Study On Magnetic Shielding Behavior Of Graded Iron-nickel Alloy

To satisfy the geomagnetic shielding requirements of magnetic-sensing sensors used in civil or military electronic equipments. Taking use of perm alloy's excellent magnetic shielding character, a method of manufacturing layered graded iron-nickel alloy was presented in this paper, which was prepared by diffusion alloying on the surface of common used DT4E magnetic shielding material. Optical microscope, scanning electron microscopy, transmission electron microscopy, vibration sample magnetometer and gauss meter et al were used to systematically investigate the microstructure, soft magnetic properties, geomagnetic shielding property of graded iron-nickel alloy. And the contribution of layered structure of the material to geomagnetic shielding was investigated.Graded iron-nickel alloy possesses three layered structure on macro level, in which DT4E layer is clamped by two perm alloy layers, and present a sandwich structure overall. The observation of graded iron-nickel alloy's microstructure shows that the composition of perm alloy on the side is gradient distributed from surface to inside according to diffusion law, the surface layer of graded iron-nickel alloy possesses fine equiaxed grains ofγ-(Fe, Ni) solid solution, the layer around the iron nickel interface possesses a fine structure caused by diffusion, and the inner layer possesses ferrite grains structure grown up from recrystallization. The level of surface alloying in graded iron-nickel alloy is controlled by holding time of 1200oC in heat treatment. The coercivity force and saturation magnetization of graded iron-nickel alloy have little to do with the surface alloying level, while the geomagnetic shielding property is grate affected by it. The coercivity force of graded iron-nickel alloy is 0.8Oe corresponded with DT4E, and saturation magnetization drops 10-20% compared with DT4E, while the graded iron-nickel alloy with surface Ni atom content of 80%, after twice heat treatment at 600oC, possesses a geomagnetic shielding factor 8 times as high as DT4E.The magnetic shielding mechanism of mufti-layered structure is investigated by the method of Maxwell 2D FEM and theory analysis. The formula of magnetic shielding factor of muti-layered structure is obtained, and its structural optimization is processed. The result shows that magnetic shielding factor has noting to do with enlarging or narrowing in structure size. There are optimum size and optimum layers number for a given shield thickness and a given material, and the optimum layers number would stepped increase with shield thickness and material permeability increasing. The analysis of muti-layered structure in graded iron-nickel alloy shows that its geomagnetic shielding property is mainly contributed by layers'role of parallel shunt, and the muti-layered structure have little to do with it.