Study On Directional Solidification Behaviour Of Mn-Sn Alloys Under High Magnetic Fields

Directional solidification technology can achieve unidirectional heat transfer and lead to crystal oriented growth.It has been widely used because of its advantages in controlling of temperature gradient,heat flow direction and solidification rate.The effects of high magnetic field can induce crystal orientation,control melt flow,influent solute distribution and microstructure in solidification process.The study of directional solidification of materials under high magnetic field is helpful to the development of solidification science,the preparation of new materials and the optimization of industrial production process.At present,the study of the mechanism of high magnetic field on directional solidification needs to be further improved.In this thesis,hypoeutectic Mn-89.7 wt.%Sb alloy and hypereutectic Mn-94 wt.%Sb alloy were chosen to be studied under magnetic field by directional solidification.The influence of directional solidification rate and magnetic induction intensity on the evolution of solidification microstructure and crystal orientation was investigated by means of optical microscopy,XRD,EBSD and other detection methods.The following conclusions were gained:(1)Without applying magnetic field,the hypoeutectic Mn-89.7 wt.%Sb alloy formed two regions of nondirectional solidification and directional solidification from top to bottom.In the unidirectional solidification region,the primary MnSb phase was equiaxed grain morphology,and the grain size decreased with the increase of solidification rate.The microstructure of directionally solidified Mn-Sb alloy without magnetic field was investigated,the results indicated that the primary MnSb phase of hypoeutectic Mn-89.7 wt.%Sb alloy and the primary Sb phase of hypereutectic Mn-94 wt.%Sb alloy are respectively and evenly distributed in the MnSb/Sb matrix and along the solidification direction.With the increase of solidification rate,the dendrite is refined,the lateral growth of MnSb dendrite is suppressed,and the primary Sb phase is changed from facetd to nonfaceted morphology.(2)When the magnetic field was applied,the ordered arrangement of the solidified structure is destroyed both in hypoeutectic Mn-89.7 wt.%Sb alloy and hypereutectic Mn-94 wt.%Sb alloy.The interaction of magnetic force and thermoelectric magnetic force caused primary phase unevenly distributed along the axis of the sample and promoted the transformation of MnSb phase from dendrite to equiaxed grain,also translated all the primary Sb phase into nonfaceted.Furthermore,high magnetic field significantly refine the primary phase and eutectic phase of hypoeutectic Mn-89.7wt.%Sb alloy in nondirectional solidification region.Compared with the conditions without a magnetic field,the 1T magnetic field increased the length of the unstable region but the length was decreased0 as the magnetic induction increases to 6T.(3)High magnetic fields can induce the orientation of the primary MnSb crystals of hypoeutectic Mn-89.7 wt.%Sb alloy with their c axis perpendicular to the magnetic field direction and the primary Sb phase of hypereutectic Mn-94 wt.%Sb alloy along the<102>direction.