| As a novel type of multi-functional smart materials for potential uses in sensor and actuator devices, Ni-Mn-Ga alloys have attracted considerable attention due to their large output and quick response under magnetic field. These alloys also demonstrate large change in magnetic entropy under the magnetic field at the vicinity of the magneto-structural transformation and thus a giant magnetocaloric effect, which could be potentially exploited for magnetic refrigeration applications.During the last two decades, numerous experimental and theoretical studies have been conducted on the composition-dependent magnetic shape memory behavior and magnetocaloric effect, the specific microstructural features and the property optimization of Ni-Mn-Ga alloys. It has been revealed that the excellent shape memory effect is originated from the reorientation of martensitic variant driven by magnetic field, and the giant magnetocaloric effect is resulted from the strong coupling between magnetic transition and structural transformation. So far, the field induced output strain and the magnetocaloric effect have almost reached the theoretical limit in single crystals. However, due to the complexity of lattice modulation of Ni-Mn-Ga martensite and the limitations of traditional characterization techniques, it is quite difficult to make a quantitative analysis on microstructural elements and crystallographic orientations, thus resulting in a lack of a direct correlation between them, which greatly hinders the further development of these alloys towards practical application through polycrystallization and texturation.In this work, based on the spatially resolved electron backscatter diffraction (EBSD) technique, an effective method for characterizing the orientation relationships between martensitic variants and the crystallographic features of inter-variant interfaces in modulated martensite was proposed. It was applied to detailed analysis on the microstructural features and the transformation crystallography of martensite in polycrystalline Ni-Mn-Ga alloys. Furthermore, possible routes for the texturation of polycrystalline alloys via external field treatment were explored, and the composition dependent magnetocaloric effect of melt-spun ribbons and the property improvement through microstructure control were demonstrated. The innovative results are summarized as follows:1. Based on the minimum shear criterion, a general method to calculate complete twinning elements existing in various crystal systems was developed. By introducing the crystallographic equivalent relation, the uncertainty in identifying specific interfaces was solved for the indirect two-trace analyses method. The superstructure information on the modulated Ni-Mn-Ga martensite was applied for automatic EBSD orientation mapping. The variant configurations, twin relationships and twin interfaces of five-layered modulated (5M), seven-layered modulated (7M) and non-modulated (NM) martensite were studied.2. By considering the lattice distortion of martensitic transformation and the structural modulation of modulated martensite, a method to predict the orientation relationship of the austenite to martensite transformation from the orientation of modulated martensite was proposed without recourse to the residual austenite, and the most favorable orientation relationship between parent austenite and martensite was fully determined. Moreover, the characteristics of microstructure evolution during the martensitic transformation were analyzed, and the self-accommodation mechanism of martensite was further elucidated.3. According to detailed microstructural and crystallographic characterizations, a long-term debate concerning the crystal structure nature of 7M martensite was clarified. The 7M to NM intermartensitic transformation was evidenced to be a structural transformation other than the branching of the nanotwinned NM variants. Moreover, the composition dependence on the phase stability of 7M martensite and its role in bridging the austenite to NM martensite transformation were demonstrated. It was revealed that the non-coherent inter-plate interface character of NM martensite and the large lattice distortion between austenite and NM martensite impose insurmountable barriers to the direct transformation from austenite to NM martensite, which could be explored to enlarge the existing temperature range of 7M martensite.4. By means of thermal-magnetic and thermal-mechanic treatment, possible processing routes for the microstructure control and texturation of polycrystalline Ni-Mn-Ga bulk alloys were explored. Results show that the thermal-magnetic treatment can effectively reduce the variant number of 7M martensite in suction-cast alloys. For directionally solidified alloys, the compression loading along the solidification direction during the martensitic transformation can induce the [010]7M of 7M martensite in parallel to the loading direction, which resulted in the formation of the strong<010>7M texture.5. By the composition tuning, the Ni52Mn26Ga22 ribbons with the co-occurrence of magnetic transition and structural transformation (i.e. magneto-structural transformation) were prepared by melt-spinning, showing the maximum magnetic entropy change of-11.4 Jkg'K"1 at a magnetic field of 5 T. After further high temperature annealing, the maximum magnetic entropy change was elevated to-29.9 Jkg-1K-1 at 5 T, due to the improvement of atomic ordering and the introduction of the intermartensitic transformation into the magneto-structural transformation,. This giant magnetic entropy change represents the highest value reported for ferromagnetic Ni-Mn-Ga ribbons.The present work has provided new insight into the crystallographic nature of Ni-Mn-Ga alloys, enriched the fundamental theory of martensitic transformation and extended the crystallographic characterization methods. On such basis, some prototypes on microstructure control could be established towards the functional property optimization. |
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