Contrastive Study On Microscopic Evolution Of Austenite And Ferrite With Cyclic Deformation

Iron and steel materials have very wide and important applications in engineering.According to the lattice type of iron,it can be divided into body-centered cubic(BCC)and face-centered cubic(FCC)structure.Because components are inevitably subjected to external loads during service,they are prone to cyclic deformation,such as fatigue and ratcheting,which will affect the service life and operation safety of components.Plastic deformation of materials is essentially macroscopic indication of dislocation movement.The difficulty of dislocation movement and its evolution form are directly affected by the crystal structure of materials through the influence of Peierls force and other factors.Therefore,discussing the difference of macroscopic deformation behavior from the point of view of microcrystalline structure,and connecting dislocation,grain boundary change with cyclic stress-strain relationship is of great significance for understanding the deformation mechanism of iron and steel materials.In this study,austenite(Z2CND18.12N austenite stainless steel)and ferrite(industrial pure iron)in submillimeter scale were used to compare and analyze microstructures and structure evolution during fatigue and ratcheting deformation based on X-ray diffraction(XRD)and electron backscattered diffraction(EBSD)technology.It would be helpful for the deep understanding of the deformation mechanism and the corresponding difference between FCC and BCC were deeply understood,which would laid a foundation for developing damage detection and life assessment methods methods.The main research contents and conclusions are as follows:(1)Comparing the cyclic deformation response of austenite and ferrite,austenitic stainless steel with face-centered cubic structure exhibits cyclic hardening first and then cyclic softening and at last secondary hardening during fatigue deformation,whereas industrial pure iron with bace-centered cubic structure only exhibits cyclic hardening.Both exhibit cyclic hardening during ratcheting deformation.The strain and variation of strain amplitude in ratcheting deformation of two materials are obviously larger than that of fatigue deformation,indicating that the former is more seriously damaged.(2)Cyclic loading surface morphology and XRD analysis of austenite and ferrite show that the total dislocation density of both materials increases with cyclic deformation,where the proportion of edge dislocation in austenite increases obviously with cyclic loading,and that of screw dislocation decreases continuously,but screw dislocation dominates in ferrite.The reason is that Peierls force and stacking fault energy of austenite are lower,dislocation slip with edge dislocation is easier.In contrast,ferrite has higher stacking fault energy and is prone to cross-slip of screw dislocation.Dislocation density corresponding to ratcheting deformation and proportion of edge dislocation are significantly higher than that of fatigue.(3)EBSD analysis shows that:during cyclic loading,the band contrast(BC)of austenite and ferrite decreases and local misorientation(M_L)increases,and the M_L and BC values of austenite are lower than those of ferrite in fatigue deformation.This is due to the stress concentration at grain boundaries and obvious localized deformation during austenite fatigue,which results in broken grains and the calculation area of grain interior becomes small and also the whole M_L value.However,solid solution strengthening effect of industrial pure iron is weak,so the slip system is easy to be activated and grains are intact easy to deform as a whole part,which results in a larger M_L value.After cyclic deformation,the number and proportion of low angle grain boundaries(LAGBs)in the two materials increase rapidly,and(50)3 grain boundaries of austenite decreases rapidly.The reason is that(50)3 grain boundaries block connectivity of high angle grain boundaries(HAGBs)network,and decreases due to the influence of dislocations during deformation.Compared with fatigue deformation,the BC value of ratcheting deformation of these two materials is smaller and M_L is larger.