Low Cycle Fatigue Behaviors And Mechanism Of Gradient Nano-Structured CLAM Steel

The service environment of structural materials for the blankets of fusion reactors is severe.The design of materials is one of the bottlenecks in the research of the blankets of fusion reactors.Pulsed plasma operation mode is used in most of the design scheme of fusion reactors,which puts forward higher requirements on the fatigue performance of structural materials.According to the requirements of fatigue performance in structure materials and the goal to improve the fatigue performance of China low activation martensitic(CLAM)steel,the fatigue behaviors and mechanism of gradient nano-structured(GNS)CLAM steel were investigated in this thesis.The main research contents of this thesis are as follows:A surface nano-submicron-micron gradient structure was formed on the surface of tempered CLAM steel by means of surface mechanical rolling treatment(SMRT).The graidient layer was analyzed and characterized.The results showed that the thickness of the gradient structure was～85 μm.The average size of the grains in the nano-grained layer was～43 nm.The main reason for the formation of gradient nanostructures on the surface of CLAM steel was that with the continuous accumulation of strain during SMRT,grains were refined due to dislocation entanglements and the conversion of sub-grains to nano-grains.The effect of holding time at 550℃,the upper limit temperature of CLAM steel,on the gradient nanostructure of GNS-CLAM was studied.The results showed that when the holding time increased from 5 min to 20 min,the average nano-grain size increased from 47 nm to 67 nm.When the holding time was continued to 60 min,the grain size remained almost unchanged.The dragging effect of the tantalum-riched MC phase in the gradient structure on the grain boundary was the main reason for the stable grain size with increasing holding time.The low cycle fatigue performance of GNS-CLAM at room temperature was studied.The results showed that when the stress amplitude was 425 MPa,the fatigue life of GNS-CLAM was 57442 cycles,which was 6.3 times higher than that of CLAM steel without SMRT.The main reason for the improvement of fatigue performance at room temperature was the high strength of the nano-grain layer in GNS-CLAM steel,which may suppress the localization of strain.Furthermore,the dislocation entanglements in the gradient layer may slow the fatigue crack growth rate.Therefore,an ideal structure resistant to fatigue crack initiation and propagation was formed.The low cycle fatigue performance of GNS-CLAM at 550℃ was studied.The results showed that when the stress amplitude was 260 MPa,the fatigue life of GNS-CLAM was 15415 cycles,which was 4.5 times higher than that of CLAM steel without SMRT.The low cycle fatigue life prediction model of GNS-CLAM at 550℃ was established.Strength at high level induced by nano grains with stable sizes and the suppressed fatigue damage during cyclic loading caused by the stable dislocation structures in the gradient layer were the reasons for the improved the high-temperature fatigue performance of GNS-CLAM steel.In summary,GNS-CLAM steel was produced.The low cycle fatigue behaviors and mechanism of GNS-CLAM steel at high temperature was studied.The fatigue life of CLAM steel was significantly improved.The results of this thesis may provide theoretical basis and reference for the development of structure materials in the blankets and optimization of fatigue performance.