Effect Of The External Magnetic Field On The Phase Transition Thermodynamics And Kinetics Of Fe-based Alloys

Under the external magnetic field, the thermodynamics and kinetics of phase transition of Fe-based alloys have become a hot topic in the field of materials science. The previous research indicates that the microstructure of Fe-based alloys will change in the external magnetic field. Consequently, the mechanical properties of Fe-based alloys can be improved by controlling the strength of external magnetic field and the temperature of heat treatment.So far, the thermodynamics and kinetics of phase transition of Fe-based alloys in external magnetic field were rarely studied. In this work, under the external magnetic field, the magnetic properties, the thermodynamics and kinetics of phase transition in pure iron and Fe-based alloys were calculated. The results are listed as following:(1) Based on the Weiss molecular field theory, the magnetization of pure iron and Fe-based alloys were calculated under various external magnetic fields. The Curie temperature was defined as the inflexion of the M-T curve, then the Curie temperature in the external magnetic field can be obtained. The results indicate that both the magnetization and Curie temperature increase with increasing external magnetic field.(2) Considering the influence of external magnetic field on the Gibbs free energy ofαphase, theα/γphase equilibrium of pure iron, Fe-C, Fe-Si, Fe-Mo, Fe-Cr and Fe-Mo-Si systems were calculated, respectively. The results indicate that theαphase become more stable in external magnetic field.(3) Based on the diffusion coefficient model without external magnetic field, the diffusion coefficient model with external magnetic field was firstly deduced. Then theα-Fe self-diffusion coefficient and the diffusion coefficient of Ni and Co inα-Fe were calculated by the deduced model. The calculated results indicate that the diffusion coefficient decreases with the decreasing of temperature and the increasing of external magnetic field intensity, and near the Curie temperature, the diffusion coefficient decreases obviously and the lgD-T curve becomes smoothly.(4) Based on the classical nucleation theory and grain growth model, and considering the influence of external magnetic field on theγ→αphase transition driving force, the critical nucleation energy, critical nucleation radius, nucleation ratio, grain growth velocity and TTT kinetics curves ofγ→αphase transition in pure iron and Fe-lC (at. %) alloy were calculated. The results indicate that under the external magnetic field, the nucleation potential energy and critical nucleation radius decrease; the nucleation ratio and growth velocity increase, and the nucleation ratio changes more obviously; the nose of the TTT curves increases and the time of phase transition shortens. These results are good fit with the condition of grain refinement, so they can explain the reason of grain refinement in external magnetic field.The calculated results can provide a guidance for the experiments of the Curie temperature, phase equilibrium and phase transition kinetics of pure iron and Fe-lC (at. %) alloy.