Structure Evolution Of Fe-B-(Ni,Si,P,Cu)Amorphous Alloys During Annealing And Its Effect On Magnetic Properties

With the rapid development of electronic power industry,the requirements for properties of soft magnetic alloys have risen.In this thesis,the structure evolution of Fe-B-(Ni,Si,P,Cu)amorphous alloys during annealing and its effect on the magnetic properties are investigated.The structural mechanism of the changes in magnetic properties is also studied,which is helpful to optimize the soft magnetic properties of the alloysThe effects of heating rate on the atom migration,the phase structure and magnetic properties in the Fe82Si2B11P4Cu1 alloy are investigated.Annealing the samples at a high heating rate promotes the formation of Cu-rich regions and delays the phase separation process to high temperature,which is conductive to increasing the volume fraction of the nanocrystalline phase and refining nanograins during the crystallization.For the nanocrystalline alloy isothermally annealed at 758 K for 180 s,the saturation magnetic flux density increases from 1.74 T to 1.79 T and the coercivity decreases from 7.8 A/m to 4.4 A/m with the heating rate increasing from 0.5 K/s to 20 K/s.Moreover,it is confirmed that the occupation of Si atoms in the bcc-Fe lattice can be suppressed by the annealing with high heating rate.However,it is found that the content of the nonmagnetic atom enriched regions rises with the increase of heating rate,which can reduce the magnetic interaction in the nanocrystalline ribbons Therefore,excessively high heating rate will deteriorate the soft magnetic properties of the alloyBased on the conventional rapid annealing,the effects of cyclic annealing on the crystallization behavior,microstructure and magnetic properties of Fe-B-Cu alloys are studied.It is found that the cyclic annealing is more effective to refine and uniform a-Fe grains by introducing multiple rapid heating than the non-cyclic rapid annealing.The saturation magnetic flux density of the Fe83.5B15Cu1.5 nanocrystalline alloy increases from 1.79 T to 1.82 T and the coercivity simultaneously decreases from 13.1 A/m to 7.5 A/m when the cyclic numbers change from C=1 to 6.Furthermore,it is confirmed that the rapid cyclic annealing can inhibit the aggregation of B atoms in the amorphous matrix,which is beneficial to enhance the magnetic interaction between a-Fe nanocrystals.However,the excessive cyclic annealing will increase the average grain size and deteriorate the soft magnetic properties of the Fe83.5B15Cu1.5 nanocrystalline alloyFinally,with the substitution of Ni for B,we study the crystallization behavior of the Fe-(Ni)-B-Cu alloys and its effect on the microstructure and magnetic properties.The substitution of Ni for B in the Fe83B16Cu1 alloy enriches Fe clusters in melt-spun ribbons and promotes the nucleation of a-Fe,which is beneficial to the precipitation of nanograins with high volume fraction during crystallization.Moreover,the addition of 1-2 at.%Ni widens the optimum annealing temperature range for the nanocrystalline alloys.Excellent nanostructure and magnetic properties are obtained by annealing the Fe83Ni2B14Cu1 melt-spun ribbons at 683 K.In addition,the nanocrystalline alloy with 2 at.%Ni possesses a saturation magnetostriction(19 ppm)lower than that of Ni-free alloy(25 ppm).