Microstructural Evolution And Influence On The Mechanical Property Of Reduced Activation Ferritic/martensitic Steel

Reduced activation ferritic/martensitic（RAFM）steel has been considered to be one of the reference structural materials for future fusion power reactor,owing to its low thermal expansion coefficients,high thermal conductivity coefficients at elevated temperatures and good irradiation resistance.The operating temperature of existing RAFM steels is limited by the mechanical strength reduction in high temperature region（>550°C）.In order to enhance the mechanical properties of the RAFM steel at elevated temperatures（>550°C）,the microstructure of it needs to be further improved on the existing basis of RAFM steels.Under this background,a series of strengthening methods have been used to control microstructures of RAFM steel in the present work,such as decreasing the normalizing temperature,warm rolling&tempering processes,and adding Ta,Zr elements.The microstructure characteristic parameter and thermal expanding were detected by the optical microscope（OM）,transmission electron microscopy（TEM）,electron backscatter diffraction（EBSD）,X-ray diffraction（XRD）,laser scanning confocal microscopy（LSCM）equipment,and mechanical properties were detected by electronic universal testing machine and electronic creep testing machine equipment.The relevance of microstructures and mechanical properties has been revealed.In addition,the correlation of annealing twin and different cooling rates on the lath martensite transformation in RAFM steel was studied.Based on a large number of experimental studies and corresponding mechanism analyses,the main results were summarized as follows:Lath martensite transformation in RAFM steel relates to the discontinuity of the transformation rate,which exhibits a series of martensite transformation rate maxima（peaks）,especially in low cooling rates.If the cooling rate was not high enough,the lath martensite transformation rate peaks will be divided into two groups by annealing twins in parent austenite.These transformation rate peaks in the first group were mainly caused by lath martensite transformation in the non-twin zones in the PAGs.Then,the last group mainly consisted of a single transformation rate peak,which was primarily caused by the martensite transformation in the twin zones in the PAGs.Decreasing the normalizing temperature refine the M₂₃C₆ carbide particles through refining the parent austenite grains（PAGs）and martensite blocks in RAFM steel.Warm rolling&tempering can refine the M₂₃C₆ carbide particles by enhancing the amount of sub-grain boundaries（SGBs）,since more M₂₃C₆ carbide particles can nucleate at SGBs.Adding Ta,Zr elements can refine the M₂₃C₆ carbide particles and increase the amount of MX carbide particle in the RAFM steel.Large number of MX carbide particles limit the mobility of dislocations in martensite lath,delay the occurrence of dynamic recovery and recrystallization of matrix in RAFM steel,and increase the stability of microstructure morphology of martensite lath during tempering or creeping at elevated temperatures.Then these refined microstructural parameters were studied to be beneficial to the stability of microstructure in RAFM steel at elevated temperature.The difference of normalizing temperature will influence the size of M₂₃C₆carbide particles,and the dislocation density of RAFM steel matrix,then the interaction of interstitial solute atoms with dislocations is different.Thus,the RAFM steel samples with different normalizing temperatures treated present different tensile mechanical properties at the same temperature.More M₂₃C₆ carbide particles nucleated at SGBs through warm rolling&tempering treated,then the stability of microstructure of RAFM steel can be enhanced.Large number of MX carbide particles in the RAFM steel increased by adding Ta,Zr elements can significantly increase the yield and tensile strength of RAFM steel at elevated temperatures.In addition,the dynamic strain aging（DSA）can influence the distribution of dislocations in the process of plastic deformation of RAFM steel in the middle temperature range,but the dislocations are mainly tangled in the lath martensite boundaries when deforming at RT.Adding Ta,Zr elements can significantly increase the stability of microstructure of RAFM steel in the creep process at elevated temperatures.In the process of creep deformation,there will precipitate C14-Laves Fe₂W phase particles（rich in Fe,W,Cr,and Si）in the RAFM steel.The influences of chromium（Cr）on the thermal properties of C14-Laves Fe₂W phase basing on the quasi-harmonic approximation calculation,and the coupling effect of Cr and He on the mechanical behavior of C14-Laves Fe₂W were studied by first principle method.Cr can decrease the Gibbs energy and bulk modulus of the C14-Laves Fe₂W phase,and can enhance the thermal expansion coefficient of the C14-Laves Fe₂W phase,especially when temperature is over 500 K.Cr can reduce the He trapping capacity of C14-Laves Fe₂W phase.In addition,Cr can decrease the tensile strength of the C14-Laves Fe₂W phase aligning to the[0001]direction,and it can also coordinate He to further decrease the tensile strength of C14-Laves Fe₂W.