Characteristics Of Physical And Energy Consumed Of Twin-boundary Motion 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 as heat flow, resistance, alternating-current magnetic susceptibility, and strain measurements. Some experimental results are obtained as follows:The co-occurrence of magnetic and martensitic transformations in Ni46Mn35Gai9 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.The magnetic-field-induced strain was measured under different temperatures in Ni52Mn24.5Ga23.5 and Ni52Mn16.4Fe8Ga23.6 single crystal. And the temperature stability of magnetic-field-induced strain have analyzed and discussed. Thus discovered that two materials both have giant spontaneous phase transformation strain and magnetic-field-induced strain, the magnetic-field-induced strain in Ni52Mn16.4Fe8Ga23.6 single crystal have the good temperature stability. 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.