Thermodynamic Analysis Of Phase Boundary Movement In NiMnGa Alloys

Ferromagnetic shape memory alloys is a new functional material. It is one of the focuses in the field of smart materials, because it is not only having large magnetic field induced strain and high-driving force of general shape memory alloys, but also possessing high frequency and response of magnetostrictive materials. As a typical ferromagnetic shape memory alloy material NiMnGa, having the largely magnetic-field-induce strain and two-way shape memory effect controlled by magnetic field, also their martensitic transformation exhibit various characteristics. Mechanism of Magnetic-field-induced strain, which obtain maximum in the temperature of the Martensitic transformation has just concluded, is from the field-induced motion of twin boundaries in martensite state. Depending on the specific composition, except the magnetic transition and martensitic transformation, this material shows also the premartensitic transformation, and intermartensitic transformation. Moreover, adjust the material composition, the martensitic transformation and magnetic transition can occur simultaneously. Due to unique physical properties, the Material shows a broad prospects.In this thesis, different single crystals are grown by Czochralski technique. According to the characteristics of magnetism and crystallography , the physical properties were characterized by various ways, such asstrain measurement, resistance measurement, X-ray diffraction, ac susceptibility measurements,and differential scanning calorimetry analysis. Some experimental results are obtained as follows.The co-occurrence of magnetic and martensitic transformations in Ni46Mn35Ga19 single crystal grown by the Czochralski method were characterized by various ways. Accompanying with this transformation behavior, a large spontaneous transition strain of -0.89% in zero field and a large magnetic-field-enhanced strain of -1.90% is observed.The experimental results are analyzed and discussed according to the characteristics of shape memory and the mechanism of preferential orientation of the martensitic variants. Based on the thermodynamical theory and some experimental data, the relevant energy contributions related to the thermoelastic martensitic transformation for three off stoichiometric Ni-Mn-Ga single crystals with the transformation temperature below, near and above room temperature, respectively, were calculated, and their influence to the transformation behavior was discussed. The results identify further that for a completely thermoelastic martensitic transformation, the slope of the transformation loop results from the elastic strain energy, and the thermal hysteresis comes from the friction of interfacial motion. It is found that in these studied alloys, the dissipated energy in the transformation from cubic phase to tetragonal phase is larger than that in the metastable phases. The result is attributed to the larger energy barriers opposing the interfacial motion during the transformation from the cubic structure to the stable tetragonal structure.The magnetic-field-induced strain was measured under different temperatures in Ni52Mn24.5Ga23.5 single crystal. martensitic thermal hysteresis of single crystals Ni52Mn24.5Ga23.5 has smaller, when it generated martensitic transformation it will produce the expansive strain along the crystal growth direction. It will generate shrinkage strain when the Inverse phase transformation started.The parent phase for the L21 alloy fcc, martensite phase, five layer (5M) in the tetragonal body-centered.The temperature stability of magnetic-field-induced strain have analyzed and disc- ssed. Thus discovered that the material has giant spontaneous phase transformation stra- in and magnetic-field-induced strain. Furthermore, based on the thermodynamics theory, the energy consumption of twin-boundary motion during the process of magnetic-field- induced strain is calculated, and the relations of the energy consumption along with the temperature change have given. The results show that magnetic-field-induced strain qualitatively reflects the energy consumption of twin-boundary motion in a cycle.