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Investigation On Microstructures, Mechanical And Magnetic Properties Of Co-Ni-Al Bases Ferromagnetic Shape Memory Alloys

Posted on:2016-05-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:J JuFull Text:PDF
GTID:1221330503977485Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
Co-Ni-Al based alloys are considered as a new kind of potential candidate materials for ferromagnetic shape memory alloys (FSMA), thanks to their rapid response and large magnetic-field-induced strain (MFIS). Compared with tradition FSMA (Ni2MnGa alloy), Co-Ni-Al alloy have several advantages, such as excellent mechanical properties and workability, etc. Now, the MFIS which Co-Ni-Al alloy obtained is relatively small. The polycrystalline Co-Ni-Al alloy can only obtained 130 ppm of MFIS in the external magnetic field, this feature seriously restrict its industrial application. Therefore, to improve the MFIS of Co-Ni-Al alloy is a research emphasis for researcher. From the point of view of the analytical model of magnetic shape memory effect (MSME), the MFIS of Co-Ni-Al alloy can be enhanced by rising the magnetocrystalline anisotropy and saturation magnetization while reducing the critical force and stress of martensite reorientation. At present, the most important and direct way is by alloying to improve the magnetocrystalline anisotropy and saturation magnetization of alloy and decreasing the critical force and stress of martensite reorientation by phase transition and phase distribution.The present of this work systematical studied the effects of the ferromagnetic alloy elements Fe, Gd, Dy and Er on the microstructures, martensite transformation, magnetic and mechanical properties by using optical microscope (OM), X-ray diffraction (XRD), scanning electronic microscope (SEM), energy dispersive x-ray (EDX), transmission electron microscope (TEM), Vibrating sample magnetometer (VSM) and electronic universal testing machine, etc. Moreover, analyses the strengthening mechanism of ferromagnetic alloy elements on the magnetocrystalline anisotropy and saturation magnetization of Co-Ni-Al system.The experimental results show that with Al of Co-Ni-Al alloy replaced by element of Fe, Gd, Dy and Er, the microstructure of Co-Ni-Al alloy can transfer from dual phase structure (i.e. β+γ phase) to β+γ+martensite ternary phase structure gradually, and finally transfer to a new dual phase structure (y+martensite). The y phase in grain boundary becomes coarse along the grain boundary as the content of alloy elements rising. But some discrete-type y phase in grain internal disappears gradually during this process. When the doping amount of alloy elements Dy and Er exceeds 0.5 at.%, the bulk-type intermetallic compound CosDy and Co2Er appears in the coarse y phase and its number continued to increase with the rise of doping amount of alloy elements Dy and Er.The valence electron concentration of alloy can be apparent increases by ferromagnetic alloy elements Fe, Gd, Dy and Er replace element Al in Co-Ni-Al alloy and result in the martensitic transformation temperature ascends. During the doping process, the valence electron concentration of alloy increases with rising of the doping concentration of ferromagnetic alloy elements, the martensitic transformation temperature of alloy gradually increased. Eventually, the martensitic transformation temperature of alloy exceeded room temperature and got martensite structure in the alloy at room temperature. In addition, the interface of martensite is spearhead-shaped and the substructure of the martensite contains high-density dislocation in it. In the process of migration, martensite interface is move along the martensite twin boundary cluster which can be form strip twin martensite.The magnetic measurement illustrates that the saturation magnetization of alloy increases with the doping amount of ferromagnetic alloy elements rising when ferromagnetic alloy elements Fe, Gd, Dy and Er doped in the ranges of 0~1.0 at.%. If the doping amount of ferromagnetic alloy element exceed 1.0 at.%, the saturation magnetization of alloy increases with the further doping of Fe. But the saturation magnetization of alloy decreases with the further doping of Gd, Dy and Er. During doping process, the magnetization of alloy is hard to saturate with the increase of the amount of the doping. At the same time, with the doping amount of ferromagnetic alloy elements increases, the magnetocrystalline anisotropy constant of alloy rises continuously, which can enhance the magnetocrystalline anisotropy energy of alloy and improves the driven force of MFIS in the alloy.The mechanical test show that the doping of ferromagnetic alloy elements Fe, Gd, Dy and Er can decrease the hardness of alloy in the initial stage. With the doping amount of ferromagnetic alloy elements increasing, the trend of alloy hardness becomes mitigating. When the doping amount of ferromagnetic alloy elements achieved and exceed 0.5 at.%, the hardness of alloy is decreasing with the further doping of Fe and Gd. Nevertheless, the hardness of alloy is increasing with the further doping of elements Dy and Er. Furthermore, the compressed rupture strength and the compressive strain of Co-Ni-Al alloy continuous rises with the increasing of the content of the doping. However, as the doping amount of Dy and Er exceed 0.5 at%, both compressed rupture strength and compressive strain of alloy decreases with continuous doping.The shape memory ratio and MFIS test show that the shape memory ratio of Co- Ni-Al alloy can be improve effectively by ferromagnetic alloy elements Fe and Gd. With the doping amount increasing, the shape memory ratio of Co-Ni-Al alloy increases consistently. However, during Dy and Er doping process, the shape memory ratio of Co-Ni-Al alloy increases first and then decreases with the doping amount rising. At the same time, Co-Ni-Al alloy has a higher shape memory ratio in lower compression or thermal mechanical cycles training repeatedly. The MFIS of Co-Ni-Al alloy increases consistently with the doping concentration of ferromagnetic alloy elements rising. When the doping concentration less than 0.5 at.%, the MFIS of Co-Ni-Al alloy increases with doping element rising. But when the doping concentration exceeds 0.5 at.%, the MFIS of Co-Ni-Al alloy gradually increases with the doping amount increasing of Fe and Gd, nevertheless, it gradually decreases with the doping amount increasing of Dy and Er.
Keywords/Search Tags:Co-Ni-Al, shape memory alloys, microstructure, martensitic transformation, magnetic-field-induced strain, shape memory ratio
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