Microstructure And Property Regulation Of Fe-Si-B-C Amorphous/Nanocrystalline Alloys With High Saturation Magnetization

As a new generation of green energy-saving materials,Fe-based amorphous/nanocrystalline soft magnetic alloys have been widely applied for power electronics,automobiles,photovoltaics,and semiconductors etc due to their excellent soft magnetic properties.However,there is still a significant gap in the saturation magnetic flux density（Bs）between Fe-based amorphous/nanocrystalline systems and silicon steel,making them difficult to meet the requirements of high power density and device miniaturization.In order to raise the saturation magnetization,high Fe content is often required,which may lead to insufficient glass forming ability（GFA）and serious brittleness of the nanocrystalline materials.Therefore,more stringent requirements have been put forward for the preparation process of amorphous/nanocrystalline alloys.How to develop Fe-based amorphous/nanocrystalline alloys with excellent GFA,processing technology and soft magnetic properties through microstructure and property regulation has become an important topic in the field of materials science.In this dissertation,ab initio molecular dynamics（AIMD）calculations were conducted to simulate the evolution of local atomic packing and the dynamic properties of liquid alloys during the transition process from liquid to amorphous of Fe-Si-B-C alloys.Based on the local structural order and dynamic characteristics of amorphous alloys,the effects of C addition on the GFA,thermal stability,and soft magnetic properties of Fe-Si-B-C series amorphous alloys were studied.For the Fe-Si-B-C system with high Fe content,Fe-Si-B-C-P-Zr system with high Bs was obtained by adding P and Zr elements.The GFA and soft magnetic properties of Fe-Si-B-C-P-Zr system were optimized by adjusting the content of various metalloid element（Si,B,C,P）.The effects of metalloid element（C,Si,B,P）and metal elements（Cu,V）on the GFA,crystallization behavior,and soft magnetic properties of Fe-Si-B-C-P-Cu amorphous/nanocrystalline alloys were studied.By optimizing and regulating the microstructure and soft magnetic properties of the alloy system,Fe-based amorphous/nanocrystalline alloys with high Bs and low Hc have been developed.The main research contents,results findings of this dissertation are summarized as follows:The radial distribution functions,bond pairs,Voronoi polyhedron types and coordination numbers of Fe₇₅Si₁₅B₁₀,Fe_83.5Si_6.5B₁₀and Fe_83.5Si_1.5B₁₀C₅ alloys were systematically studied by using AIMD method.It has been clarified that the shortrange ordered structure and atomic stacking characteristics are increased during the liquid-solid transition of alloys.The results indicate that the short-range ordering tendency in ternary alloys is significantly stronger than that in quaternary alloys.The addition of C element increases the short-range ordering tendency between Fe and metalloid elements,shortens the distance between Si-B bond pairs,and increases the atomic arrangement density.The contents of intact icosahedron of<0,0,12,0>and icosahedron like of<0,1,10,2>in amorphous alloys are the highest.Fe-Si-B-（C）alloys have strong icosahedron characteristics of amorphous configuration.The C addition forms the cluster of<0,3,6,0>with three caps of triangular prism,which plays a key role in the structural stability of amorphous soft magnetic alloys.In terms of the kinetics,the increase in Fe content improves the diffusion ability of atoms,while C element effectively reduces the self-diffusion coefficients of Fe,Si,and B atoms in the liquid alloy.Therefore,whether from the perspective of local ordered structure of amorphous alloys or the kinetics,the addition of C into Fe-Si-B alloy is beneficial for improving the GFA of the alloys.Based on the localized ordered structure of the amorphous and the dynamic obtained by AIMD calculation results,Fe_83.5Si_6.5-xB₁₀C_x（x=1,2,3,4,4.75,5,5.25,6,6.5 at.%）alloys was designed.The effects of C replacing Si on the GFA,thermal stability and soft magnetic properties in the alloy system were studied.And the intrinsic mechanism of the excellent soft magnetic properties of the amorphous alloys was discussed.It indicates that replacing Si with C can effectively improve the GFA of the alloys.When the C content is between 4-6 at.%,a full amorphous structure can be formed.A neatly ordered magnetic domain structure is formed in amorphous alloys.As the C content is increased,the Bs of both as-quenched and annealed alloys at 330℃ show a trend of increasing in the first place,then decreasing,and finally increasing,while Hc shows a trend of first decreasing and then increasing.Based on the above investigation,Fe_83.5Si_1.5B₁₀C₅ amorphous alloy with Bs of 1.79 T and Hc of 3 A/m was successfully developed.In Fe-Si-B-C system with the increasing the Fe content and the addition of P and Zr elements simultaneously,the effects of elements substitution on the GFA,thermal stability and soft magnetic properties were systematically studied.It has been shown that replacing B with C or P,or replacing Fe with B in appropriate portion is beneficial for improving the GFA of this alloy system.The partial substitution of Si for B has little effect on the GFA and thermal stability of the alloys.Meanwhile,after annealing,the Bs of this alloy system reachs above 1.75 T.However,due to the presence of residual hard magnetic phases and the inability to inhibit the growth of α-Fe（Si）at high temperatures,the Hc of this alloys system is relatively high.The effects and mechanisms of metalloid elements（C,Si,B,P）and the ratio between metalloid and metallic elements on the GFA,thermal stability,and soft magnetic properties of Fe-Si-B-C-P-Cu nanocrystalline alloys were studied.Fe_85-xSi₂B_9.2P_2.5+xC_0.5Cu_0.8（x=0,0.5,1,1.5 at.%）alloy system was designed.Fe_83.5Si₂B_9.2P₄C_0.5Cu_0.8 alloy with excellent GFA and soft magnetic properties was detected.Based on this alloy,the P/C ratio,C/Si ratio,P/Si ratio,and Fe/Cu ratio were regulated.It has been found that an appropriate P/C ratio and C/Si ratio both contribute to the improvement of GFA.The increase in P/Si ratio not only improves the GFA but also increases the Bs and reducing Hc of this alloy system.The addition of Cu is beneficial for the precipitation of soft magnetic phase α-Fe（Si）and suppresses the formation of Fe₃（B,P）hard magnetic phase.By replacing Fe with Cu,two alloy compositions with excellent soft magnetic properties were obtained.Fe_83.5Si₁B_9.2P₅C_0.5Cu_0.8 nanocrystalline soft magnetic alloys annealed at 470℃,achieve the Bs and Hc of 1.82 T and 7.8 A/m,respectively.the Fe_84.3Si₁B_9.2P₅C_0.5 amorphous alloy annealed at 370℃ achieve the Bs and Hc of 1.71 T and 3.5 A/m,respectively.The GFA and soft magnetic properties of Fe_83.5Si₁B_9.2P₅C_0.5Cu_0.8-xV)x（x=0,0.1,0.2,0.3,0.4,0.6,0.8 at.%）alloy were studied.It has been shown that the addition of V improves the GFA of the alloy.When the V content is above 0.6 at.%,a fully amorphous ribbon with a thickness of 35μm can be prepared.By adjusting the V/Cu ratio and annealing temperature,amorphous/nanocrystalline alloys with excellent soft magnetic properties were obtained.When the V content is below 0.4 at.%,Fe based nanocrystalline alloys exhibit Bs above 1.8 T and Hc lower than 10 A/m,respectively.When the V content reaches above 0.6 at.%,Fe based amorphous alloys with Bs higher than 1.73 T and Hc lower than 4.1 A/m can be successfully prepared.