Creep-Fatigue Properties And Failure Mechanism Of China Low Activation Martensitic Steel At 550? In Vacuum Environment

Withstanding the constant load during the stable operation in International Thermonuclear Experimental Reactor(ITER),Test Blanket Module(TBM)may also be subjected to the combined effect of complex loads such as pulse loads,which will induce creep-fatigue interaction damage.Therefore,the study of creep-fatigue damage behavior and mechanism is significant for the design and optimization of fusion blanket.In this paper,China Low Activation Martensitic(CLAM)steel was selected as the research object.Low Cycle Fatigue(LCF)and Creep-Fatigue(CF)tests were mainly conducted under vacuum of 5×10-3 Pa at 550? with the total axial strain control mode.To ensure the safe operation in ITER and even the future fusion reactor,the paper explored the effect of hold time,strain amplitude and peak stress holding mode on the creep-fatigue mechanical behavior and failure mechanism of CLAM steel.Firstly,the thesis studied the LCF mechanical behavior at 550? under different strain amplitudes.The test results showed that the peak stress continued to soften under cyclic load.After fatigue failure at 0.5%strain amplitude control,the dislocation density reduced from 2.4×1014 m-2 to 0.2×1014 m-2,the mean lath width coarsed from 0.7 ?m to 1.3 ?m.Based on the theory of dislocation strengthening and the sub-grain strengthening,the dislocation density decreasing and lath coarsening were the main reasons for the decrease of the peak stress.According to the relationship of fatigue life and the elastic/plastic strain,the manson-coffin model of CLAM steel was established,and the fatigue life decreased with the increase of strain amplitude.Comparing with the fatigue life in atmospheric environment,higher fatigue life was obtained in vacuum.Meanwhile,the fatigue life of CLAM steel was similar to that of other RAFM steels in vacuum.Secondly,on the basis of LCF in vacuum,the creep-fatigue interaction behavior under compression holding mode was analyzed.Cyclic softening and stress relaxation were occurred,and they were caused by the decrease of dislocation density and the coarsening of the laths and the precipitates.Meanwhile,the mechanisms of lath coarsening and large angle grain boundary formation were analyzed.The creep-fatigue life was lower than pure fatigue life due to the promotion of the load holding to the initiation of cracks and the nucleation of creep cavities.In addition,the increase of load holding time and strain amplitude would result in more cyclic softening and stress relaxation,and lead to the decrease of the creep-fatigue life.The creep-fatigue interaction controlled by low strain amplitude was more serious than that controlled by higher strain amplitude.The analysis showed that the decrease of dislocation density and the coarsening of laths were more obvious under low strain amplitude.Finally,the tension holding mode was used to analyze the effect of load holding methods on creep-fatigue interaction behavior.The results indicated that the cyclic softening and stress relaxation were also produced by tension load holding.The creep-fatigue life of the CLAM steel was more sensitive to tensile loading,which was different from the compression load sensitivity in atmosphere environment.Fracture behavior analysis showed that,except for the influence of oxidation on crack propagation of tension and compression holding,the stress relaxation was the main reason of the sensitivity.