Phase Transition Characteristics And Theorial Analysis Of NiMnGa Crystals

NiMnGa alloys are intermetallic compounds possessing the characteristic of ferromagnetism and thermoelasticity of martensitic transition. It is a ferromagnetic intermetallic compound undergoing amartensitic transformation from a cubic L21 structure to a complex tetragonal structure.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.The resistance, alternating-current magnetic susceptibility, martensitic transformation strain with and without a biasing magnetic field of 1.2 T applied along the [010] and [001] directions on the Ni51.6Mn23.4Ga25 single crystal was measured. A large spontaneous strain of -1.15% in the [001] direction, relating to the two-way shape memory effect, is detected without a biasing magnetic field. The strain is enhanced up to -2.35% and 0.56% with a biasing field 1.2 T applied along the strain measuring direction and perpendicular measuring direction, respectively. Using elastic model and growth thermodynamics theory, the mechanism of the large spontaneous transformation strain and magnetic field induced transformation strain is analyzed in detail. It is showed that the magnetic field and stress can induce the preferential orientation of the matenstic variants. According to the optical microscopy observation and transformation strain measing, there is a residual oriented internal stress which originate from the directional solidification during the NiMnGa crystal growth process by the Czochralski method.The microstructural evolvement of martensitic and intermartensitic transformation induced by the single axis in the Ni_53.2Mn_22.6Ga_24.2 single crystal have been observed by the method of the metallography. It has been found that the mechanism of rearrangement of martensitic variants is different with that of intermartensitic variants. The rearrangement of martensitic variants is completed through the twin boundary motion. Contrary, the rearrangement of intermartensitic variants is accomplished by means of the growth of the preferential oriented twin-variants. According to the stress-strain curves, the energy storage, output work and energy consumption have been calculated. The results show that the energies increase with increasing the deformation temperature. However, when the deformation temperature is increased to the higher value, the energy consumption decrease.