Proton Irradiation Effects Of Ni-Mn-Ga Magnetic Shape Memory Alloy Thin Films

Ni-Mn-Ga ferromagnetic shape memory alloys?FSMS?have huge potential application for spacecraft as novel smart materials showing large magnetic field induced strain and high response frequency.Charged particle irradiation such as proton irradiation is an important factor of performance degradation or even failure of spacecraft materials and devices during their orbital service.However,the effect of space charged particle irradiation on Ni-Mn-Ga alloy has not been studied systematically until now.Aiming at this problem,in this work,the microstructural,martensitic phase transformation,magnetic properties,mechanical behavior and shape memory effect of Ni-Mn-Ga films were thoroughly studied by X-ray diffraction?XRD?,transmission electron microscopy?TEM?,differential scanning calorimetry?DSC?,superconducting quantum interference device?SQUID?and nanoindenter,and so on.It is found that the microstructure of Ni-Mn-Ga films undergoed proton irradiation and there was significant difference.For the austenite Ni-Mn-Ga film,the phase structure remained cubic L2₁ when the proton fluence was low as 1×10¹⁵p/cm².However,for the 1×10¹⁶ p/cm² irradiated films,a certain amount of irradiation-induced martensite had found at local area.Crystal structure of the martensite made a detailed study by application of the TEM.The martensite has stacking sequence,which had crystal structure with orthogonal 7M modulation.And the two martensitic variants had a twinning relationship of?202??40?.The results of positron annihilation experiment show that there are vacancy defects in the film after irradiation,and the defect density increases with the increase of irradiation fluence.For 7M martensite Ni-Mn-Ga film,non-modulated NM martensite was found irradiated by 120 keV proton with 2×10¹⁶ p/cm² fluences.The reorientation of the variants was found in films with 5×10¹⁵ p/cm²,when 7M martensite films irradiated by 3 MeV;the phenomenon of reorientation continued to occur and NM martensite formed at local region of the 2×10¹⁶ p/cm²-irradiated film;the amount of irradiation-induced NM martensite increased and the nanoscale amorphous zones could be introduced in irradiated film with 5×10¹⁶ p/cm² fluences.The martensite Ni-Mn-Ga film exhibited a single-step phase transition after 3MeV and 5×10¹⁵ p/cm² proton irradiation,corresponding parent?7M martensite;when the fluence reached 2×10¹⁶ p/cm²,the film exhibited a two-step martensite phase,which corresponding to parent?7M martensite and 7M?NM martensite,respectively.The martensitic transformation temperature and reverse phase transition temperature of the irradiated film increased with the increase of the proton fluence.Curie temperature of Ni-Mn-Ga decreased significantly after proton irradiation.When the martensite film was irradiated with 120 keV protons,its Curie temperature decreased linearly with the proton fluences.After 3 MeV proton irradiation with 5×10¹⁵ p/cm² fluences,the Curie temperature decreased rapidly,and remained stable when continue to increased proton fluences.Saturation magnetization and magnetocrystalline anisotropy constant of martensite increased first and then decreased after 120 keV proton irradiated Ni-Mn-Ga film.However,they were decreased when the martensite Ni-Mn-Ga film irradiated by 3 MeV proton.The hardness of Ni-Mn-Ga martensite increased after proton irradiation,and the elastic modulus and shape memory effect decreased.The effect of proton irradiation on the properties of Ni-Mn-Ga films is closely related to the crystal defects induced by proton irradiation.Stress field was formed due to the introduction of a large number of defects?such as vacancy and dislocation?which was the main reason for the phase transition of parent?7M and 7M?NM induced by proton irradiation.The reducing of shape memory effect attributed to point defects hindered the movement of the twin interface between the martensite variants and forming NM martensite with low interface mobility.The essence of proton irradiation on the saturation magnetization lies in the competition between introduced Ga vacancy,which increasing atomic moment of Mn and Ni atoms,and forming 7M martensite with low magnetization,which should result in the saturation magnetization of the Ni-Mn-Ga films increasing first and then decreasing with the increase of the irradiated fluences.