Investigation Of The Microstructure,Thermal-induced And Magnetocaloric Characteristics In Ni-Mn-Ga Microwires

In the present thesis, one-dimensional small size Ni-Mn-Ga microwires were fabricated using the modified melt-extraction method, the composition and processing parameters were systematically investigated and optimized. The microstructural evolution, martensitic transformation and magnetic properties were investigated. Chemical ordering annealing, grain growth heat treatment and training processes were carried out and the corresponding properties, including the magnetic properties, superelasticity and twin boundar y motion, were studied. Fully and partial coupled magnetic and structural transition were obtained and the corresponding magnetocaloric effects were investigated.It was found that during the melt-extraction process, the shape of the product was affected by the wheel velocity and the heating power and t he uniformity of the microwires was found to be related to th e Rayleigh waves. Here, continuous and uniform microwires with semicircular D-shape cross-section were fabricated under a medium wheel velocity of about 24 m/s, a feed rate of 60-90 ?m/s and heating power circa 20 k W. The diameter values were typically 45 to 65 μm. The nucleation was heat dependent that one nucleation region transformed into two during the fabrication process. Unique grain growth behavior was observed along the radial direction with columnar grains initially growing along the crystal <001> direction with a fan-like texture and then gradually evolving into <001> direction in perpendicular to the flattened surface in the late stage o f crystallisation. Such grain structure and texture may favour the magn etic-field-induced strain in NiMn-Ga microwires. The TEM observation demonstrated that the austenite of the as-extracted microwires showing tweed-like strain contrast as well as high density of dislocations and the martensite displaying stripe-like martensite twin plates.It was demonstrated that the chemical ordering annealing did not lead to substantial grain growth and the composition loss was negligible. The results indicate that annealing increase the composition homogeneity, transformation temperature and decrease the transformation hysteresis. The martensite structure was changed after annealing and the magnetic properties as well as magnetocrystalline anisotropy were improved. The EBSD results indicated fine twin structure in the annealed microwires. Bamboo structure microwires were successfully prepared using the optimized grain growth heat treatment parameters. It was found that the orientation of the neighb ouring grains were different.Chemical annealing decreases the critical stress and the hysteresis, and improves the reversibility and stability during superelastic cycling. After annealing, the stress change amplitude during stress induced martensitic transformation process is smaller and the temperature dependences of the superelastic stresses become weaker. The theoretical strain of annealed microwires was calculated to be 0.018, which is similar as in the as-extracted one. Fully recovered one way shape memory effect was obta ined in microwires and heat treatment changed the orientation of the original martensite of the microwire, leading to various of shape memory recovery paths. It was found that the c-axis was prone to align parallel to the wire axis after chemical ordering annealing, leading to a strain increasement during hea ting, and this phenomenon was weakened with increasing maximum applied stress or after training. The c-axis was prone to align perpendicular to the wire axis in oligocrystalline microwires, leading to negative strain value during recovery.Perfect two way shape memory effect was also found in the microwires with excellent stability. The oligocrystalline microwire exhibited larger transformation strains at lower applied stress. After stud ying the training processes, it was demonstrated that both stress-assisted thermal cycling training and superelaticity training could improve the twin boundary mobitity and decrease the superela stic critical stress. During the tensile tests of the oligocrystalline microwire, multiple stages with different strain amplitude were found in the stress-strain curves and the twin stress decreased from ~20MPa to ~9MPa after stress-assisted thermal cycling training, which would favor the magnetic field induced strain.Different magneto-structural coupling states were obtained using compositional tuning(e/a=7.7) and Cu substitution(Ni50Mn25Ga25-x Cux, x=3.8 at.%) and subsequent annealing(NMG4C and NMGC1C). Cu substitution increased the martensitic transformation meanwhile decreased the magnetic transition temperature, leading to a lower coupled temperature. Vacuum annealing without Mn addition was carried out, which led to a gradient element distribution from the surface to the inner part of the microwires, thus, a wide martensitic transformation temperature range was obtained. Together with proper transition temperature difference of ~28K, parti al magneto-structural coupling was obtained in microwires(NMG2C). Due to the high specific surface area of the microwires and the annealing process that reduced the internal stress and defects, both thermal and magnetic hysteresis decreased in microwires. Magnetic field induced transition from paramagnetic austenite to ferromagnetic martensite and ferromagnetic austenite to ferromagnetic martensite were found in NMG4 C and NMG2 C, respectively, leading to different magnetic entropy change during structural transformation.It was found that NMG4 C show an extremely high magnetic entropy change of ~-24.9 J/kg K, which are comparable to or even superior to that of other Ni-MnGa alloys. However, the working temperature range was ~3K, leading a small refrigeration capacity value of ~48.3J/kg. After widening the martensitic transformation temperature, the entropy change of NMGC1 C was decreased to ~-8.3 J/kg K, while leading to a wide working temperature range of ~13K, thus, an improved refrigeration capacity of ~78J/kg. The partial coupled microwire NMG2 C showed two peaks with entropy change of ~-5.2 J/kg K and ~-5.3 J/kg K, together with the high value of the transition area, a widened working temperature range of ~60K was obtained, which lead to a well improved refrigeration capacity of ~240J/kg, showing great advantages than other Ni-Mn-based alloys.