Effect of a weak transverse magnetic field on solidification structure during directional solidification

doi:10.1016/j.actamat.2013.10.050

Acta Materialia

Volume 64, February 2014, Pages 367-381

https://doi.org/10.1016/j.actamat.2013.10.050 Get rights and content

Abstract

Six alloys were directionally solidified at low growth speeds (1–5 μm s⁻¹) under a weak transverse magnetic field (⩽0.5 T). The results show that the application of a weak transverse magnetic field significantly modified the solidification structure. Indeed, it was found that, along with the refinement of cells/dendrites, the magnetic field caused the deformation of liquid–solid interfaces, extensive segregations (i.e., freckles and channels) in the mushy zone, and a change in the mushy zone length. Further, in situ monitoring of the initial transient of the directional solidification was carried out by means of synchrotron X-ray radiography. It was observed that dendrite fragments and equiaxed grains were moved approximately along the direction perpendicular to the magnetic field. This result shows that a thermoelectric magnetic force (TEMF) acted on the liquid or the solid during directional solidification under a weak magnetic field. The TEMF during directional solidification under a transverse magnetic field was investigated numerically. The results reveal that a unidirectional TEMF acted on the solid and induced thermoelectric magnetic convection (TEMC) in the liquid. Modification of the solidification structure under a weak magnetic field is attributed to TEMC-driven heat transfer and interdendritic solute transport and TEMF-driven motion of dendrite fragments.

Introduction

Solidification in a magnetic field is an interesting topic and has attracted much attention from researchers. However, the effect of a static magnetic field on solidification has not been well understood, mainly because the experimental observations were made in different configurations, such as ingot solidification or directional solidification. In ingot solidification, the magnetic field brakes the convection in the liquid and reduces the heat-transfer rate [1], [2]. In directional solidification, the magnetic field also brakes the convection, but in directional solidification in the dendritic regime some unexpected behaviors are observed [3], [4], [5], [6], [7]. These behaviors depend on the composition of the alloy and the experimental conditions. Youdelis and Dorward [3], [4] applied a 3.4 T transverse field on the directionally solidified Al–Cu alloy. The result showed that the value of the effective partition coefficient decreased with the presence of the field, as if the magnetic field enhanced mass transport in the liquid. Tewari et al. [5] found that the cellular array was severely distorted, and stripes of freckles on the plane perpendicular to the magnetic field formed when a Pb–Sn alloy was solidified vertically at very low growth speeds under a 0.45 T transverse magnetic field. The experiment were performed by Alboussière et al. [6] and Lakar [7] on Bi–60 wt.% Sn and Cu–45 wt.% Ag alloys, solidified vertically under solutally and thermally stabilizing conditions with a 0.6 T transverse or 1.5 T axial magnetic field. Large freckles appear in this case, showing that a new movement has been created. Alboussière et al. [6] suggested that this new flow was induced by the interaction between the magnetic field and thermoelectric (TE) effects. Subsequently, Lehmann et al. [8] offered some experimental evidence for thermoelectromagnetic convection (TEMC).

Regarding the forces induced by magnetic fields, three extra forces in the liquid as well as the solid may normally be introduced under the magnetic field. One is the magnetic force arising from the interaction of the magnetism of a material and the magnetic field [9]. Moreover, when the liquid is moving, there is a second force, the Lorentz force, generated by the interaction between the induced electric current and the applied magnetic field [10]. The third force is the thermoelectric magnetic force (TEMF) induced by the interaction between the TE current and the magnetic field, which was first noticed by Shercliff [11]. The magnetic force and Lorentz force have been widely investigated [12], [13], [14], whereas little attention has hitherto been paid to the TEMF, and there is a lack of sufficient experimental evidence to prove the TEMF.

