Study On Microstructure Evolution Of Al-Fe Alloys By Directional Solidification Under High Magnetic Field

Directional solidification(DS)technique attracted much attention in materials preparation because of its advantages in controlling of heat transfer and crystal growth.Study on directional solidification laid the foundation for the modern solidification theory.Magnetic fields affect solidification tremendously,attracting much attention recently,though the effects of magnetic fields on solidific ation are undoubtedly complicated.However,DS under magnetic field and its effects were not widely studied yet.In this thesis,the Al-Fe alloy was chosen to be studied under magnetic field by DS.The microstructure was observed by optical microscope and electron microscope,the crystal orientation was investigated by X-ray diffraction,and the effects of solidified rates,magnetic field and magnetic field gradient on morphology were investigated.The microstructure of hypereutectic Al-Fe alloy was investigated under magnetic field gradient during directional solidification.The results indicated that the magnetic field significantly affected solidification process,the Al₃Fe phase broke down and twisted,and some of the primary Al₃Fe phases grew perpendicular to the solidification direction.A eutectic area appeared at the tops of the samples,and as the magnetic flux density increased,the eutectic area clearly enlarged.Further,the volume distribution of the primary Al₃Fe phase was also investigated,and revealing that applying the magnetic field during solidification caused dramatic disorder in the solute and phase distributions.Analysis suggested that the magnetic force induced by the interaction between the magnetic field gradient and the magnetic materials appeared to be the main reason not only for the occurrence and enlargement of the eutectic area but also for the movement of Fe-enriched zones during directional solidification.Otherwise,the deformation and fracture of the Al₃Fe phase,the fluctuations in the interface between the eutectic area and the Al₃Fe phase,and the random distribution of the aggregated Al₃Fe phase appeared to come from the thermoelectric magnetic force(TEMF)/thermoelectric magnetic convection(TEMC)under the magnetic field.Directional solidification of eutectic Al-Fe alloy with and without magnetic field was investigated,respectively.In the case of no magnetic field,the ?-Al fully separated and grown as dendrites with 150 ?m/s because of the single instability of Al.In addition,the application of magnetic field aggregated the single instability of ?-Al and its separation.Analysis suggested that the magnetic force not only enhanced the aggregation of Fe atoms which were ejected from the growth of ?-Al,at the same time,but also aggregated the Fe atoms around the ?-Al dendrites.In another word,the magnetic force suppressed the redistribution of solute at the solidification front and made the ?-Al grow with dendrites.In addition,the faceted eutectic Al₃Fe phase transformed to non-faceted and the sizes of Al₃Fe phase in three dimensions were all dramatically decreased because of the increasing of growth rates and magnetic flux densities.In the case of high growth rates,with the increasing of magnetic field,many granular or dotted eutectic Al₃Fe phase occurred in the microstructure.In this case,the magnetic energy provided the external energy for the eutectic Al₃Fe phase to keep the unstable state in such a small size and conditions.The shape transformation of Al₃Fe phase were from plate-like,fiber to granular or dotted.