Effects Of External Magnetic Field On Microstructure Evolution During Solidification Process In Mn-Bi And Nd-Fe-B Alloys Investigated By Phase-field Method

Permanent magnetic alloys are indispensable basic functional materials for modern industry and technology,which can provide stable and persistent magnetic flux and realize the function of electrical signal transformation and transmission of electrical energy and mechanical energy.They are widely used in many fields,such as energy,transportation,machinery,electronics,medical treatment,national defense and so on.At present,the Nd-Fe-B alloys are the most representative permanent magnets,which are usually prepared by sintering or bonding.In order to reduce the product cost,research and development of new high-performance permanent magnetic alloys without rare earth elements（such as Mn-Bi alloys）,or preparation of Nd-Fe-B magnets and even soft/hard phase nanocomposite magnets by direct casting process have recently become hotspots.The magnetic properties of materials are closely related to their microstructures.To further improve the magnetic behaviors of the above permanent magnetic alloys,it is necessary to control and optimize the microstructure of the alloys by technological means.As an effective auxiliary processing method,external magnetic field has produced a significant effect in the casting field.However,the effects of external magnetic field on the above permanent magnetic alloys as well as their regularity and mechanism still need further study and explanation.Therefore,the effects of external magnetic field on the microstructure evolution of the Mn-Bi alloy and the Nd-Fe-B alloys during solidification process have been studied systematically by phase-field method in this thesis.Firstly,different magnetic terms were respectively introduced to phase-field model to clarify the effects of various magnetic energy terms induced by external magnetic field on the MnBi grain during the process of peritectic reaction.Then all the magnetic energy terms were introduced to research the combined effects of external magnetic field.The results were as follows:Zeeman energy could promote the growth of ferromagnetic MnBi grain.Magnetocrystalline anisotropy energy would make the magnetic moments of the ferromagnetic phase MnBi turn to be oriented along the direction of the easy axis.The MnBi grain was prompted to be elongated along the direction of the magnetic field under the effect of demagnetizing field and the shape changed from sphere to olive whose major axis was parallel to the external magnetic field.The influence of external magnetic field on MnBi grain during peritectic reaction were the sum of above effects.Then,the solidification of Nd-Fe-B supercooled melt was simulated by phase-field method.With magnetic energy terms introduced to free energy of system and nucleation energy,and amorphous phase that was always found in rapid solidification microstructures considered,the effects of external magnetic field of different intensity on solidification structures in Nd_7.57Fe_88.65B_3.78（at.%）alloy were investigated.The simulation results showed that external magnetic field could refine the grains.Combined with experimental results,it was further deduced that grain refinement was the concurrent result of the decrease in nucleation energyΔG and diffusion activation energy Q.The nucleation of grains whose easy axes were along the external magnetic field would be promoted so that（00L）texture along external magnetic field was found in solidification structures.Comparing the effects of external magnetic field on competitive growth of T₁,α-Fe and amorphous phase,it was found that external magnetic field could restrain the formation of amorphous phase.Magnetic properties of microstructures from phase-field simulation could be attained by micromagnetic simulation.It was proved that the coercivity of Nd-Fe-B magnets could be improved with the effects of magnetic field on solidification structures（grain refined,grain orientation optimized and amorphous phase decreased）.Finally,we researched on the melt spinning process of Nd₉Fe₈₅B₆ alloy.Microstructure evolution of the Nd₉Fe₈₅B₆ melt during the continuous cooling at the temperature range of400 K-600 K was simulated by phase field method.The effects of the ordered clusters in the initial liquid,cooling rates and external magnetic field on solidification structures were investigated.It could be concluded that the formation of core/shell-like Nd₂Fe₁₄B/α-Fe nanocomposite structures were caused by the ordered clusters in initial liquid.The structures were beneficial for the magnetic properties of the magnets.With the increase of cooling rates or external magnetic field intensity,the grain sizes of T₁ decreased,and the nucleation numbers and volume fraction ofα-Fe grains increased.The low volume fraction of T₁ phase in the solidification structures would be caused by the excessive cooling rates or external magnetic field intensity,which would make the magnetic properties of the alloy worse.In summary,the effects of external magnetic field on microstructure evolution of Mn-Bi and Nd-Fe-B alloys were investigated by phase-field method.Some experimental observations were explained based on the simulation results and thereby some new perspectives were presented.This work provided theoretical supports and guides for the actual preparation processes.