Thermodynamic Mechanism Of Microstructure And Carbide Evolution Of Iron-based Materials During Tempering Under High Magnetic Field

Because of its cleanness and renewability,nuclear fusion energy has become the most potential new energy for the development of modern society.The preferred structural material for supporting the superconducting coil in the fusion reactor is low activation ferrite/martensite steel,which is working under high temperature and high magnetic field during its service,and its microstructure evolution is strongly affected by the magnetic field.Therefore,it is of great scientific significance and engineering value to research the effect of high magnetic field on the formation and evolution mechanism of matrix microstructure and alloy carbide during tempering of low activation steel.In order to simplify the influence of complex composition on the research,Fe-C-Cr alloy heat-resistant steel and Fe-C-W alloy heat-resistant steel were respectively designed,which were based on the chemical composition（9Cr-WVTa）of the candidate low activation steel.The effects of 12 T high magnetic field on the matrix structure and alloy carbides of two kinds of heat-resistant steels at different tempering temperature and time were systematically studied by means of scanning electron microscopy,back scattering electron diffraction,transmission electron microscopy and calculation of the first principle.The main results were as follows:（1）When Fe-C-Cr heat-resistant steel was tempered at 200?C for 3600 s,a large amount of transitional carbide M₂C was precipitated in the matrix,and the number density of carbide was increased by the magnetic field.The reason was that the magnetic field reduced the nucleation barrier of M₂C and increased its nucleation rate.（2）The activation energy of dislocations was low when Fe-C-Cr heat-resistant steel was tempered at low temperature.So the majority of dislocations were not able to move due to the necessary conditions of dislocation movement（the effective stress must be greater than Peierls stress）.The number of movable dislocations was small and the dislocation density was high.The high athermal yield stress produced by high dislocation density in martensitic laths resulted in the increase of strain energy.The increase of strain energy in the matrix reduced the possibility that the low angle boundaries with low energy transformed into the high angle boundaries with high interface energy.The percentage of low angle boundaries consequently increaseed.The higher percent of low angle boundaries resulted in the more stable the martensite laths and the slower recovery of the martensite.（3）When the tempering temperature increases to 500?C,the transition carbide M₂C gradually decreased with gradually increased M₃C type carbide.And the total density of alloy carbides was increased with the application of magnetic field.With the increasing of tempering temperature,the effect of temperature on dislocations increased,resulting in the increase of effective stress.Larger effective stress of dislocations increased the number of movable dislocations,which finally improved the dislocations recovery and reduced the dislocation density.However,when the 12 T high magnetic field was applied,the total carbide number density increased,which hindered the movement of movable dislocations and thus restrains the dislocations recovery.The high dislocation density increased the percentage of low angle boundaries and improved the stability of martensitic lath.Therefore,the recovery of martensite is inhibited by the application of high magnetic field,and the effect was stronger than that at low temperature.（4）When Fe-C-Cr heat-resistant steel was tempered at 700?C for 3600 s,the number of movable dislocations greatly increased by the influence of high temperature.The movement of movable dislocation highly promoted,which consequently improved the dislocation recovery and decreased the dislocation density.The critical unpinning stress increased because that high density carbides of M₃C by the magnetic field interacted with the movable dislocations,which greatly hinderd dislocation movement.Therefore,magnetic field inhibited the dislocation recovery and increased the density of residual dislocation in the matrix.The high density dislocations increased the strain energy in the matrix,which finally increased the percentage of low angle boundaries and the stability of martensitic laths.The inhibition effect of high magnetic field on martensite recovery was much more obvious when the Fe-C-Cr heat-resistant steel tempered at higher temperature.（5）When Fe-C-W heat-resistant steel was tempered at 200?C for 3600 s,a lot of M₃C alloy carbides are precipitated in the matrix.When the 12 T magnetic field was applied during tempering,the carbide number density increased.High density dislocations at low tempering temperature resulted in high percentage of low angle boundaries and thinner average width of martensitic laths.（6）When Fe-C-W alloy steel was tempered at 500?C for 600 s,only M₃C precipitates in the matrix.With the application of 12 T high magnetic field,M₆C type carbide Fe₃W₃C precipitated in the matrix.The application of high magnetic field promoted the precipitation of M₆C in advance.The reason was that when the high magnetic field was applied,the changes of magnetic free energy of M₆C was larger due to its greater magnetization.Therefore,the application of the 12 T high magnetic field promoted the precipitation of M₆C at early stage,and with the extension of tempering time,the application of high magnetic field resulted in the increase of its volume fraction.（7）With the prolongation of tempering time when the Fe-C-W alloy steel tempered at middle temperature,the number of movable dislocations increased,which resulted in a higher dislocation vibrations and a larger frequency for crossing obstacles.Therefore,much more dislocation movements aggravated the dislocation recovery and reduced the final dislocation density.（8）When the Fe-C-W alloy steel was tempered at 700?C,the number of movable dislocations greatly increased due to the significant influence of the high temperature.More movable dislocations accelerated its motion,which led to faster dislocation recovery and smaller dislocation density.The density of carbide increased by the application of high magnetic field.High density carbides increased the critical unpinning stress,which greatly hindered dislocation movement.Therefore,high magnetic field inhibited dislocation recovery and increased the density of residual dislocation in the matrix.The high density dislocations increased the strain energy in the matrix,and finally increased the percent of low angle boundaries and the stability of martensitic laths.Therefore,the application of 12 T high magnetic field obviously inhibited the recovery of martensite.