Effect Of High Static Magnetic Field On Microstructures In Directionally Solidified Hypereutectic Alloys

High static magnetic field plays an important role in directionally solidifiedalloys. The application of high static magnetic field can modulate the properties ofmaterials by influencing the solidification structure. Hypereutectic alloys haveattracted a great of attention as wildly-used structural materials. However, the effectof magnetic field on directionally solidified hypereutectic alloys has not beensystematically studied yet. In this paper, three hypereutectic alloys, Al-40wt%Cualloy, Al-12wt%Ni alloy and Bi-Mn alloy have been studied in order to investigatethe effect of high axial magnetic field on the morphology of the solid/liquid interfaceand the growth behavior of primary phases. The major contents of this dissertationare epitomized as follows:The high axial magnetic field has great effects on the solid/liquid interfacemorphology and the growth behavior of primary phases Al2Cu in Al-40wt%Cu alloyduring directional solidification. With the high axial magnetic field, long strip-likeAl2Cu primary phase becomes irregular and breaks. Moreover, it has been found thatthe high magnetic field promotes a transition of the primary phase morphology fromfaceted growth structure to irregular cellular structure. The magnetic field causes theoccurrence of a considerable radial segregation in the macroscopic scale. Besides,the high axial magnetic field makes the length of mushy zone decrease. Also, theprimary phase spacing and crystalline grain size both decrease when the magneticfield is applied. The EBSD results show that the [001] direction of cracked Al2Cugrain still align alone the magnetic field direction under magnetic field. Theseexperimental results may be attributed to the effects of thermoelectric magnetic force(TEMF) in the solid and thermoelectromagnetic convection (TEMC) in the liquid.Under high axial magnetic field, the considerable TEMF is the major factor topromote the morphology of primary phase to transform from faceted growth structure to irregular cellular structure. Meanwhile, the TEMC causes the occurrenceof a considerable radial segregation.The application of the high axial magnetic field has affected Al-12wt%Ni alloysignificantly during the directional solidification. It makes the Al3Ni primary phaseincline from the preferred direction. Moreover, when the magnetic field intensityreaches a certain magnitude, the Al3Ni phase grows perpendicular to the magneticfield direction and forms a plane parallel to the Al/Al3Ni eutectic plane. As aconsequence, the regular sandwich structure is formed. This phenomenon isattributed to the remarkable magnetocrystalline anisotropy of Al3Ni phase. The[001]-crystal direction is the easy magnetization axis of Al3Ni crystal and therotation is driven by the magnetic force.The high axial magnetic field also has a great influence on Bi-Mn hypereutecticalloys. When solidified under high magnetic field, the MnBi phase turns to themagnetic field direction. MnBi phases assemble and arrange alone the magnetic fielddirection after the application of high magnetic field. This is because ferro-magneticMnBi phase has remarkable magnetocrystalline anisotropy and it is induced by themagnetic moment. The rotation is driven by the magnetic moment. The neighbouringcrystal grains draw each other. As a consequence, the crystal aggregates and grows,then finally forms chain-like structure.