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Influence Of The Static Magnetic Field On The Directional Solidification Of Ni-Mn-Ga Alloy

Posted on:2015-05-05Degree:MasterType:Thesis
Country:ChinaCandidate:Q Y LiFull Text:PDF
GTID:2181330422489430Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
It has been found that magnetic field plays an important role in directionalsolidification structure. As one of the most promising function materials,Ni-Mn-Ga alloys have been extensively studied recently. In present work, threealloys with different crystal structures, which are Ni48Mn30Ga22, Ni54Mn24Ga22and Ni50Mn28Ga22alloys, have been investigated under transverse magnetic fieldand longitudinal magnetic field during directional solidification with constanttemperature gradient respectively. In order to illustrate the effect of magneticfield, the morphology of the solid-liquid interface, the macro-segregation andthe crystal growth behavior have been observed. Besides, the crystal orientationand texture have been analysed by electron backscatter diffraction (EBSD).The transverse magnetic field deeply influences the tilted degree of thesolid-liquid interface and the dendrite growth of Ni-Mn-Ga alloys during thedirectional solidification. With the application of the magnetic field, lots of Mnsolute atoms aggregate on one side and the solid-liquid interface is tilted towardthis side of the specimen. The tilted degree of the solid-liquid interface firstincreases and then decreases with the increasing magnetic field intensity. Thisshould be attributed to the solute migration caused by the thermoelectricmagnetic convection (TEMC) on the specimen scales and the dendritic scalesinduced by the transverse magnetic field during the directional solidification,and the effect of the TEMC is different in each scale. Moreover, the transversemagnetic field can influence the primary dendrite spacing. Indeed, it is foundthat the primary dendrite spacing decreases with the increasing magnetic fieldintensity. The possible reason is that the TEMC on the dendritic scalesstrengthens with the increase of the magnetic field intensity and therefore refinesthe dendrites.The longitudinal magnetic field has a remarkable influence on the interfacemorphology and the dendrite growth of Ni-Mn-Ga alloys as well. After applyingthe longitudinal magnetic field, the solid-liquid interface becomes smoother andthe dendrite length turns shorter with the increase of longitudinal magnetic fieldintensity (B≤1T). When the longitudinal magnetic field intensity reaches2T, some dendrites become larger because of the magnetic damping. Moreover,when the longitudinal magnetic field intensity is large enough, the thermo-electric magnetic force can twist off the dendrite. It induces the originalmonocrystalline becomes polycrystalline. These are caused by the thermo-electric magnetic force (TEMF).The crystal orientation and texture of the directional solidification structureare obviously effected by the static magnetic field as well as the orientation ofmartensite variant. The EBSD results show that the crystal-selected phenomenonbecomes obvious gradually with the increase of the growth rate. And the crystalorientation is just right for the preferential growth orientation <001>Cof theoriginal austenitic Ni-Mn-Ga alloys. The transverse magnetic field has a weakinfluence on the crystal orientation during directional solidification, which issimilar to the phenomenon of non-magnetic field. Nevertheless, when thelongitudinal magnetic field is applied during directional solidification, theTEMF would be large enough to twist off the dendrites. As a consequence, thespecimen becomes polycrystalline. Accordingly, the crystal orientation of thissample is consistent along the axial direction and disordered along the radialdirection. In addition, the crystal direction [001]Mof some martensitic variants,instead of the hard magnetization direction [110]M, tends to align with thelongitudinal direction after the application of the longitudinal magnetic field.
Keywords/Search Tags:Static magnetic field, Directional solidification, Thermoelectricmagnetic effect, Ni-Mn-Ga alloys, EBSD
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