Study On Thermodynamic Effect And Application Function Of The NiMnGa Alloys

The Heusler alloy NiMnGa has been attracting investigation as a potential smart material and also as a new functional material. This alloy is interesting for several reasons as follows. First, it is a ferromagnetic intermetallic compound undergoing a martensitic transformation from a cubic L21 structure to a complex tetragonal structure. Second, associated with the martensitic transformation, it exhibits a two-way shape-memory effect. Third, depending on the specific composition this material shows also the intermartensitic transformation, and premartensitic transition, except the martensitic transformation. This work is a systemic investigation on the micro-structural, different phase transformation behaviors, magnetic properties, shape memory effects, magnetic field-induced-strains and stress-strain characteristics for the NiMnGa alloys.A stress-free and two-way thermoelastic shape memory, with 1.2% strain measured in the [001] direction and 6 K temperature hysteresis, has been obtained in single crystal of Ni52Mn24Ga24. In particular, the deformation can be enhanced more than three times, up to 4.0%,shrinkage with a bias field 1.2 T applied along the measurement direction, or changed to 1.5% expansion by the 1.2 T applied perpendicular to the measurement direction. It was found that the residual oriented internal stress caused by the directional solidification during the crystal growth plays an important role in achieving the reorientation of the variants, and resulting in a large transformation strain. Based on the experimental data on transformation strains with or without bias field, the oriented internal stress in single crystal Ni52Mn24Ga24 was calculated to be about 2.45 MPa. Furthermore, A large magnetic field-induced-strain (MFIS) of about 1.2% is obtained at nearly martensitic transition finishing temperature. It is found that the MFIS is dependent on the temperature. According to the difference of both effective elastic constants and the distributing of magnetic domain, the different characteristics of the MFIS between the [001] and [010] directions of the crystal parent phase caused by the preferential orientation of the martensitic variants during the martensitic transformation, such as maximum strain values, saturated fields, hysteresis effect and initial magnetic fields, et al, have been discussed.A complete thermoelastic two-step martensitic transformation has been found in single crystalline Ni53.2Mn22.6Ga24.2. The intermartensitic transformation in this two-step transformation provides a much larger strain of about 1.0%, which is larger than that of the martensitic transformation of about 0.1%. With a biasing magnetic field, the intermartensitic transformation strain is inhibited and the martensitic transformation strain is enhanced. Compressive stress-strain characteristics can be affected greatly by a static magnetic field. At low deformation temperature, the stress level necessary for starting first deformation stage decreases and the irreversible transformation strain induced by the stress becomes reversible, when a static magnetic field is applied. Further, it is found that the magnitude of the stress necessary for rearrangement of martensitic variants is dependent on the direction of the biasing magnetic field. This shows the possibility of using a NiMnGa single crystal as a magnetically controlled superelastic element. In addition, the optical micrographic observation has found that the twin boundaries motion during loading and unloading exhibits a well-defined character of soliton motion. The premartensitic transition strain in free samples of Ni50.5Mn24.5Ga25 single crystals was first reported. A large magnetostriction up to 505 ppm in the [001] direction of parent phase is obtained in a pulse magnetic field of about 1 kOe applied along the [010] direction at premartensitic transition point, which is more five times larger than that in parent phase. It was found that not only the premartensitic transition temperature pronounces a more rapid decrease, but also the premarten...