| Magnetic shape memory alloy with large strain output and fast response under magnetic field is a new type of smart material. This kind of alloy is promising to be candidate material for actuators and sensors. Owing to the large magnetocrystalline anisotropy and excellent machinability, Ni-Fe-Ga alloy has become especially concerned research hotspots of functional materials. In this thesis, the Ni55-x Fe18Ga27 Cox alloys with x=5.5, 5.7, 6, 6.5, 9, 10 and 12, respectively, were prepared by arc-melting and suction-casting and annealed partially. Moreover, the single-crystals with x=5.5, 5.7, 6 and 6.5 were grown and obtained in an optical floating-zone furnace. Some significant characteristics of these single-crystals were studied such as crystal structure, phase transformation, superelasticity, magnetic property, and magnetic-field-induced strain. In addition, the influence of adding Co and Cr elements on structural transformation and mechanical properties was also investigated.In Ni55-x Fe18Ga27 Cox quaternary alloy system, the Co content has an enormous effect on the crystal structure and phase transformation. In the alloys with x=9 and 10, the transformation from austenite to single martensite was induced during the cooling. The microstructure, phase transformation and mechanical behavior are sensitive to the various Co content in alloys with x=5.5-6.5, in which the metastable state mottled structures were produced during martensitic transformation. In the strain glassy alloy with x=12, no martensitic transformation was observed during the cooling.The crystal structure and phase transformation of Ni46Fe18Ga27Co9 and Ni45Fe18Ga27Co10 alloys were studied by in-situ neutron diffraction technique. It was shown that the two kinds of alloys had the same crystal structure for both parent and martensitic phase, i.e. cubic L21 Heusler and tetragonal L10 structure, respectively. Meanwhile, no intermartensitic transformation was detected in these alloys. The results provided clear and direct evidence on the stress-induced transformation from parent phase to martensitic phase at room temperature in the Ni46Fe18Ga27Co9 alloy.The microstructure, phase transformation and mechanical behaviors in the alloys with x=5.7 and 6 were investigated. The magnetic properties and magnetic-field-induced strain in the alloy with x=6 were further studied. It was shown that the structures of the pre-martensitic phase in the two kinds of alloys were different. The martensite phases with superlattice diffraction spots were produced during transformation in the alloys with x=5.7 and 6. The superlattice structures were named mottled structures. Moreover, the superlattice diffraction spots of the mottled structures of the two alloys were different. The mottled structures were metastable and transformed to more stable structure as the temperature continued to decrease. A small part of mottled structure transformed to 7-layered modulated martensite in stable state at 133 K in the alloy with x=5.7. However, in the alloy with x=6, the mottled structure transformed to more stable two-phase coexistence structure(non-modulated and 7-layered modulated martensite). The single crystal with x=6 undergone two-step transformations and one-step variant selection during loading at 150 K, transformation from 7-layered modulated martensite to non-modulated martensite, the variant selection in the non-modulated martensite, transformation from non-modulated martensite to 4-layered modulated martensite. The stress–strain curves for the single crystals with x=5.7 and 6 at various temperatures were measured by mechanical test. The experimental results showed that the yield stress decreased with decreasing temperature when the performance temperature is above the starting temperature of martensitic transformation(Ms) in these alloys. However, the yield stress increased with the decrease of temperature when the test is conducted below the starting temperature of martensitic transformation in these alloys. At 243 K, the yield stresses with the minimum value of the alloys with x=5.7 and 6 were 20 MP and 5.5MPa respectively. The alloy with x=6, compared to x=5.7, exhibiting a smaller yield stress, is more likely to have a strain output under the magnetic field. Thus, the magnetic property and magnetic-field-induced strain were studied for this alloy with x=6. The experiment results showed that the thermo-magnetization(M-T) curves for the zero field cooling(ZFC) and field cooling(FC) were seperated at the low temperature. The strain accompanying the martensitic transformation was weakened by the application of a magnetic field of 1 T. The path of the transition in the field can be remembered by the alloy after removal of the field. The time dependence of magnetization at the constant temperatures was observed during the phase transformation. The time dependence of magnetic-field-induced strain was also observed. Meanwhile, the value of magnetic-field-induced strain decreased with decreasing temperature. These behaviors can be explained by the theory that the transformation from mottled structure to non-modulated martensite was induced by the magnetic field. Furthermore, we used the in-situ high-energy X-ray technique to study the crystal structure and phase transformation in the alloys with x=5.5 and 6.5. The experimental results showed that the high-temperature cubic phase, which was dominant, and pre-martensitic phase coexisted at room temperature in these alloys. In the alloy with x=5.5, in which the austenite failed to transform completely, coexisting non-modulated martensite, modulated martensite, and residual austenite can be observed at 150 K. In the alloy with x=6.5, in which the austenite transformed completely, the modulated and non-modulated martensitic structures coexisted at 150 K.In situ neutron diffraction and high-energy X-ray technique was used to study the deformation behavior in a Ni43Fe18Ga27Co12 alloy with strain glassy state in order to reveal the intrinsic physical nature of the quasi-linear superelasticity. A significant modulus softening prior to the stress-induced martensitic transformation was observed during compression test in the studied alloy, which is similar to the characteristics exhibited in the tweed precursor phenomena prior to temperature-induced martensitic transformation. Moreover, the diffraction peak broadening was further evidenced during the elastic stage of deformation for both single-crystal and polycrystalline samples, which mainly stems from the short-range fluctuations in the strain field inside each grain based on the Williamson-Hall analysis. We believe that there exists a spatial heterogeneity in modulus in the confined martensitic transformation alloy.The substitution of B for Ni in Ni46-x Fe18Ga27Co9Bx(x=0, 0.1, 0.5) alloys is proved to be effictive in increasing the compressive strength and ductility. Meanwhile, the martensitic transformation temperatures decreased with increasing B content in these alloys. The substitution of Cr for Ni in Ni46-x Fe18Ga27Co9Crx(x=0.5) alloy, followed by proper heat-treatment process, can cause abnormal grain growth. The grain size can reach 2mm. This discovery provides a new way for the preparation of single crystals. The addition of Cr element resulted in the increase of the yield strength. |
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