Experimental And Numerical Simulation Study On Microstructure Evolution Of Industrial Pure Iron With Low Cycle Fatigue

Industrial pure iron is composed of a single ferrite structure with good comprehensive strength and toughness.It is a good soft magnetic material and is widely used in electronic industry and other fields.At the same time,as a typical element and structure of iron and steel materials,industrial pure iron has a basic research significance for Body-centered cubic(BCC)metals.There have been a lot of researches on the micro dislocation structure,macro stress-strain and fracture behavior of industrial pure iron after fatigue deformation,but the research on the mechanism of fatigue deformation at the meso level is still insufficient.Therefore,electron backscattered diffraction(EBSD)analysis and crystal plasticity finite element method(CPFEM)simulation were combined to study the microstructure evolution of industrial pure iron under low cycle fatigue;At the same time,the dislocation density was analyzed by X-ray diffraction(XRD),and a comprehensive analysis was made to study the early deformation mechanism of annealed industrial pure iron with low cycle fatigue.The main research contents and conclusions are as follows:(1)According to the macroscopic stress-strain response and surface morphology changes,the fatigue deformation of 1000 cycles(6%fatigue life)of industrial pure iron can be divided into three stages:at the initial stage of loading(0-2 cycles),rapid hardening occurs in industrial pure iron,and slip bands appear in individual grain;In the middle stage of loading(2-100cycles),the hardening rate gradually slows down and the surface morphology has no obvious change;At the later stage of loading(100-1000 cycles),a large number of wavy slip bands appear in the industrial pure iron,the internal deformation of the grain is serious,the surface undulation at the grain boundary increases obviously,and the roughness Ra increases rapidly from 0.27?m to 0.52?m.(2)XRD analysis shows that after the fatigue deformation of industrial pure iron,the strongest peak(110)shiftes to the left as a whole,the residual stress inside the material increases,which is most obvious at one cycle fatigue.The dislocation density is characterized by the modified Williamson-Hall integral width method and the modified Warren-Averbach Fourier integral method.The initial dislocation density is 2.6×10¹⁴m^-2,which increases to6.2×10¹⁴m^-2 after 1000 cycles of fatigue loading.(3)During the fatigue period of 0-1000 cycles,the grains of industrial pure iron are complete without obvious preferred orientation.The trend of local misorientation(M_L)is consistent with dislocation density.At the beginning of loading,0.6-0.9°small angle grain boundary increases;In the middle stage of loading,the change of small angle grain boundary is not obvious;After loading,0.6-1.2°small angle grain boundary increases rapidly again.The M_L changes of the three characteristic orientations<001>,<101>and<111>were analyzed.It is found that the change trend of the mean values of the three orientations is consistent at the early and middle stages of cyclic loading.The<001>orientation increases rapidly at the late stage of loading,and the damage is the most serious.(4)CPFEM 2D polycrystalline simulation shows that the strain LE11 in the loading direction increases rapidly in the initial stage of loading,but the distribution of LE11 remains unchanged in the middle stage of loading,and the Von Mises stress in the grain increases gradually,mainly concentrated in the grain boundary and the junction of three grains.At the same time,the<001>oriented grains are more prone to deformation and stress concentration,which is consistent with the experimental results.