| Since Ullakko et al reported0.2%magnetic-field-induced strain (MFIS) in Ni2MnGa single crystals in1996, NiMnGa ferromagnetic shape memory alloys have attracted lots of attention as a new member of potential actuator materials for approximately20years. Up to now, maximal MFIS of9.5%was found for modulated martensite in NiMnGa single crystals, which almost reach the maximal theoretical MFIS of10.6%. However, there are some shortcomings for NiMnGa alloys. Firstly, the preparation of single crystalline is a slow process of crystal growth. Secondly, it is difficult to get homogeneous modulated NiMnGa single crystals because the chemical segregation occurs easily for multicomponent alloys during single crystal growth.Some methods may be useful for NiMnGa alloys overcoming these shortcomings shown above. Fabricating non-modulated structure NiMnGa alloys with a low twinning tress can avoid the chemical segregation of modulated structure alloys. For example, non-modulated NiMnGa alloys may show low twinning tress after doped of other element. Otherwise, pores NiMnGa polycrystalline alloys show MFIS similar to single crystalline. Since grain boundaries hinder twinning boundary motion in MFIS, and grains boundaries can eliminated via amount of pores, adding porosity to polycrystalline may be a solution for the problem. By developing effective technique associated with the preparation of metal porous materials, one can get pores NiMnGa alloys which exhibit MFIS similar to single crystalline.The aim of the present work is to advance the non-modulated NiMnGa alloys with porous structure towards magnetic-field-induced strain effects. The basis of this object are that there is excellent production technology of porous structure and the non-modulated porous NiMnGa alloys show low twinning tress.Since one of the aims of the present thesis is to seek NiMnGa alloys with low twinning stress, the structure and martensitic transformation of Co/Cu-doped NiMnGa alloys were investigated. It was found that, by doping Co for Ni, NiMnGa alloy martensitic transformation temperatures decrease. By doping Cu for Mn, martensitic transformation temperatures increase. The martensitic transformation temperature change (ΔT) induced by Cu doping is larger than that by Co doping. It was revealed that not only the martensitic transformation temperatures, but also the lattice parameters could be adjusted by Co and Cu doping into NiMnGa alloys. Meanwhile, Ni46Mn28-xGa22Co4Cux alloys have a typical V-shaped change for the function relation between the crystal distortion ratio c/a and x. The minimal crystal distortion ratio (c/a=1.1445) and martensitic transformation temperature range (about1K) were found for x=4alloy. The alloy exhibits about1000ppm thermal-induced strains in martensitic transformation during cooling and reverse martensitic transformation during heating.In this thesis work, it also is found that the grains of as cast NiMnGaCoCu alloy ribbons prepared by melt spinning technique show some preferred orientations, which cannot be completely reduced after the ribbons was annealed at1173K. There are different grain size distributions in wheel surface and inside the ribbons. The grain sizes in wheel surface don’t change up to973K. When the ribbons annealed at1173K, a drastic grains growth was found. However, the grains inside the ribbons show a gradual growth pattern. The different grain growth behaviors lead to an inconsistence between the microstructure change and the magnetic domain structures change in wheel surface.In this work, martensite variants states of NiMnGaCoCu alloys ribbons were studied by the magnetic force microscopy (MFM) measurements. By annealing at1173K, large columnar grains were obtained for non-modulated NiMnGaCoCu alloy ribbons. Due to the large grain size, the magnetic domains structure changes corresponding to different martensite variant states could be clearly observed when a magnetic field was applied to the ribbon at ambient temperature. The martensite variants state change and "training" of the variants under magnetic field were studied. It was found that a small magnetic field (1kOe) can induce martensite variant rearrangement.In order to get pores NiMnGa alloys, a composite of NaAlO2powders and NiMnGa alloys was prepared by the method that Ar gas compress liquid metal into the gap of powders. The NaAlO2powders can be removed from the compound by ultrasonic cleaning. It was found the ultrasonic cleaning is time-consuming and often leads to the cracks of sample. In order to overcome these shortcomings, we put forward the hydrothermal pore architecture. Because NaAlO2transform to α-A12O3when heat treated at high temperature during the preparation of pores NiMnGa alloys, the chemical properties of α-Al2O3were studied. Experiment results show that cc-A12O3can be removed by NaOH in reaction kettle at optimized temperature. It was found that the NiMnGa alloys prepared by the hydrothermal pore architecture maintain original properties such as crystal structure and transformation temperatures. Moreover, the hydrothermal pore architecture can perform these benefits of speediness, easy way and low cost to the pores NiMnGa-based shape memory alloys manufacturing process. |
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