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Preparation and Properties of Tb-Dy-Fe Magnetostrictive Materials

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Tutor: TuGanFeng LiHongWei
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: magnetostriction,directional solidification,powder sintering,strip-casting,ion n
Type: PhD thesis
Year:  2008
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Tb-Dy-Fe magnetostrictive materials are the core materials used for producing underwater acoustic transducer, precision actuator, intelligent sensor and the like devices, and they are widely used in high-tech fields such as military equipment, ultrasonic detection, precision automation and robot etc.. In china, the research on Tb-Dy-Fe magnetostrictive material is done with a later start. Up to now, the preparation technologies are not advanced enough, the main problems are the higher producing cost, the lower performance than international level, the less application scale and the inferior properties which can not meet the requirements of the high-grade devices. In this paper, the research involving the effects of the processing parameters on microstructures and properties was carried out by using the " one-step " directional solidification process, casting-powder sintering method and strip-casting binding method, thus it can provide the experiment foundations for preparing the materials with high quality. Furthermore, the surface-modified treatment for Tb-Dy-Fe material is also investigated in this paper.The effects of directional solidification rate, alloy composition and heat treatment systems used in the "one-step" directional solidification process on the microstructures and properties are studied, and the preferred processing parameters are obtained. When the directional solidification rate is 70mm/h, the crystal growth mode is a cellular morphology, and the material has a<110> preferred orientation and thus can obtain better properties. When the directional solidification rate is 110-150mm/h, the crystal growth mode is still a cellular morphology, but the<110> orientation becomes weak while the<113> orientation enhance. When the directional solidification rate is 200mm/h, the crystal growth mode is a developed dendrite morphology, thus the material fails to get a preferred orientation and thereby obtain an inferior performance. For the (TbxDy1-x)Fe1.95 alloys, the anisotropism of the magnetocrystalline is the lowest when x=0.3, and the properties of materials in which x is deviated from the value are worsened to some extent. By increasing the ratio of Tb/Dy, the anisotropy constant can be increased and the spinning reorientation temperature Tr can be decreased, thus the material has better properties at lower temperature. When x is 0.3, the magnetic field is 160kA/m and there is no compression stress, the temperature coefficients for this magnetostrictive material is very small(-1.9¡Á10-6/¡æ), so this material is suitable for0 using in the specific working circumstance which has lower and sharply changed temperature(e.g.-50¡æ-40¡æ). After the heat-treatment at 900¡æand 950¡æ, the RE-riched phase in this alloy is changed from netlike distribution to spherical distribution, thus the phase interface between this phase and the main phase is decreased, and the magnetostrictive performance is markedly enchanced. The heat-treatment at higher temperature (e.g.>1000¡æ) can lead to a segregation of the second-phase REFey(y=3-5) in the alloy, and the performance is barely enchanced. By using the metal with higher purity and the preferred processing parameters(e.g.V=70-85mm/h, heat-treatment at 950¡æfor 2hs), a rod-shaped (Tb0.3Dy0.7)Fe1.95 material (¦Õ50mm) with advanced world standard is obtained, wherein, it¡¯s¦Ëis up to 1380¡Á10-6 under lOMPa and 80kA/m, and it can be used in high power underwater-acoustic-transducer and magnetostrictive driving-valve etc..Moreover, the relationship between the casting-powder sintering producing process and the material properties therefrom is also studied. When the particle size is smaller than 0.147mm, the performance and the density of the sintered sample is higher, and the magnetic field has the most effect on the orientation of<111> crystal orientation, furthermore, the degree of the orientation is up to 40.56%. Increasing the isostatic-pressure and extending the pressing time are helpful to improve the performance and the density of the sample. When the sintering is carried out at 1200¡æ, the material with higher performance can be obtained. With the external magnetic field of 240kA/m and the pre-stressing force of 8.0MPa, the magnetostriction for this sample can up to 1613¡Á10-6 which is the leading national level.The strip-casting technology for preparing TbDyFe bonded magnetostrictive material is put forward in this paper. And the relationship between the process and the material properties is studied. As the cooling speed increase, the crystal growth mode will transform from the dendrite morphology to the cellar morphology, and the preferred orientation changes from<112> to<110>, wherein, the sample with a preferred orientation<112> has the better performance than that with a preferred orientation<110>. The binder with small elastic modulus (e.g. epoxy resin) is helpful for the performance, and the optimized concentration of it is 4%.The particle size of the powder has some effect on the performance, and the preferred particle size of the powder is¡Ü100-150¦Ìm. The optimized curing temperature is 150¡æand the curing time is 2hs. The reorientation resulted by the magnetic field has a good effect on the performance. The binding material with higher performance is obtained through this research, and the¦Ëis up to 1380¡Á10-6 under the magnetic field of 240kA/m.The surface modifying on the magnetostrictive material by ion nitriding process is performed for the first time. By ion nitriding process, a FeN compound layer is formed on the surface of the material, and thus it can delay the corrosion speed of RE-riched phase under acid circumstance. And the anti-corrosion property, surface hardness and wearable property of the sample are remarkablely improved, wherein, the said surface hardness can be increased. from 587HV to 622HV. Ion nitriding process on the surface nearly has no effect on the property of the material, so it is a better surface-treating method to modifying Tb-Dy-Fe magnetostrictive material.
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