Effect Of Magnetic Field Heat Treatment On Magnetic Properties And Microstructure Of Fe-Nd-Nb-B-M(M=Ga,Zr) Bulk Alloys

Fe-based amorphous alloys have received widespread attention for their excellent soft magnetic properties.However,the Fe-based amorphous alloys containg rare earth elements（Re-Fe-based）exhibit excellent permanent magnetic properties after crystallization,which have aroused the research enthusiasm of many scholars at home and abroad.Though a variety of Re-Fe-based amorphous alloy systems were developed,there are still some problems in these alloys,such as weak amorphous forming ability and poor magnetic properties after crystallization.Moreover,the uncertain relationship between magnetic properties,phase composition and microstructure of the alloys,as well as the lack of effective methods to enhance their magnetic properties,seriously hinders their industrial applications.To solve above problems,the Fe_69.5-xNd₇B₂₁Nb_2.5M_x（M=Ga,Zr）bulk amorphous alloys were produced,and the magnetic properties,phase composition,microstructure,and atomic distribution of the as-cast and annealed alloys were investigated using vibrating sample magnetometer（VSM）,X-ray diffractometer（XRD）,transmission electron microscope（TEM）and atomic probe tomography（APT）.In addition,the influence of Ga and Zr elements addition on structure and properties of the alloys were studied.Specially,the effect of magnetic field heat treatment on the magnetic property,phase composition and microstructure of the alloy was comprehensively analyzed.What`s more,the correlations between temperature,magnetic field intensity and the magnetic properties,phase composition and microstructure of these alloys were established.We can draw the following conclusions:（1）Fe_69.5-xNd₇B₂₁Nb_2.5M_x（M=Ga,Zr）bulk amorphous alloys were prepared by a suction cast method,and it was found that an appropriate amount of Ga（x=0.6）and Zr（x=1.0-2.0）addition can effectively improve the amorphous forming ability of the alloys.Furthermore,the saturation magnetization（M_s）of the as-cast alloy increases with the addition of Ga,while it decreases with the addition of Zr.（2）Fe_69.5-xNd₇B₂₁Nb_2.5M_x（M=Ga,Zr）bulk alloys change from soft magnetic to hard magnetic after crystallization annealing treatment at different temperatures.In the Fe_69.5-xNd₇B₂₁Nb_2.5Ga_x（x=0-1.0）alloys,the hard magnetic phase Nd₂Fe₁₄B,soft magnetic phaseα-Fe and non-magnetic phase Nd_1.1Fe₄B₄,precipitate after annealing.Moreover,the intrinsic coercivity(H_ci)decreases gradually,while the remanence（B_r）,squareness(H_k/H_ci)and maximum magnetic energy product（BH）_max increase first,and then decrease with the increase of Ga content after annealing.When x=0.3（alloy Ga-0.3）,it shows the best comprehensive magnetic properties:H_ci=368.68 k A/m,B_r=0.64 T,and（BH）_max=33.73 k J/m³.In the Fe_69.5-xNd₇B₂₁Nb_2.5Zr_x（x=0-3）alloys,the addition of Zr inhibits the precipitation of Nd_1.1Fe₄B₄,and only Nd₂Fe₁₄B andα-Fe precipitate after annealing.Furthermore,B_r decreased gradually,while H_ci,H_k/H_ci and（BH）_max all increase first,and then decrease with the increase of Zr content.When x=2（alloy Zr-2.0）,it displays the best comprehensive magnetic properties:H_ci=513.92k A/m,B_r=0.57 T,and（BH）_max=37.59 k J/m³.（3）The alloy Ga-0.3,alloy Zr-2.0 and the annealed Fe_69.5Nd₇B₂₁Nb_2.5（alloy 0）were selected to conduct magnetic field heat treatment near the Curie temperature（T_c）.It was found that the（BH）_max of the three alloys after magnetic annealing below T_creaches 39.23 k J/m³,50.05 k J/m³ and 35.40 k J/m³,respectively,and compares to the alloys without magnetic annealing,it increases 16.3%,33.5%and 22.8%,respectively.Therefore,preparation of nanocrystalline permanent magnets by magnetic field heat treatment bulk amorphous alloys not only improves the magnetic properties of permanent magnets,but also provides a new way for the preparation of high-density permanent magnetic alloys,which shows a new direction for the practical application of bulk amorphous alloys.（4）XRD and TEM results show that the phase composition of the alloy 0,alloy Ga-0.3 and alloy Zr-2.0 remains unchanged,but the relative content of each phase changes significantly,and their microstructure is more uniform after magnetic field heat treatment.Compared with the the alloy whitout magnetic field heat treatment,I_（110）Fe/I_{（214）2:14:1}(the ratio of the intensity of Fe to Nd₂Fe₁₄B diffraction peaks)of the alloy 0,alloy Ga-0.3 and alloy Zr-2.0 increases by 11.7%,40.3%and 123.9%,respectively.This means that the relative content ofα-Fe obviously increases after magnetic field annealing,which may be the reason for the large increase of H_k/H_ci,（BH）_max after magnetic field heat treatment.（5）The results of APT show that the distribution of elemental atom of each phase of the alloy Ga-0.3 and alloy Zr-2.0 is more uniform and the relative content ofα-Fe obviously increased after magnetic field heat treatment.In addition,the regions of B-rich narrows,and the content of the B element also decreases.In the alloy Ga-0.3,magnetic field heat treatment promotes Ga atoms in the B-rich regions enter into the grain boundary regions,forming Ga-rich regions.In the alloy Zr-2.0,no Nd element is found in the grain boundary region,but Nb,Zr and B are enriched,while Fe is depleted in the grain boundary region.Moreover,the magnetic field heat treatment can enhance the enrichment and dilution degree of different elemental atoms in this region.（6）The effect of magnetic field intensity（H）on the magnetic properties of the alloy Zr-2.0 was studied.When H≤0.5 T,the magnetic properties of the alloy change a little;when H>0.5 T,B_r,H_ci,H_k/H_ci and（BH）_max of the alloy all increased with the increase of H.When H=1 T,the（BH）_max is 50.05 k J/m³.Moreover,with the increase of H,the grains in the alloy Zr-2.0 are finer with moreα-Fe particles precipitated,which enhances the exchange coupling effect and leads the improvement of its magnetic properties.