Study Of Structure And Hard Magnetic Properties On The Non-rare-earth Containing Co-Zr System Alloys

In this thesis, formation, structural and magnetic properties of the non-rare-earth containing Co-Zr system alloys have been investigated, based on the review of the research and development of permanent magnetic materials, using melt-spun technology and element additive method. The purpose of this paper is to search for new permanent magnetic materials without rare earth. Furthermore, the coercivity mechanism of such permanent magnets is studied also.1. Co-Zr alloy ingots were prepared by arc melting and the melt-spun ribbons were successfully produced by rapidly quenched technology. We had systematically studied the structure and magnetic properties of those alloys.From x-ray diffraction analysis of Co_100-xZr_x(x=16～20)alloy ingots and those melt-spun ribbons, we find that alloy ingots are composed of face-centered cubic Co, Co₂₃Zr₆,Co₅Zr and Co11Zr2 phase, while the melt-spun ribbons are composed of face-centered cubic Co, Co₂₃Zr₆ and Co₅Zr phase. Further analysis show that the more Co₅Zr phase the ribbon contains, the larger the coercivity it has. This indicates that the hard magnetic phase in Co-Zr alloy is Co₅Zr phase. This result has clarified the controversy for the hard magnetic phase in Co-Zr alloys and provided a basic research for further study of Co-Zr alloys. By the analysis of magnetic properties on the binary Co-Zr alloy samples with different components and quenching rates, we have obtained the optimized coercivity values of the component and quenching rate. The purpose of this work is to further study of Co-Zr alloys by additive. We had attempted to study magnetic properties of Co-Zr alloys by transition metal additive. It has been first found that the coercivity of Co-Zr alloy could be significantly enhanced by a small amount of Mo additive. From x-ray diffraction analysis of Co_82-xZr₁₈Mo_x(x=1～5) alloy ribbons, we find that the melt-spun ribbons were composed of face-centered cubic Co and Co₅Zr phase. With the increasing of Mo content, the mount of Co₅Zr phase in those ribbons increases also. When the Mo content is up to 5%, the ribbons with nearly single Co₅Zr phase structure had been first found.2. We had systematically studied the effects of Mo additive on structure and magnetic properties of Co-Zr alloy.From thermo magnetic analysis of Co-Zr-Mo system alloy ribbons, we find that the Curie temperature of Mo doped Co₅Zr phase is about 450°C. The high Curie temperature suggests that Co-Zr permanent materials have potential applications for high temperature. The x-ray diffraction analysis and magnetic measurement of those ribbons show that Mo additive leads to a decrease of the magnetization measured in the field of 10kOe. While Mo additive plays an important role in increasing the mount of Co₅Zr phase, refining the grain size and enhancing coercivity. Those results confirm that the hard magnetic phase in the Co–Zr-Mo alloys is of Co₅Zr phase. The influence of quenching rate on the magnetic properties of Co-Zr-Mo melt-spun ribbons had been investigated. It is found that with the increasing of quenching rate, the coercivity increases linearly. The reason for this case is that with the increasing of quenching rate, the mount of minor phase (Co₂₃Zr₆) and the grain size of major phase (Co₅Zr) will decrease sharply. The influence of annealing on the magnetic properties of Co-Zr-Mo melt-spun ribbons had been investigated. It is found that annealing will lead to coercivity change by the change of the mount of Co₅Zr phase and the grain size of this phase. According to magnetizing field dependence of coercivity, the coercivity mechanism is controlled by the nucleation of the reversed domain.We had broadened and refined the range of compositions studies to systematically study the structure and magnetic properties of Co-Zr-Mo melt-spun ribbons. From x-ray diffraction analysis of Co_100-x-yZr_xMo_y (x=15.5～18.5,y=4.0～6.5) melt-spun ribbons, we find that the major phase is Co₅Zr and minor phase is Co together with Co₂₃Zr₆. The composition range of the ribbons with nearly single Co₅Zr phase structure is x=18.0 y=5.0～6.0, x=16.5～18.0 y=5.5 and x=18.5 y=5.0. Coercivity, magnetization measured in the field of 10kOe and mean grain size of Co₅Zr phase as a function of composition of Cobalt-rich Co-Zr-Mo melt-spun ribbons were investigated. We can see that when the atomic proportion of Co Mo and Zr is near at 5:1, the ribbons will have the maximum coercivity value (4.7kOe). In comparison with the maximum coercivity value (3.6kOe) of the binary Co-Zr alloy, the maximum coercivity value increases near 30%. We had systematically analyzed the factors which affected the coercivity. We find that the effect of the mount of Co₅Zr phases is much less important than that of grain size. The meaning of this result is the confirmation that decrease of grain size is a effective method to increase coercivity. This study provides a method to further improve their magnetic properties.We had investigated the low temperature magnetic properties of some Co-Zr-Mo melt-spun ribbons. We find that the coercivity of the ribbons at low temperature are significantly higher than that of the ribbons at room temperature, that is to say, temperature dependence of coercivity is abnormal. According to magnetization measured in the field of 10kOe at low temperature (-196℃) dependence of composition, we find that experimental results can be in good agreement with the virtual bound state model.3. According to the theory of amorphous, the larger the difference in the size of constituent atoms, the easier the amorphous formed. So, small B atom is introduced into the Co-Zr-Mo alloys in an attempt to reduce the grain size and increase consequently the coercivity. The structure and magnetic properties of Co-Zr-B-Mo melt-spun ribbons had been studied.The largest coercivity of 7.0kOe ever for Co-Zr based alloys is observed on Co₇₇Zr₁₆B₂Mo₅ ribbons by the melt spinning. In addition, magnetization measured in the field of 10kOe increases with the increasing of the B content. From x-ray diffraction analysis of Co₇₇Zr_18-xB_xMo₅(x=0.5～4.0)melt-spun ribbons, we find that the ribbon of x=4.0 is amorphous. In the other ribbons major phase Co₅Zr and minor phase Co of face-centered cubic structure are coexist. According to thermo magnetic analysis and AC magnetic susceptibility measurements of Co-Zr-B-Mo melt-spun ribbons, we find that the Curie temperature of major phase is about 450°C, which is nearly unchanged with the increasing of the B content. The influence of annealing on the magnetic properties of Co-Zr-B-Mo melt-spun ribbons had been investigated. It can be seen that annealing at low temperature will lead to coercivity increase insignificantly while annealing at high temperature will lead to coercivity decrease significantly.The microstructure of Co₇₇Zr₁₆B₂Mo₅ melt-spun ribbons had been investigated by the transmission electron microscopy. The ribbons are of multi-crystalline structure. There are many blocky grains in the ribbons. Length and width of the grains are about a few hundred nanometers and the thickness of the grains is about several tens of nanometers. Each grain has the alternating light and dark lamellar structure, which suggests that the grains have high density of planar defects. The thickness of the planar defects is 2～5 nm. The spots in the Selected area electron diffraction(SAD)pattern demonstrate the characteristics of single crystal diffraction, but there is a broadened ring, suggesting that the ribbons is a partial crystal structure. The high coercivity may be related to the unique micro-structure of Co₇₇Zr₁₆B₂Mo₅ melt-spun ribbons.The coercivity indicates the ability of resisting the reversal field for a permanent magnets and the coercivity mechanism is key problem in permanent magnets. According to magnetizing field dependence of reduced remanence and coercivity and Henkel Plot, the coercivity mechanism is controlled by inhomogeneous domain pinning with planar defects. Unfortunately, it is impossible to calculate the theoretical coercivity according to the planar defects model because many parameters, especially in the planar defects, are unknown.