Irradiation Effects Of Magnetic Materials And Structure Evolution

The irradiation effects of aircraft materials in space environment are important for the realization of reinforcement design of aircraft radioprotection. In the present work, the change of magnetic properties and irradiation damage of several types of magnetic materials exposed to neutron, proton and electron irradiations were investigated. The effects of different irradiations on the defects, crystal structures, electric structures and magnetic domain structures were systematically studied with the utilization of synchrotron radiation technique and advanced structure analysis methods. The changes of atomic local structure, atomic magnetic moments and hyperfine fields after irradiation were quantitatively characterized. The irradiation damage mechanisms were discussed on the base of the change of magnetic properties and the observation of magnetic domains. The evolution of magnetic domains of NdFeB permanent magnets during thermal demagnetization was also studied.The damage effects of typical magnetic materials under different irradiation environments were summarized as follow: The technical magnetic properties of FeAl soft magnetic alloy changed after proton and electron irradiation, i.e. the magnetic permeability decreased and the coercivity increased after irradiation. The irradiations led to the decrease of magnetic flux of FeCrCo and NdFeB permanent magnetis, but had no obvious effect on the technical magnetic properties. The magnetic flux of NdFeB magnets was rather sensitive to the temperature increase during irradiations.The amount of point defects in FeAl alloy increased after irradiation. The vacancy clusters of individual vacancy with pretty much the same size are dominant, while relatively larger vacancy groups from the coalescence of individual vacancy appeared seldom after electronic irradiation. While in the case of proton irradiation, the aggregation of individual vacancy and the growth of vacancy group were apparent.The proton irradiation resulted in the precipitation of ordered phase Fe3Al in FeAl alloy, accompanying with the increase of structural disorder degree. The irradiation had no apparent effect on the crystal structure of the main magnetic phases in NdFeB and FeCrCo alloys, but significantly affected the atomic local structures.The disorder degree around atoms increased and the coordination number changed after irradiation. Atomic diffusion occurred during irradiation process. The atomic local structure of Fe atoms within 0.36 nm range changed after irradiation, which also led to the increase of disorder degree around Fe atoms and the decree of Fe-Nd coordination number. The atomic diffusion betweenα1 andα2 phases occurred in FeCrCo alloy during electron irradiation. The diffusion of Fe atoms was dominant.M?ssbauer spectrum study showed that the hyperfine structure of permanent magnets changed after irradiation, and the hyperfine magnetic field decreased. The proton irradiation resulted in the increase of vacant state density of Fe 3d electrons, indicating the alteration of bonding environment of Fe atoms. X-ray magnetic circular dichroism results indicated that electronic irradiation led to the variation of orbital-spin coupling effect and magnetic moment of magnetic atoms in FeCrCo alloy. Charge transfer occurred among magnetic atoms during irradiation, leading to the change of vacancy density of 3d electrons of magnetic atoms.Magnetic force microscopy study of NdFeB permanent magnets showed that the domain structures of thermal demagnetization state magnets were multi-domain consisting of plate-like domains with average width of 2.1μm. Each grain of the magnets was saturation magnetized single domain. The impact of proton irradiation on the magnetic domain structure of NdFeB magnets was the coupled action of both temperature rising and stress affect. The temperature rising during irradiation resulted in the rotation of magnetization vector in single domain and the formation of reversal domain. The inter stress induced by proton irradiation leds to the deflection of original magnetization vector, resulting in the formation of fine stripe-like magnetic domains.The changes of magnetic properties in soft magnetic alloy were caused by that the stress induced by the irradiation and the variation of magnetic hysteresis expension coefficient due to phase transformation. For permanent magnetic magnets, the structural defects induced by the irradiation leds to the fluctuation of micro-region magnetocrystalline anisotropy. It promotes the nucleation of reversal magnetic domains and hinders the motion of domain wall, resulting in the change of magnetic properties.