| In this work,the effect of a static magnetic field on the microstructure in four directionally solidified peritectic alloys(Fe-Ni,Pb-Bi,Cu-Sn and Zn-Cu)has been investigated experimentally.3D numerical simulations for the thermoelectric magnetic force(TEMF)acting on the solid and thermoelectric magnetic convection(TEMC)in the liquid at the sample and dendrite scales during directional solidification under the magnetic field are performed.The effect of a transverse magnetic field(B<1 T)on the formation of banded and island structures,segregation,and the refinement and orientation of primary dendrites in four peritectic alloys has been studied.Furthermore,the effect of an axial magnetic field(1<B<6 T)on macrosegregation,eutectoid transformation,and the dendritic growth in Cu-Sn peritectic alloy has been studied.3D numerical simulations for thermoelectric magnetic effect during directional solidification under the transverse and axial magnetic fields have been constructed.The thermoelectric power difference at the solid/liquid interface of Pb-Bi and Zn-Cu peritectic alloys at different growth speeds during directional solidification are measured in situ,and the results show that the thermoelectric power difference decreases as the growth speed increases.The TEMC in the liquid in four directionally solidified peritectic alloys under a transverse magnetic field is simulated.The results show that the TEMC in the liquid increases with the magnetic field and after a critical field it decreases.In addition,different types of the four peritectic alloys have a different TEMC in the liquid under the magnetic field,and the magnetic field corresponding to the maximum value of the TEMC in the liquid is different.The TEMF acting on the solid during directional solidification under an axial magnetic field is simulated.The results show that the TEMF acting on the solid increases with the magnetic field continuous increases.Directional solidification experiments for four peritectic alloys under a transverse magnetic field have been carried out,and the results show that the magnetic field can cause the formation of a stable banded structure in Fe-Ni and Pb-Bi peritectic alloys at low growth speeds and high temperature gradient.The magnetic field corresponding to the formation of the stable banded structure for Fe-4.2 at.% Ni and Fe-4.5 at.% Ni peritectic alloys is approximately 0.1 T and 0.5 T,respectively.For Pb-35 at.% Bi peritectic alloy,the magnetic field causes the spacing of bands decrease with the magnetic field and after a critical field it increases.The above results are attributed to the effect of the TEMC in the liquid at the sample scale on the distribution of the solute under a transverse magnetic field.Further,a theory of the convection parameter ? under a magnetic field has been developed.It can be found that,owing to the produce of the TEMC in the liquid,the composition range for stable banded structure forms in the sample has been changed.The magnetic field also can cause the formation of an island structure in Fe-Ni and Pb-Bi peritectic alloys at high growth speeds.For Fe-4.2 at.% Ni peritectic alloy,the magnetic field induces the formation of the island structure of primary phase in the matrix of peritectic phase.For Pb-25 at.% Bi peritectic alloy,the magnetic field induces the formation of the island structure of peritectic phase between the cells of primary phase.The above results are attributed to the effect of the TEMC in the liquid at the dendrite scale on the re-melting of primary dendrites under a transverse magnetic field.During directional solidification for Pb-Bi and Zn-Cu peritectic alloys,the magnetic field causes the formation of macrosegregation at one edge of the sample,and refines primary dendrite at the other edge.The refinement of primary dendrite near the macrosegregation tends to be limited.The above results are attributed to the effect of the TEMC in the liquid on the solute transport between the dendrites under the transverse magnetic field.During directionally solidified for Cu-Sn peritectic alloy,the magnetic field enhances the alignment of <001> crystal direction of primary phase along solidification direction.This result is attributed to the TEMC in the liquid and corresponding secondary flows enhance heat transfer ahead of the solid/liquid interface and induce the growth of primary dendrite along the preferred orientation during directional solidification under the transverse magnetic field.Directional solidification experiments for Cu-20 wt.% Sn peritectic alloy under an axial magnetic field have been carried out,and the results show that the magnetic field can cause the formation of macrosegregation,reduce eutectoid ? phase,and disrupt primary ? dendrites and then induce a transition from arrayed growth to nonaligned growth.The above results are enhanced with the increase in the magnetic field.Magnetohydrodynamic effects at the sample scale and thermoelectric magnetic effects at the dendrite scale are studied.3D numerical result shows that a ring of TEMC forms in the liquid around the primary dendrite and the velocity of the TEMC in the liquid decreases as the magnetic field continuous increases.The formation of macrosegregation and reduction of eutectoid ? phase under the axial magnetic field are attributed to the magnetohydrodynamic effects hinders solute transport in the liquid at the solid/liquid interface and between the dendrites during directional solidification,respectively.Moreover,the eutectoid ? phase retains the orientation relationship of the peritectic ? phase after the eutectoid reaction.Further,3D numerical result also shows that a TEMF imposes on the solid and induces a torque at the dendrite and that the value of the TEMF acting on the solid increases as the magnetic field continuous increases.The disruption and transition between arrayed and nonaligned growth of primary dendrites under an axial magnetic field is attributed to the increase of the total number of heterogeneous substrate particles available per unit volume caused by the TEMF acting on the solid,which enhances the formation of equiaxed grains during directional solidification. |
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