| In the current economic tendencies of environmental protection,energy conservation and emission reduction,power electronic devices are developing towards the directions of energy-saving,high efficiency and miniaturization,hence the soft magnetic materials widely used therein should possess low coercivity(Hc),low core loss,high permeability and saturation magnetic flux density(Bs).In comparison with the traditional materials,the novel Fe-based nanocrystalline alloys show superior soft magnetic properties,which originate from the low average magnetocrystalline anisotropy produced by the exchange interaction between the nano-sized ?-Fe grains in the alloys.The Fe-Si-B-Nb-Cu nanocrystalline alloys(FINEMET)have been used as magnetic cores in industrialization,whereas their relatively low BS of 1.24 T is unfavorable for the device miniaturization.The Fe-Si-B-P-Cu and Fe-Si-B-Cu nanocrystalline alloys which were successively developed in recent years exhibit high BS of over 1.80 T,while their poor amorphous forming ability(AFA)make it difficult to produce amorphous precursor with a stable quality in the mass production,and the fine a-Fe grains and excellent magnetic softness can only be obtained by annealing under a high heating rate that is hard to achieve in industrial production.Alloying is an effective way to enhance the AFA of the alloys and tune the crystallization process of the amorphous alloys,thus it might improve the manufacturability of the Fe-(Si,B,P)-Cu nanocrystalline alloys,meanwhile maintain their excellent soft magnetic properties.The work on the effects of alloying elements on the AFA,crystallized structure and magnetic properties on the Fe-(Si,B,P)-Cu alloys is still insuficient,hence it is necessary to study the influence rule of the alloying and clarify the related mechanism to provide theoretically guidelines and candidate compositions for the development of high-BS nanocrystalline soft magnetic alloys with good industrial applicability.In addition,the Fe-based nanocrystalline alloys possess a severe annealing embrittlement,which increases the difficulty to the processing and forming of the magnetic cores,and also causes the devices prone to failure in service.It is important to investigate the influences of alloying elements on the annealing embrittlement of the Fe-(Si,B,P)-Cu alloys and explore the method to reduce the brittleness.In this work,the effects of early transition metals(ETMs,Ti,V,Cr,Mn,Zr,Nb,Mo,Hf,Ta,W)and ferromagnetic elements(Co,Ni)additions and Cu content on the AFA,melt-spun structure,thermal stability and crystallized structure,soft magnetic properties and brittleness of the Fe-(Si,B,P)-Cu alloys were systematically investigated,the mechanism related to the a-Fe grain refinement and magnetic softness improvement were clarified by studying the crystallization behaviors of the melt-spun alloys,and the corresponding nanocrystallization models were put forth.The correlation between the nanostructure and brittleness was established,and the reason of the reduction in the brittleness was explained.The research results are summarized as follows:1.Addition of Mo in the Fe85Si2B8P4Cu1 alloy enhances the AFA and thermal stability.2 at.%Mo increases the critical thickness for amorphous formation(tc)and the onset temperature of primary crystallization from 14 and 659 K to 20 ?m and 678 K,respectively.The Mo alloying also refines the a-Fe grains and improves the magnetic softness of the nanocrystalline alloy,and the 2 at.%Mo lowers the average ?-Fe grain size(D)and HC from 42 nm and 105.3 A/m to 30 nm and 37.6 A/m.respectively,meanwhile the BS is higher than 1.70 T.2.Alloying minor(2 at.%)ETMs into the Fe-(Si,B,P)-Cu alloys enhances the AFA and thermal stability,refines the nanostructure by inhibiting the growth of the a-Fe grains during the crystallization process,and hence improves the magnetic softness meanwhile maintaining the high BS.Additions of Nb,Mo,Hf,Ta and W which possess large atomic sizes and strong chemical affinities with the mian components in the alloys show a prominent improvement effect.3.Additions of ferromagnetic Ni or Co enhance the AFA of the Fe-(Si,B,P)-Cu alloys.Adding a proper amount(5 at.%)of Ni or Co increases the number density(Nd)of nucleation sites for the a-Fe phase by properly delaying the Cu clustering process,thus refines the a-Fe grains and enhances the magnetic softness,while the excessive addition decreases the Nd due to the overinhibition of the Cu atoms clustering,resulting in the grains coarsening and magnetic softness deterioration.Addition of 5 at.%Co increases the BS,but adding Ni decreases it slightly.The Fe80Ni5Si2B8P4Cu1 nanocrystalline alloy with a tC of 21 ?m possesses a small D of 22 nm,low Hc of 14.3 A/m and high BS of 1.77T.4.Increasing the Cu content to over 1.5 at.%in the Fe-(Si,B,P)-Cu alloys changes the melt-spun structure from a single amorphous phase to a composite of a-Fe nanoparticles(D=6 nm)with a high Nd dispersing in amorphous matrix.The competitive growth among these nanoparticles along with new nuclei formed during crystallization process refines the a-Fe grains and hence improves the magnetic softness of the nanocrystalline alloys,and then the nanocrystallization model of the alloys with high Cu content is put forth.The Fe81.3Si4B13Cu1.7 nanocrystalline alloy possesses a smaller D of 14 nm,lower Hc of 7.1 A/m,higher effective permeability(at 1 kHz)of 16500 and BS of 1.77 T compared with those of 53 nm,379.3 A/m,550 and 1.73 T,respectively,for the Fe81.7Si4B13Cu1.3 alloy.5.Alloying minor Ti,Zr,Nb,Hf and Ta in the Fe81.3Si4B13Ci1.7 alloy enhances the AFA,decreases the Nd of the a-Fe and weakens the competitive growth effect,thus coarsens the a-Fe grains and deteriorates the magnetic softness of the nanocrystalline alloys,while adding the above ETMs with over 3 at.%eliminates the ?-Fe nanoparticles,but effectively suppresses the growth of the ?-Fe grains during the crystallization process,hence refines the nanostructure and improves the magnetic softness.6.Additions of alloying elements or increase of Cu content reduces the annealing embrittlement of the Fe-(Si.B,P)-Cu nanocrystalline alloys by refining the ?-Fe grains.Adding 2 at.%Mo or 5 at.%Ni in the Fe85Si2B8P4Cu1 alloy increase the fracture strain(?f)from 1.17%to 1.38%and 1.50%,respectively;increasing the Cu content to 1.7 at.%in the Fe81.7Si4B13Cu1.3 alloy enhances the ?f from 1.33%to 1.63%.In addition,increase in the heating rate also effectively lowers the annealing brittleness of the nanocrystalline alloys by refining the nanostructure. |
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