux density?B_s?of the Finemet alloy is relatively low,which is adverse to the miniaturization and light weighting of the equipment.In recent years,Fe-based nanocrystalline soft magnetic alloys with higher Bs have been developed in the alloy systems of Fe-Si-B-Cu,Fe-Si-B-P-Cu,and so on,which will further generalize the application in industry.The current research on the Fe-based nanocrystalline soft magnetic alloys with high B_s mainly focuses on the composition design,microstructure tailoring,and magnetic properties optimization,while the research on the mechanical properties and annealing embrittlement is not so adequate.The Fe-based nanocrystalline soft magnetic alloys are usually fabricated by annealing the amorphous ribbons.During annealing,the ribbons become brittle,which increases the difficulty to the processing and forming of the iron cores,and also causes the devices prone to failure in service,restricting the applications in industry.Therefore,it is of significance to investigate the correlation between the annealing embrittlement and microstructure of the Fe-based nanocrystalline soft magnetic alloys,and then suppress the annealing embrittlement of the alloys.In this work,a Fe_81.7Si₄B₁₃Cu_1.3?in at.%?alloy with high Fe concentration was selected and the microstructure,magnetic and mechanical properties of the alloy before and after annealing were systematically investigated,and the correlation between the annealing embrittlement and the microstructure was analyzed.In addition,the effects of Nb addition and heating rates on the microstructure,magnetic and mechanical properties of Fe_81.7-xSi₄B₁₃Nb_xCu_1.3?x=0,2,4?alloys were studied as well.The results are as follows:1.The melt-spun Fe_81.7Si₄B₁₃Cu_1.3 ribbon has a fully amorphous structure,and the relative strain at fracture??_f?value is 1,which indicates a ductile feature.After annealing at 608 K for1 h,a nanostructure with?-Fe nanocrystalline grains dispersing in the amorphous matrix was formed.The B_s and H_c of the nanocrystalline alloy is 1.65 T and 6 A/m,respectively,The?_f of the nanocrystalline alloy is 0.0240,and gradually decreases with further increase of annealing temperature,indicating the aggravation of embrittlement.2.The Nb addition enhances the thermal stability of the Fe_81.7Si₄B₁₃Cu_1.3 amorphous alloy and widens the temperature interval of heat treatment for obtaining optimum H_c.As the Nb content increases,the grain size of Fe_81.7-xSi₄B₁₃Nb_xCu_1.3?x=0,2,4?alloys decreases from 52.6to 10.4 nm after annealing at their respective optimum annealing temperatures for 1 h,and the H_c lowers from 382.1 to 2.4 A/m,indicating the improvement of magnetic softness although the B_s decreases from 1.72 to 1.42 T,as well.The?_f increases from 0.0133 to 0.0172,indicating that the annealing embrittlement of the alloys is suppressed.3.As the heating rate increases from 10 to 400 K/min,the grain size and H_c of the Fe_81.7-xSi₄B₁₃Nb_xCu_1.3 alloys gradually decrease,indicating the improvement of magnetic softness,while the B_s changes slightly.The?_f of the alloys gradually increases,indicating that the annealing embrittlement is suppressed.The Fe_77.7Si₄B₁₃Nb₄Cu_1.3 alloy has the minimum H_c and the maximum?_f after annealed at 783 K for 10 min under a heating rate of 400 K/min,which are 1.2 A/m and 0.0179,respectively.However,the dependence of the?_f of Fe_77.7Si₄B₁₃Nb₄Cu_1.3 alloy on the heating rate is the minimum.4.The annealing embrittlement of the Fe_81.7-xSi₄B₁₃Nb_xCu_1.3 alloys has a strong correlation with the microstructure.As the grain size decreases,the?_f of the alloys gradually increases.