| 1 | Thermo-Electric Motions and Structures Generated by Static Magnetic Fields During the Solidification of Metallic Alloys显示文摘Thermo-electric currents in the presence of static magnetic fields generate significant electromagnetic forces(TEM forces).The thermo-electric currents are due to the Seebeck effect when temperature gradients exist in the material.Those forces may produce various phenomena like pumping,stirring in liquid metals as well as solid motions,stresses in the solid metal.Those effects may be encountered especially during the solidification of metallic materials because of the existence of significant temperature gradients.In liquid metals the application of a static magnetic field enhances TEM convection at moderate intensity but also damps it when it is strong enough.This means that there exists a maximum of the convection which occurs when the Hartmann layers are comparable to the considered length scale.However,the smaller the length scales are(for example when primary or secondary dendrite arm spacings are considered),the higher the magnetic field strength which is needed to damp the TEM convection.So far,many solidification experiments on various types of alloys(e.g.,Sn-Pb,Al-Cu,Al-Si,Al-Ni etc.)have been carried out.The experiments have shown that TEM convection occurs both in the liquid bulk but also in the deep mushy zone.TEM convection may strongly influence the meso-macrosegregation patterns,the solidification structures and the grain boundaries leading to the striking grain boundary structure of the mushy zone.The flow pattern and accordingly the segregations may be controlled by changing the orientation of the applied magnetic field,i.e.,axial or transverse.We have shown that very high magnetic field strengths,up to 16 T,are needed to damp the TEM convection.Heuristic numerical and analytical investigations show that the TEM force density is very important in the liquid,but can be even more important in the solid.Indeed,for high Hartmann number,the electric currents are confined in a small skin layer near the solid boundaries leading to a decay of the flow.However,the situation is different in the solid,since the force is still important and is not confined.This is confirmed experimentally,since the solidified structures are subject to a much greater rate of dislocations and twinning effects.Furthermore,the TEM forces on the solid promote the appearance of direct Columnar-to-Equiaxed Transitions thanks to the possible enhanced fragmentation of the dendrites.Such mechanisms have been recently partly observed by X-ray in situ observations,where channel formation effects as well as detachment of grains along with horizontal motions were observed. | LI Xi REN Zhong-ming Gagnoud Annie Budenkova Olga Bojarevics Andris Fautrelle Yves | 2012 | Journal of Iron and Steel Research(International)2012，19，S1： | 0 |
| 2 | Direct Observation of Thermoelectric Magnetic Convection in Unidirectionally Solidified Al-Cu Alloys显示文摘This study performed the time-resolved and in-situ observation of unidirectional solidification of A1-Cu and Sn-Bi alloys under a static magnetic field up to 0.45 T.Irt the case of Sn-Bi alloy,the static magnetic field reduced the local convection around the primary dendrite arms.No thermoelectric convection was detected.On the other hand,the thermoelectric convection was clearly observed in Al-Cu alloy.The observed convection was consistently explained by the thermoelectric magnetic convection.According to a simplified model,the thermoelectric magnetic convection can be induced if the difference in the thermoelectric power between the solid and the liquid phases is in the order of 1μV/K and temperature gradient is several K/mm. | Hideyuki Yasuda Keisuke Inoue Yudai Minami Tomoya Nagira Masato Yoshiya Kentaro Uesugi Keiji Umetani | 2012 | Journal of Iron and Steel Research(International)2012，19，S1： | 0 |
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