| Nd-Fe-B permanent magnets have been widely used in various fields due to their excellent magnetic properties.The conventional fabrication of Nd-Fe-B magnets is based on the powder metallurgy process.However,all those methods have complicated processes,and the prepared magnets generally exhibit some powder metallurgical imperfections which are harmful to the magnetic properties.On the other hand,the requirement of the minor magnets with size of 110 mm is increasing due to the rapid development of the electronic information industry.Hence,it is important to develop a new preparation method with low cost and simple process to prepare fully dense rare earth magnets.Since last century,preparing minor magnets of amorphous Nd-Fe-Al and nanocrystaline Nd-Fe-B based alloys by copper mold direct-cast process has been proposed,and some great progress has been achieved.However,up to now,the hard magnetic properties of the directly cast alloys are still in need of improvement and some physical mechanisms associated with the amorphous and nanocrystalline magnets are still not clear.In this thesis work,the Nd-Fe-Al and Nd-Fe-B magnets with the amorphous,nanocrystalline and microcrystalline structures and various sizes were successfully prepared by copper mold casting.The relationships among the processing,composition,microstructure and properties of the cast magnets were systemically investigated.The magnetic characteristics and the coercivity mechanisms for various alloys were also studied.The effects of Ce substitution on the magnetic characteristics and metallurgical behaviors of the cast Nd-Fe-B alloys were deeply studied.Firstly,the hard magnetic Nd70-xFe30Alx?x=010?magnets with a size of?2 mm and amorphous or partly amorphous structure were prepared by injection casting.Their room-temperature coercivity mechanisms and the magnetic characteristics of nano-clusters in the alloys are investigated.The results indicate that high coercivity originates from the ferromagnetic nano-clusters distributed in the amorphous matrix.Strong exchange coupling among the clusters improves their thermal stability,and thus,increases the Tblcok to higher than room temperature,which is responsible for the ferromagnetic properties at room temperature.The temperature dependence of the coercivity could be well explained by the strong pinning model of domain walls of Gaunt.Nanocrystalline hexagonal Nd phase works as the pinning center.Glass forming ability of the alloys and the amorphous content increase with the increasing Al content,which leads to the lower content of crystallized hexagonal Nd phase,less pinning centers,and lower coercivities.It was also found in these alloys that the 5K M-H loops after field cooling shift upwards compared with the loops without field cooling due to that parts of the clusters'moments are blocked.Secondly,Co and B were added into the Nd-Fe-Al alloys,and the effects of the Nd and B contents on the microstructure and magnetic properties of the Nd-Fe-Co-Al-B alloys prepared by injection casting were investigated.Among the?2 mm Nd25Fe40Co20Al15-xBx?x=715?alloys,the as-cast x=11 alloy has the highest coercivity of 1140 kA/m.Then,after heat treated at 750?for 10 min,the x=10 alloy has the highest coercivity of 1437 kA/m.The nanocrystalline 2:14:1 grains,whose sizes are close to the single domain size,should explain its high coercivity.The magnetic anisotropy of the cast alloy is successfully induced by the thermal gradient and hot deformation.Cast within a magnetic field refines the microstructure of the alloy,which,in turn,improves the magnetic properties.Thirdly,nanocomposite Nd-Fe-Co-Ti-Nb-B-C alloys were prepared by suction casting to further improve the magnetic properties.The effects of C and Nd contents on the magnetic properties and microstructure of those nanocompiste alloys were studied.The cast alloy is composed with 2:14:1 and amorphous phases.The addition of C element enhances the glass forming ability of the alloys and helps to form nanocrystalline 2:14:1 grains in the surface area of the alloys,which is the reason for the increased coercivity.For the as-cast?1 mm Nd9.5Fe61.5Co10Ti2.5Nb0.5B15.5C0.5 alloy,the magnetic properties are Mr=60.2 A·m2/kg,Hc=1068 kA/m and?BH?max=42 kJ/m3.While,the increasing Nd content in the alloy decreases the Ms of the big area grain boundary amorphous phase,which is beneficial for the magnetic properties.For the as-cast?1 mm Nd11Fe60Co10Ti2.5Nb0.5B15.5C0.5 alloy,the magnetic properties are Mr=64.0 A·m2/kg,Hc=600 kA/m and?BH?max=52 kJ/m3.Both experimental and micromagnetic simulation results indicate that the thin and non-magnetic grain boundary phase is beneficial for the magnetic properties of nanocomposites alloy.In addition,a novel anisotropic nanocomposite structure with nanoscale amorphous phase remaining inside the micro-sized 2:14:1 grains was observed.It may provide an alternative approach to design and fabricate anisotropic nanocomposite Nd-Fe-B-based permanent magnets with enhanced magnetic properties.At last,to further decrease the cost of the magnets,rare earth Ce was used to substitute the Nd in the cast Nd-Fe-B alloys.Both melt spinning and injection casting methods were used to prepare the RE-rich Nd-Ce-Fe-B alloys with different Ce contents.Magnetic characteristics and metallurgical behaviors of the Ce-containing magnets were investigated.Hyperfine structures of the(NdxCe1-x)2Fe14B phases were studied by 57Fe Mossbauer spectrometry.Magnetic properties of both as-spun and as-cast alloys decrease with the increasing Ce content.However,among the as-spun(Ce1-xNdx)16Fe78B6?x=00.7?alloys,high coercivities of 623 kA/m and 1206 kA/m are achieved in the high Ce-contained x=0.1 and x=0.4 alloys.Among the as-cast?2 mm(Nd1-xCex)25Fe40Co20Al4B11?x=00.7?alloys,the coercivity of x=0.3 alloys shows an abnormal increase,from 641 kA/m of x=0.2 to 863 kA/m of x=0.3 alloy.This abnormal increase could be attributed to the phase separation in the2:14:1 phase.Both the experimental and micromagnetic simulation results prove that the existence of phase separation can improve the coercivity of the alloy.In addition,elemental segregation of RE is observed in both as-spun and as-cast alloys.It is found that the Nd element prefers to enter into the 2:14:1 phase and the Ce element prefers to enter into the grain boundary phases.What's more,the existence of the 1:2 phase will enhance the elemental segregation.This effect will increase the intrinsic properties of the 2:14:1 phase,which,in turn,increases the magnetic properties of the alloys. |
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