Previous work [15] investigated the effect of the TEMF on solidification structure during directional solidification under an axial magnetic field. It was found that the TEMF significantly affects solidification structure during directional solidification. In this work, the effect of a transverse magnetic field on the solidification structure in six different alloys was been investigated experimentally. The results show that, along with refinement of the cell/dendrite, the magnetic field caused the deflection of the liquid–solid interfaces, extensive segregations (i.e., freckles and channels) in the mushy zone and a change in the mushy zone length. Furthermore, the processing of solidification experiment under the magnetic field was recorded by in situ synchrotron X-ray imaging. Dendrite fragments and equiaxed grains were observed to be moved approximately along the direction perpendicular to the magnetic field. Modification of the solidification structure under a weak magnetic field is attributed to the TEMC-driven heat transfer and interdendritic solute transport and the TEMF-driven motion of dendrite fragments. The aim of the present work is twofold: (1) to investigate the effect of a transverse magnetic field on the solidification structure during directional solidification; and (2) to study the effect of interdendritic TEMC on the solidification structure, thereby gaining a deeper understanding of the effect of interdendritic convection on the solidification structure.

Section snippets

Description of experiments

Six alloys (i.e., Al–2.5 wt.% Cu, Sn–20 wt.% Pb, Ni-based DZ417G (C 0.18, Cr 8.96, Mo 3.08, Co 9.72, V 0.86, B 0.015, Al 5.41, Ti 4.50, Fe 0.23, P 0.002, S 0.002, Si 0.04, Mn 0.05, and Ni as balance, wt.%), Al–7 wt.% Si, Sn–20 wt.% Bi and Al–40 wt.% Cu alloys) were solidified directionally under a weak transverse magnetic field. Cast samples were enveloped in tubes of high-purity corundum with an inner diameter of 3 mm and a depth of 200 mm for directional solidification. The experimental device is

Results

Six alloys (i.e., Al–2.5 wt.% Cu, Sn–20 wt.% Pb, Ni-based DZ417G, Sn–20 wt.% Bi, Al–7 wt.% Si and Al–40 wt.% Cu alloys) were solidified directionally with and without a 0.5 T transverse magnetic field. Fig. 1 shows the longitudinal structures near the quenched liquid–solid interface obtained in the directionally solidified above-mentioned alloys at low growth speeds (1–5 μm s⁻¹). The growth length at quenching was 60 mm. The dendrite morphology without the magnetic field is typically columnar, and the

Effect of a transverse magnetic field on the liquid–solid interface shape and macrosegregation

The above experimental results show that the application of a transverse magnetic field during directional solidification can modify the liquid–solid interface shape and cause the formation of segregations (i.e., freckles and channel) in the alloys examined. This result should be attributed to the effect of convection on the distribution of the solute in the bulk melt ahead of and in the mushy zone. When the magnetic field is applied during directional solidification, a TEMF and TEMC will form.

Conclusion

The influence of a weak transverse magnetic field (⩽0.5 T) on the morphology of the liquid–solid interface and the microstructures of the solid was investigated during Bridgman growth of six alloys. The experimental results show that the application of a weak transverse magnetic field during directional solidification significantly modified the shape of the liquid–solid interface and the cellular/dendritic array in these alloys. Indeed, along with the refinement of the cell/dendrite, the

Acknowledgements

This work is supported partly by the European Space Agency through the Bl-inter 09_473220, National Natural Science Foundation of China (Nos. 51271109, 51171106 and 2011CB610404) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. The authors are indebted to Prof. Thierry Duffar in EPM/CNRS, Grenoble, for helpful and fruitful discussions.

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Effect of a weak transverse magnetic field on solidification structure during directional solidification

Abstract

Introduction

Section snippets

Description of experiments

Results

Effect of a transverse magnetic field on the liquid–solid interface shape and macrosegregation

Conclusion

Acknowledgements

Acta Mater

Acta Mater

Acta Mater

Acta Metall

Trans Metall Soc AIME

ISIJ Int

Can J Phys

Can J Phys

Metall Mater Trans

Magnetohydrodynamics