Investigation On Phase Diagram Thermodynamics And Phase Transformation For Binary Systems Containing Fe,Co,Ni,Mn Under An External Magnetic Field

Magnetic field as contactless physical field,has been successfully applied to tailor the microstructure and properties in the preparation of many material systems.The essence is that external magnetic field can change the phase transition temperature,as well as thermodynamics and kinetics of phase transformation.For systems without external fields such as magnetic field,thermodynamic models,thermodynamic databases for phase diagram calculation,and also simulation method for microstructure evolution are well developed.However,scarce studies on thermodynamics and structural simulation under magnetic field have been found.To be specific,there is no accurate thermodynamic model or computational method or microstructural simulation method considering the effect of external magnetic field.To improve the current research status,the analytical solution of the contribution from magnetic field to Gibbs free energy was obtained numerically based on Weiss molecular field theory.The thermodynamic description without applied magnetic field was firstly established by means of experimental investigation and CALPHAD（CALculation of PHAse Diagrams）approach.Then the Gibbs energy contribution from magnetic field was integrated in order to obtain the thermodynamic description under external magnetic field.The predicted phase diagrams and phase transitions for binary systems containing elements Fe,Co,Ni,Mn under the influence of magnetic field were compared with experimental data.The main innovative results are as follows:（1）A general thermodynamic model under the influence of external magnetic field was developed and applied to Fe-X（X=Ni,Co,Rh）systems.Integrating Weiss molecular field theory,numerical calculation and CALPHAD method,a general thermodynamic model and phase diagram calculation method under applied magnetic field were established.The thermodynamic description under magnetic field of Fe-X（X=Ni,Co,Rh）system was obtained based on the developed magnetic thermodynamic computational model and thermodynamic description without magnetic field.The effect of magnetic field on the bcc/fcc phase transition temperatures were predicted using the self-developed CALPHAD software.The calculated transition temperatures for bcc+fcc→fcc and bcc→fcc+bcc agreed well with the experimental data.（2）Co-Ge phase diagram under magnetic field was predicted using the established general thermodynamic model for external magnetic field.First,the thermodynamic description of Co-Ge system was established.Co-Ge phase diagram was systematically reinvestigated,the liquidus in Co-rich and Ge-rich portions has been measured and Co₃Ge phase was determined to be Fcc＿L1₂ ordered phase.Substitutional solution model and associated model were applied to describe the liquid phase,Scheil solidification simulation for Co-Ge alloys could reasonably describe the microstructure of the as-cast alloys.The Co-Ge phase diagram under magnetic field was predicted according to the magnetic thermodynamic model.The calculation indicated that Co₃Ge phase was destabilized and Co₂Ge phase was suppressed by magnetic field.The phase boundary lines between（αCo）and（εCo）shifted towards higher Co content and higher temperature.Both calculation and experiment showed that the undercooling of Co-Ge alloys was not altered by magnetic field.（3）The Mn-Bi phase diagram under external magnetic field and the existence of four-phase equilibrium was predicted.The thermodynamic description of Mn-Bi system subjected to magnetic field was obtained via the proposed thermodynamic calculation approach with magnetic field,with only magnetic parameters for Bi Mn and Bi Mn1.08 phases.Phase diagrams and phase transitions at various magnetic fields were predicted and the results highly agreed with experimental data.The calculation indicated that the invariant four-phase equilibrium Liquid+Mn+Bi Mn→Bi Mn1.08 occurred at 48.56 T and 719.6 K.The predicted univariant reaction temperature related to the four-phase equilibrium under the effect of magnetic field agreed well with the experimental data.（4）Phase-field model coupled with Gibbs free energy under magnetic field was developed to quantitatively simulate the microstructure evolution of solid-state phase transformation for Fe-C alloy.Based on the established thermodynamic description of Fe-C system,the Fe-C phase diagram under magnetic field and the magnetic field dependence of temperature for bcc+fcc→fcc and bcc+Fe₃C→fcc were calculated.The thermodynamic parameter under magnetic field of this system was coupled with phase-field model to quantitatively simulate the microstructure evolution of Fe-0.4 wt%C and Fe-0.6 wt%C alloys during thermo-magnetic treatment,the obtained microstructure agreed with experimental observation.This thesis contains 78 figures,18 tables and 234 references...