| With the forward growth of new energy vehicles、information records、avigation、aerospace、national defense and other fields,the application scale of soft magnetic materials devices is widening,which also introduce greater requirements for the magnetic properties、resistivity、hardness and other comprehensive properties of soft magnetic materials.In recent years,multi-principal element alloys have shown good application prospects in engineering materials,among which FeCoNi based multiprincipal element alloys can show high resistivity 、 hardness and saturation magnetization through composition design,and have become a new hotspot in research of metallic material and condensed matter physics.However,regulating the comprehensive properties of FeCoNi based multi-principal element alloys by composition design often leads to the phenomenon that some properties increase and some properties decrease,it is difficult to optimize multiple performance parameters at the same time.Therefore,it is of great theoretical significance and application value to improve the comprehensive properties of FeCoNi based multi-principal alloy.The microstructure,magnetic domain structure,soft magnetic properties,and hardness evolution of FeCoNi-X(X=Al,Si,AlSi and Al Mn)alloy solidified under high pressure were studied by Walker 6-8 high temperature and high-pressure device with two-sided top in this paper.The enhancement mechanism of magnetic properties of the alloy were revealed,as well as strengthening mechanism.After high pressure solidification of FeCoNi alloy,in-coherent nanophases with FCC structure appeared in the matrix.Similarly,FCC+BCC dual-phase in-coherent nanophases also appeared in FeCoNiAl alloy after high pressure solidification.These in-coherent nanophases can effectively reduce the magneto crystalline anisotropy of the alloy,which is beneficial to the increase of the maximum permeability and the decrease of the coercivity.Therefore,compared with the atmosphere pressure,when the solidification pressure reaches 7 GPa,the relative maximum magnetic permeability of FeCoNi alloy and FeCoNiAl alloy increases from 67728 and 68550 to 171071 and 190877 respectively,and the coercivity decreases from 36 Oe and 40 Oe to 7Oe and 10 Oe respectively.In addition,a large number of nanophases can increase the scattering effect of electrons and improve the resistivity of the alloy.The formation of non-coherent nanophase is closely related to the short-range ordered(SRO)structure and diffusion dynamics of atoms.The molecular dynamics calculation shows that the BCC+FCC short-range ordered structures are uniformly distributed in the FeCoNiAl alloy melt at 1900 K and 7 GPa pressure.Moreover,AlCo atom pairs exhibit stronger binding force under pressure,and the short-range ordered structure formed by Al-Co atom pairs provides the basis for the formation of in-coherent nanophases.In addition,when the solidification pressure is 7 GPa,the diffusion rate of FeCoNiAl alloy atoms is 2-4 orders lower than that under atmosphere pressure,which makes the short-range ordered structure unable to meet the growth dynamics conditions,and finally in-coherent nanophases formed.Therefore,the short-range ordered structure in alloy melt and the extremely slow diffusion kinetics of atoms during high pressure solidification promote the formation of non-coherent nanophase.After solidified under 7 GPa,the shape of the grain of FeCoNiSi alloy becomes equiaxed,the grain and the primary phase size are refined obviously.As the solidification pressure increases from 0.1 MPa to 7 GPa,the matrix/primary phase interface changes from in-coherent to semi-coherent.The high solidification pressure can form the closer contact between the molten metal and the primary phase,and significantly reduce the interfacial free energy between the primary phase and the metal liquid.The matrix phase nucleates on the surface of the primary phase with semi-coherent interfacial heterostructures.Moreover,as the solidification pressure increases,the lattice mismatch between the matrix and the primary phase decreases,which is beneficial to stabilize the semi-coherent interface until the end of solidification.After solidified at 7 GPa,the coercivity of FeCoNiSi alloy decreases from 62 Oe to 14 Oe,and the initial permeability is increased from 3190 to 6350.In addition,the primary phase was significantly refined after solidified at 7 GPa,and its scattering at the interface with the matrix was significantly improved,and the resistivity of the alloy increased from 82μΩ·cm to 96μΩ·cm.The solidification pressure also introduces compressive stress inside the FeCoNiSi alloy,resulting in an extremely high nanometer hardness(12.06GPa)of the alloy solidified at 7 GPa pressure.As the solidification pressure increases from 0.1 MPa to 7 GPa,the curie temperature of FeCoNiAlSi alloy increases from 420 K to 535 K,and the alloy magnetization changes from unsaturated magnetization to saturation magnetization under the action of 30000 Oe external magnetic field,and the magnetization increases from 0.18 T to 0.36 T.The main reason is that after high pressure solidification,the distribution of elements in the FeCoNiAlSi alloy grains is more uniform,and the content of Fe and Co elements is close to 1:1 obviously,which enhances the super exchange of Co and Fe atoms.In addition,the FeCoNiAlSi alloy solidified at 7 GPa pressure has extremely high resistivity(342μΩ·cm)and hardness value(12.74 GPa),which is the highest compared with the soft magnetic alloy that have been used so far.Similarly,after solidified under 7 GPa,compared with the atmospheric pressure state,the saturation magnetic polarization strength of FeCoNiAlMn alloy increases by 1.7T from 1.21 T,and the Curie temperature increases from 715 K to 904 K.After high pressure solidification,the content of Al and Mn elements in the FeCoNiAlMn alloy grain increases and the distribution is more uniform,which makes more Mn elements show ferromagnetism,and improves the curie temperature and saturation magnetic polarization strength of the alloy.When FeCoNiAlSi and FeCoNiAlMn alloys are solidified at high pressure,the volume fraction of grain boundary phase in the alloy decreases,which significantly reduces the pinning effect of grain boundary on magnetic domain motion,reduces the coercivity of the alloy,and increases the maximum magnetic permeability.For example,after solidified under 7 GPa,the volume fraction of grain boundary phase of FeCoNiAlMn alloy decreases from 5.86% to 0.87%,the coercivity decreases from65 Oe to 15 Oe,and the maximum magnetic permeability increases from 52600 to116000. |
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