Microstructure And Functional Properties Of Electron Irradiated NiMnGa Alloys

NiMnGa alloys can produce large strain with fast response frequency in magnetic field via martensite variants reorientation,hence can be widely used in aerospace applications.However,the designed microstructure of the NiMnGa alloy will be altered as being exposed to outer space irradiation such as electrons,which might further lead to function deterioration and reliability issues of the spacecraft under service.In this thesis,PAT,TEM,DSC,PPMS and else were adopted to systematically study the effect of electron irradiation on the microstructure,martensitic transformation,magnetic properties,magnetic field induced strain and damping properties of NiMnGa alloy.The density function theory（DFT）calculations and positron annihilation spectroscopy show that with the irradiation fluence increasing the defect evolution in NiMnGa alloy is Ni vancancies→increase of vacancy density and formation of vacancy clusters→high density dislocationWith the increasing of irradiation fluence,the martensitic transformation temperature first decreases and then increases.The effect of irradiation defects on martensitic transformation is reflected in two aspects:one is that defects hinder the shear process of martensitic transformation;the other one is to provide nucleation sites for defects to promote martensitic transformation.When the irradiation fluence is low,the vacancy type point defects gather at the interface.The hindering effect plays a major role.The martensitic transformation temperature decreases.With the increase of irradiation fluence,the vacancy clusters and dislocations formed to provide nucleation sites have the more obvious effect on the promotion of martensitic transformation.Then the martensitic transformation temperature increases.In addition,the sequence of martensitic transformation is also affected by electron irradiation.When the electron dose is lower than 2×10¹⁷ e/cm²,the NiMnGa alloy presents a one-step martensitic transformation（parent?7M martensite）.However,with higher irradiation intensity,two-step martensitic transformation namely parent?7M martensite?NM martensite occurs.The induced defects break the local symmetry of 7M martensite;the weakened stability of such structure lead the transformation from 7M martensite to NM martensite during cooling process.After electron irradiation,the vacancies in NiMnGa alloy aggregate at the twin interface reducing interface coherence with the twin interface bent.When the irradiation fluence is higher than 2×10¹⁷ e/cm²,the phase is 7M+NM martensite at room temperature.Although the interface between 7M and NM martensite still maintain s a coherent relationship,due to the segregation of vacancy,vacancy cluster and other defects introduced by irradiation at the interface,it presents and irregular bending feature.After electron irradiation,the saturation magnetization,magnetocrystalline anisotropy constant and magnetic susceptibiligy of the NiMnGa alloy are first increased and then decreased.Irradiation introduced Ni vacancies increase the intrinsic magnetic moment of Mn and Ni atoms and enhance the exchange effect between Mn atoms;these two factors lead to the increase of saturation magnetization.After electron irradiation,the symmetry of the crystal structure decreases and overlapping density of electron cloud increases.The number of unoccupied 3d orbits increased which provides more opportunity of the transition from 2p to 3d orbits leading to an increase in the magnetocrystalline anisotropy constant of the alloy.The increase of the saturation magnetization and magnetocrystalline anisotropy constant increases the driving force of magnetic domain motion.With the increase of electron irradiation intensity,the magnetic induced strain of NiMnGa alloy first increases and then decreases,when the irradiation fluence is2×10¹⁷ e/cm²,the magnetic field induced strain reaches maximum.In situ observation in Lorentz microscope shows that the martensite variant interface of the unirradiated NiMnGa alloy moves at 5000 Oe.After irradiated with 2×10¹⁷ e/cm²,the martensite variant interface moves at 4000 Oe.The mechanism of the effect of electron irradiation on the magnetically induced strain from two aspects:on the one hand,the electron irradiation enhances the Zeeman energy difference between the martensites across the twin interface,and the driving force of the twin interface movement;on the other hand,the defects introduced by irradiation pin the twin interface,and the resistance of interface movement increases.At the initial stage of irradiation,the saturation magnetization and magnetocrystalline anisotropy constant increase rapidly,the Zeeman energy difference on both sides of the twin interface increases.The driving force of the interface plays a major role.At this time,the microstructure of the alloy is 7M martensite with good interface mobility,so the magnetic field induced strain increases significantly after electron irradiation.When the irradiation fluence reaches 3×10¹⁷ e/cm²,saturation magnetization and magnetocrystalline anisotropy constant decrease then the driving force of twin interface movement decreases.Besides,NM martensite is formed in the alloy.The random bending interface movement between 7M and NM martensite makes it harder for reorientation of the martensite variant,so the magnetic induced field decreases.After electron irradiation,the internal friction of NiMnGa alloy increases due to the pinning effect on interface.