Investigation Of Nucleation And Growth Of Metals In High Magnetic Fields By Differential Thermal Analysis

Application of high magnetic field to materials processing has received much attention in recent decades and some significant results have been reported. Since phase transformations have a decisive effect on the resulting structures and performance, Phase transformation in a magnetic field has been a hotspot in the field of electromagnetic processing of materials (EPM). The present work aims to investigate phase transformation in non-magnetic metallic systems, e.g., pure bismuth, pure aluminum, Al-Cu alloys and Al-Ni alloys by differential thermal analysis (DTA). It is explored how nucleation, crystal gowth, structural morphologies and orientation are affected by the magnetic field. Some new results were obtained as following:Thermal analysis which accurately detects phase transformation has been widely applied to various fields. Based on principles of DTA, the DTA apparatus adaptable to the superconducting magnet has been designed. It was found that a magnetic field could affect the output voltage of Pt/Pt-10%Rh thermocouples when some pure substances were used to calibrate the apparatus. Primary results showed that the DTA apparauts could examine the phase transformations in metallic systems regardless of a magnetic field.The melting and solidification of pure metals were investigated by DTA. It was found that nucleation temperature of pure aluminum was reduced in magnetic field, namely, the undercooling level increased. The value of undercooling in 12T magnetic field reached 21.7℃compared with 0.1℃in zero magnetic field and the growth rate increased in magnetic field, for example, the growth rates of pure aluminum in 12T nearly doubled in comparison with that without a field. For pure bismuth, its nucleation temperature decreased from 31.8℃without a field to 20.7℃in 12T. However, growth rates seem not to be influenced. The DTA curves of both of pure metals for melting showed that the melting is not affected by a magnetic field. It implied that the effect of a magnetic field on nucleation could be attributed to the change in interfacial free energy, not Gibbs free energy. It was inferred that the solid-liquid interfacial energy could be changed in a magnetic field and further influenced nucleation. Consequently, the interfacial energy increased for pure aluminum and decreased for pure bismuth in a magnetic field.For Al-Cu alloys, the solidification of hypoeutectic, eutectic, hypereutectic alloys has been studied with and without a magnetic field by DTA. The nucleaiton temperature of primary phase was reduced but growth rates were not nearly affected in a magnetic field. The resulting structures of hypoeutectic alloy showed that the dendrites were from disorder without a magnetic field to order in magnetic fields, which was attributed to the suppression of natural convection and therefore heat transer from convection to conducting. The DTA curves indicated that the nucleation of Al-Al2Cu eutectics was suppressed and the growth rates were lowered in a magnetic field. For both of hypereutectic alloys, the nucleation temperature of primary Al2Cu phases was decreased with increasing a magnetic field. Microstructures showed that the number of primary phases markedly increased with a magnetic field and they aligned along a magnetic field. The X-ray diffraction (XRD) patterns further demonstrated that the c-axes of Al2Cu crystals oriented. The formation of bar-like primary phases could be caused by the mutual interplay between thermoelectric magnetohydrodynamic flows and magnetic torque.For Al-Ni alloys, it was found that the nucleation of primary phase in the Al-3wt%Ni alloy was similarly inhibited while the rates of crystal growth seem not to be affected in a magnetic field. Structural morphologies showed the dendrites became regular in a magnetic field, i.e., secondary arms grew along the direction perpendicular to primary dendritic stems, instead of seaweed-like shapes without a field. For Al-12wt%Ni hypereutectic alloy, the morphologies of primary Al₃Ni phases were from rough interfaces without a field to smooth interfaces with a field. The phenomena could be explained by the interface controlled growth in a magnetic field. However, it was not observed that the plane of primary Al₃Ni phases was perpendicular to a magnetic field. The DTA curves showed that the nucleation of Al-Al₃Ni eutectics significantly was suppressed but the growth was accelerated in magnetic field. The microstructure indicated that eutectics were from fiber-like without a field to granular with a field.