Research On Phase Transformation, Damping And Magnetic Field Induced Strain Of Ni-Mn-Ga/Epoxy Resin Composites

Ni-Mn-Ga based ferromagnetic shape memory alloys (FSMA) have been an potential smart material as they can convert among thermal energy, magnetic energy and mechanical energy. Single crystal Ni-Mn-Ga possesses excellent SME and magnetic-field-induced strain (MFIS), but single crystals are costly and shaping is still a problem. Polycrystalline Ni-Mn-Ga is brittle and with small MFIS. Moreover, bulk samples for single and polycrystalline Ni-Mn-Ga have large eddy current losses under an alternating magnetic field. The problems in single crystal and polycrystalline Ni-Mn-Ga alloys limit their applications. Ni-Mn-Ga/polymer composites inserted with orientated single crystal Ni-Mn-Ga powders can not only improve the brittleness of polycrystalline Ni-Mn-Ga, but also avoid eddy current losses of bulk alloys. Recently, Ni-Mn-Ga/polymer composites have been attracting increasing attention.In this paper polycrystalline Ni-Mn-Ga alloy bulks and single crystal Ni-Mn-Ga alloy powders were fabricated and their phase transformations were investigated. Ni-Mn-Ga/epoxy resin(EP) composites were fabricated. The damping behavior and MFIS of Ni-Mn-Ga/EP composites were investigated.The relationship between the martensitic transformation temperatures and the ratio of electron concentration over atom (e/a) in Ni-Mn-Ga alloys was analyzed. The results show that the martensitic transformation temperatures increase with the e/a value of the alloys. The martensitic transformation temperatures of Ni_52.3Mn_23.7Ga_24.0 with an e/a value of 7.61 are near room temperature(RT). The structure of Ni_52.3Mn_23.7Ga_24.0 is composed of two phases, austenite structure and seven-layered martensite (7M) at RT. Ni_52.5Mn_23.7Ga_23.8 with an e/a value of 7.62 exhibits 7M at RT. Ni₅₃Mn_23.5Ga_23.5 with an e/a value of 7.65 and Ni₅₅Mn_20.6Ga_24.4 with an e/a value of 7.67 are non-modulated martensite(T) at RT.A thermally induced intermartensitic transformation, 7M?5M, was observed in polycrystalline Ni52.5Mn23.7Ga23.8 alloy. Internal stress has an effect on the intermartensitic transformation. The intermartensitic transformation of Ni_52.5Mn_23.7Ga_23.8 Phase transformation of Ni-Mn-Ga single powders in Ni-Mn-Ga/epoxy resin composite has been analyzed and is found not affected by the existence of epoxy resin matrix. A damping peak appears for Ni-Mn-Ga/EP composites accompanying with the martensitic transformation or reverse martensitic transformation of Ni-Mn-Ga powders during cooling or heating. The damping capacity for Ni-Mn-Ga/epoxy composites increases linearly with the increase of volume fraction of Ni-Mn-Ga powders and, decreases as the test frequency alloy is held byinternal stress from quenching and occurs after annealing at 500?for 4 hours. The hysteresis temperature and enthalpy of the intermartensitic transformation is about 30?and 0.8J/g, respectively. The critical transition temperatures and the transformation temperature ranges of 7M?5M is strongly affected by parent?7M martensitic transformation.Polycrystalline Ni-Mn-Ga alloys possess good damping property. IF peak associated with martensitic transformation is lower than that of reverse martensitic transformation. The damping property of polycrystalline Ni-Mn-Ga alloys was not only affected by vibration frequency and strain amplitude, but also depending on the phase structure of Ni-Mn-Ga alloys. The damping peak decreases obviously with the increase of vibration frequency. The damping capacity of Ni-Mn-Ga alloys with martensitic structure increases with strain amplitude. IF value of Ni-Mn-Ga alloys with martensitic structure is bigger than that of austenitic structure. increases. A crystal reorientation phenomenon was observed in 30vol% Ni-Mn-Ga/epoxy resin composite fabricated under an orientation magnetic field. The damping capacity of 30vol% Ni-Mn-Ga/epoxy resin composite is improved by the orientation magnetic field applied along the long direction of sample during sample fabricating. The IF value of magnetic field orientated composite increases, when the magnetic field is applied during the damping test along the orientation magnetic field.The crystal textured Ni-Mn-Ga powders/ epoxy resin composite was not obtained for composites with high volume fraction of Ni-Mn-Ga powders, for the pressure applied on the mixture slurry of Ni-Mn-Ga powders and epoxy resin during sample fabrication. However, a magnetically orientated damping behavior was achieved in these Ni-Mn-Ga powders/ epoxy resin composites. A threshold of magnetic field is observed. Little MFIS can be detected when the applied magnetic field is lower than the threshold, while the MFIS increases largely when the magnetic field is higher than the threshold. MFIS was almost completely recovered when the magnetic field was removed. The MFIS curves of composites are stable, repetitive and symmetrical around an axial line at 0T. MFIS of Ni-Mn-Ga/epoxy resin composites increases with the volume fraction of Ni-Mn-Ga powders.