| Fe-based nanocrystalline soft magnetic alloys are known as a new generation of green soft magnetic materials for manufacturing and application of double energy saving because of their short manufacturing process,high initial permeability(?i),low coercivity(Hc)and core loss(Pc).At presently,they have been used in reactors,transformers,inverters,sensors,filters,transformers and other devices.However,Fe73.5Si13.5B9CuiNb3(FINEMET)nanocrystalline alloy widely used in industry has a relatively low saturation magnetic flux density(BS)of 1.24 T,which can not meet the requirments of China's "new infrastructure”and”5G communication" for the miniaturization of electrical components.Although the new developed Fe-rich(Fe>83 at.%)Fe-Si-B-P-Cu(NANOMET)nanocrystalline alloys show a high Bs above 1.80 T,the low glass forming ability(GFA)of this alloy system makes the quenched structures of alloys highly sensitive to the preparation process parameters.The optimal soft magnetic properties of alloys rely on the heating rate of up to 300-400 K/min and the short optimal annealing time about 5-10 min.Therefore,how to improve the GFA of Fe-rich nanocrystalline soft magnetic alloys and reduce the sensitivity of soft magnetic properties of alloys to heat treatment process parameters are still an open question.In this thesis,a new Fe-B-P-C-Cu nanocrystalline alloy system with low cost,high GFA and Bs,and wide heat treatment process window is successfully developed by innovation of alloy system and optimization of composition design.The alloy ribbons were prepared by singe roll melt spinning method.The influence of metalloid elements C,B and P as well as Cu content on the GFA,thermal stability,crystallization behavior,soft magnetic properties and heat treatment process parameters of the alloys were systematically studied.The main research contents and results were summarized as follows:(1)Based on the theory of amorphous alloy formation,the Fe-B-P-C-Cu alloy system with high GFA was successfully developed by controlling the atomic mismatch and mixing enthalpy.In Fe83.3Si4-xB9P3Cu0.7Cx(x=0,1,2,3,4 at.%)alloys,the thermophysical parameter analysis shows that subsitution of C for Si reduces the stability of residual amorphous phase of the alloys.After annealed at 783 K for 6 min,the Bs,Hc and ?e of Fe83.3C4B9P3Cu0.7 nanocrystalline alloy reach 1.86 T,9.8 A/m and 8108,respectively.(2)To solve the common problem that the soft magnetic properties of high Bs Fe-based nanocrystalline alloys are highly sensitive to annealing temperature,the content of Cu element in Fe-B-P-C-Cu alloys was optimized based on the positive mixing enthalpy and mutual immiscibility of Fe and Cu elements.It was found that the increase of Cu content can promote rapid nucleation of ?-Fe grains when annealed near the peak value of the first crystallization,and the competition of nucleation and growth of ?-Fe grains can refine ?-Fe grains,resulting in the Reduction of the sensitivity of soft magnetic properties of the alloys to annealing temperature.It can be found that when the Cu content is between 0.9-1.1 at.%,the Bs and Hc of Fe82.9B9P3C4Cu1.1 nanocrystalline alloy annealed at 703-763 K for 6 min reach 1.84-1.85 T and 8.9-13.0 A/m,respectively.(3)To solve the common problem that the soft magnetic properties of high Bs Fe-based nanocrystalline alloys are highly dependent on annealing heating rate and holding time,the effects of P/B ratio on GFA,crystallization behavior,heat treatment process parameters and soft magnetic properties of Fe-B-P-C-Cu alloys were systematically studied based on the primary crystallization mechanism of Febased amorphous alloys.It is found that the increase of P/B ratio can effectively reduce the sensitivity of soft magnetic properties of Fe-B-P-C-Cu alloys to heat treatment process parameters.The microstructure analysis indicates that the increase of P/B ratio can effectively promote the nucleation of a-Fe grains and hinder the growth of grains.However,the thermophysical parameter analysis shows that excess P content will reduce the thermal stability of residual amorphous phase.Among them,the Bs and Hc of Fe83B7P5C4Cu1 alloy heated to 708 K at the heating rate of 40 K/min for 30 min are 1.79 T and 8.44 A/m.And the Bs and Hc of Fe83B5P7C4Cu1 alloy heated to 708 K at the heating rate of 20 K/min for 30 min are 1.74 T and 8.09 A/m,respectively.(4)In Fe84+xB6P6C3Cu1-x?(x=0,0.2,0.4,0.6,1 at.%)alloys,the GFA of alloy system increases with the decrease of Cu content.Microstructure analysis shows that when the Cu content is less than 0.6 at.%,the precipitation of ?-Fe grains will aggregate due to the reduction of effective nucleation Cu clusters of ?-Fe grains,which leads to the increase of magneto-crystalline anisotropy contant and worsening of the soft magnetic properties of the alloy system.The GFA of Fe84.4B6P6C3Cu0.6 alloy is 26 ?m.And its Bs,Hc and ?e annealed at 723 K for 30 min are 1.82 T,8.1 A/m and 10820,respectively.(5)Fe83+x(B4P8C4)16-xCu1(x=0,1,1.5,2 at.%)alloys with gradually enhanced surface crystallization intensity were successfully developed by tuning Fe content.It was found that the weak surface crystallization of melt-spun samples will not have a significant effect on the soft magnetic properties of the alloys during annealing.However,the strong quenched surface crystallization of melt-spun samples will induce a magnetocrystalline anisotropy constant in a plane,making the Hc of melt-spun sample rapidly increases.It is also found that the pre-exsiting?-Fe clusters in melt-spun sample will hinder the movement of magnetic domain during the stress relief annealing process and refine the magnetic domain structure,which leads to the rapid increase of Hc of the alloys.The rapid decrease of Hc after nanocrystalline annealing is due to the precipitation of a large number of ?-Fe grains,which is conducive to the decrease of magnetocrystalline anisotropy constant induced by surface crystallization.Among them,the Bs,Hc and ?e of Fe85B3.5P7C3.5Cu1 nanocrystalline alloy annealed at 723 K for 6 min reach 1.86 T,5.7 A/m and 12737,respectively. |
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