Thermo-electric motions and structures generated by static magnetic fields during the solidification of metallic alloys

摘要

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.