| Nanocomposite permanent magnetic materials are composed of hard magnetic phases with high anisotropic field and soft magnetic phases with large saturation magnetization. In compared with the traditional sintered Nd-Fe-B magnets, Nd-Fe-B based nanocomposite magnets have received much attention for their potential applications because of their enhanced remanence and maximal energy product, low rare earth content, low cost and high corrosion resistance.In this dissertation, nanocomposite Nd2Fe14B/α-Fe(Fe3B) type hard magnetic alloys have been prepared by melt-spinning technique and subsequent crystallization annealing. With the help of X-ray diffraction (XRD), vibrating sample magnetometer (VSM), differential scanning calorimeter (DSC), transmission electron microscope (TEM), and thermal magnetic analyzer (TMA), the effects of Ti and C additions on the microstructure and magnetic properties of the alloys have been investigated, and the influences of Nd content, B content, partial substitution of Nd by Pr, and the addition of refractory elements such as Nb, Zr, and Cr together with C on the microstructure and magnetic properties of the alloys have also been researched. At the end of this dissertation, the magnetization and demagnetization behavior, the magnetic properties of magnetic powders and their bonded magnets, of Ti and C doped nanocomposite alloys have been studied.The results show that the formation of unfavorable soft Nd2Fe23B3 and Fe3B phases can be suppressed effectively by the addition of Ti. When Ti content reaches a certain amount, Ti may precipitate as TiB2 from the Nd9.4Fe79.6B11 alloys, which can refine the structure, thus the exchange coupling between the hard and soft phases is enhanced. As a result, amazing magnetic properties of Br=0.87T, iHc=931.1kA/m, and (BH)max=115.4kJ/m3 are achieved for Nd9.4Fe75.6Ti4B11 alloy ribbons. Additional C addition in Nd9.4Fe75.6Ti4B11 alloys can suppress the formation of TiB2 by the preprecipitation of TiC. This can release abundant boron from TiB2 to insure the formation of hard Nd2Fe14B phases. The results also show that Ti and C addition changes the crystallization behavior of Nd9.4Fe79.6B11 alloys, leading to the simultaneous precipitation ofα-Fe and Nd2Fe14B phases from amorphous Nd9.4Fe75.6Ti4B10.5C0.5 alloys. This behavior can avoid the growth of the first precipitating phases and let the soft and hard phases grow together, which is of avail for attaining a fine and uniform microstructure. Excellent magnetic properties of Br=0.91T, iHc=975.6kA/m, and (BH)max=135.4kJ/m3 , together with a fine microstructure with a average grain size of about 15nm have been attained in Nd9.4Fe75.6Ti4B10.5C0.5 ribbons. The relationship between the microstructure and the crystallization behavior of the alloys has been discussed by using the kinetic theories. The results point out that the values of kinetic parameters are altered with the addition of Ti and C, which change the crystallization type from difficult nucleation and easy growth pattern to easy nucleation and difficult growth pattern, and this difficult nucleation and easy growth pattern may be the main reason why fine and even microstructure is gained in Ti and C doped alloys.The influences of Nd content, B content, partial substitution of Nd by Pr on the microstructure and magnetic properties have also been researched. The results reveal that the distribution, content, and grain size of hard and soft phases are remodeled with the change of Nd and B contents, thus alter the exchange coupling between hard and soft phases, resulting in ribbons with different magnetic properties. The ribbons with nominal composition of Nd9Fe76Ti4B10.5C0.5 have the best magnetic properties. Partial substitutions of Nd by Pr do not change the constitution of alloys after crystallization. But microstructures with an inhomogeneous and coarse grain size are gained with the substitution of Pr, which is of disadvantage for the increase of coercivity. Pr has little effect on the magnetic properties of (Nd1-xPrx)9.4Fe75.6Ti4B10.5C0.5 ribbons, ribbons with Br between 0.86T and 0.90T, iHc about 1000kA/m, and (BH)max between 130kJ/m3 and 136kJ/m3 are all achieved with different ratio of Nd and Pr following their optimal crystallization annealing, which implies that it is possible to prepare high performance, low rare earth content nanocomposite Di-Fe-Ti-B-C permanent alloys with didymium directly. For nanocomposite Re-Fe-B-Ti-C alloys with 89.5at.% rare earth metal, 4at.% titanium, 0.5at.% carbon, and 8.512.0at.% boron, an average energy product of about 130kJ/m3 would be attained, following over melt spinning and subsequent crystallization annealing. Especially when the rare earth metal content is 9at.%, and boron content is 10.5at.%, a high coercive above 1000kA/m could be achieved.Stable particles can appear in all nanocomposite Nd9.4Fe79.6B11 alloys with Ti, Nb, Zr, and Cr addition, which can avoid the formation of Nd2Fe23B3 and Fe3B phase, refine the structure, and thus increase the exchange coupling interaction between the soft and hard phases. The addition of Ti, Nb and C are found to be particularly effective in increasing the coercivity without sacrificing much remanence, but the effects of Zr, Cr and C additions are negative.Through the investigation of the initial magnetization curves, the relationship between coercivity and the magnetized field, the recoil curves, the reversible and irreversible portions of demagnetization curves of the ribbons, it founds that the coercivity mechanism of Nd9.4Fe79.6B11 alloys is dominated by both nucleation and domain wall pinning, and the larger domain wall pinning field determines the coercivity. But in Ti or Ti and C doped alloys, the coercivity mechanism is only controlled by domain wall pinning. The thin interface phases between the crystal grains may be responsible for the magnetic hardness for alloys with Ti and C additions.Excellent thermal stability has been attained in magnetic powders containing Ti and C, whose temperature coefficient of remanence and coercivity between 25100℃are about the same with that of MQP-C and MQP-D magnetic powders, but better than that of MQP-15-7 and MQP-16-7 nanocomposite magnetic powders. The excellent thermal stability may due to the change of microstructure by Ti and C additions. The results also reveal that nanocomposite magnetic powder containing Ti and C has more outstanding antioxidation properties than that of MQP magnetic powders, and is more suitable for use in high temperature environment.At last, bonded magnets are prepared with magnetic powders containing Ti and C and MQ-D. The remanence and maximum energy product of bonded Nd9Fe76B10.5Ti4C0.5 and MQ-D magnets are about the same, but the former has a larger coercivity. In contrast with MQ-D, Ti and C doped magntic powders has only 9at.% rare earth, and does not contain cobalt, which presents a property of high performance with low cost.
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