Study On Multi-functional Properties Of Ni-Mn-Ga-Based Microwires

Ni-Mn-Ga ferromagnetic shape memory alloy has the advantages of large output strain and high power factor,which makes it satisfy the application requirements of microsensors and actuators.Multi-functional properties of Ni-MnGa alloys are derived from the martensitic transformation.However,the realization of the two-way shape memory effect of alloys requires training due to the selfaccommodated behavior of martensites,which hinders realizing output strain.There is limited research on the resistance and impedance of shape memory alloys under magnetic fields and lacks theoretical mechanisms of magnetoresistance and magnetoimpedance effect related to martensitic transformation.To solve the brittleness of bulk alloys,the one-dimensional Ni-Mn-Ga-based micro wires were selected as the object in this paper by the design strategy of bamboo-like structures,which has grain sizes similar to diameters.The capacity of grain coordinated deformation was improved by increasing the surface area ratio and reducing the grain boundary constraint.On this basis,the superelasticity,shape memory effect,magnetoresistance,and magnetoimpedance effect of microwires were systematically studied,and effective methods for improving magnetic properties were explored.Ni54.71Mn22.14Ga23.15microwires with axial preferred orientation were prepared by the glass-coated method,and the shape memory effect was investigated.Excellent intrinsic two-way shape memory effect(TWSME)was achieved in the as-prepared microwire by inducing preferred orientation martensite,which was generated by the oriented internal stress in austenite.The intrinsic TWSM strain was 8.8%and the strain was up to 14.4%under 10 MPa constant stress,which reaches the reported highest level.The stability of the shape memory effect was improved by superelastic training,the TWSM strain was 7.3%,and the maximum output energy was 31 J/cm3,which exhibits the advantages of low stress and high stroke in the working window near room temperature.Mechanisms and transition sequence of magnetoresistance of Ni-Mn-Gabased microwires were investigated by co-doping of(Co,Cu)and(Fe,Cu).A jump phenomenon appeared in the initial magnetization of the Ni48Mn22Ga22Co4Cu4 microwire,which resulted in an irreversible negative magnetoresistance.The mechanism is a typical s-d scattering model and magnetic-induced phase transformation related to residual martensites,which could be effectively regulated by Fe.Results show that residual martensites at room temperature and hysteresis decreased with the increase of Fe and the magnetoresistance transformed from irreversible to completely reversible with hysteresis due to the mechanism from magnetic-induced phase transformation to the pinning effect of precipitates on martensites.Furthermore,the influence of training on magnetoresistance was revealed and the transition sequence of magnetoresistance was from completely reversible with hysteresis to irreversible and finally reversible with no hysteresis.The effect of superelastic training on the magnetic properties of Ni46Mn23Ga22Co5Cu4 microwires was systematically investigated.Over 10,000 tensile cycles were achieved at room temperature with no strain attenuation phenomenon.The order degree of austenite and magnetic anisotropy of martensite was enhanced by training,which strengthened the magnetic order and strong ferromagnetic coupling between martensite and austenite.Thus the improvement of magnetic properties was realized as follows:the difference in magnetization between the two phases and the saturation magnetization increased by 6.5 emu/g and 10.8 emu/g,respectively.The magnetoimpedance response of martensitic transformation of Ni46Mn22Ga24Co4Cu4 microwires was investigated from the transition of magnetic anisotropy.The appearance of an impedance peak during the phase transformation reflects the feasibility of impedance response.The single peak of the magnetoimpedance reflects the uniaxial anisotropy of the microwire and the difference in magnetoimpedance of the two phases is distinct and strongly dependent on the excitation frequency.The magnetoimpedance of martensites with higher anisotropy exhibits higher sensitivity to the magnetic field and hysteresis at low frequency which is due to the pinning effect of the nanoscale variant interface on the domain wall.The maximum value of the magnetoimpedance ratio of the two phases,16.48%,was obtained at 50 MHz.In summary,the relationship between microstructure and multi-functional properties was established and the effect of training on properties was explored,which provides a theoretical basis for the application of Ni-Mn-Ga alloys.