Studies On Preparation Of High Fraction Of Twins In Martensite Stainless Steel And Twins Strengthening Mechanism

To synergistically promote strength and toughness is always a key issue in the fields of research and engineering application for metallic structure materials.Abundant formation of deformation twins inside the matrix by deformation-working is one feasible method for it.In this study,two kinds of martensitic stainless steels with high density of dislocations inside(A-steel and B-steel)were selected to experience a repetitive high stress loading-unloading(RHSL)treatment at room temperature.The effects of the loading rate of force,holding stress and loading cycle number in the RHSL treatment on the formation of twins and mechanical properties were investigated by microstructure observation and mechanical property tests.The optimized conditions for preparing high fraction of deformation twins at room temperature and low strain rate were obtained by the RHSL treatment with only micro-plastic deformation,and,new strengthening and toughening models of martensitic stainless steel were constructed on the basis of the significant variations of dislocation density and twin fraction in martensitic stainless steel during the RHSL treatment.The innovations of this study are as follows:(1)At room temperature and low strain rate,high fraction twins are successfully generated in martensitic stainless steel by RHSL treatment with micro-plastic deformation.By adjusting the parameters of RHSL treatment,the loading rate,load retaining stress,and loading cycle number,an optimized process of RHSL treatment on martensite stainless steel is obtained,namely,at room temperature,with the yield strength of the martensite stainless steel as the holding stress,0.64 s of the loading time and 30 of the loading cycle number.This treatment can generate the highest fraction of twins(13.58%TBs)in martensite stainless steel and hence synergistically improve the yield strength(1060 MPa)and plasticity(elongation 19.0%).(2)The deformation twins undergo a 3-stage evolution during the RHSL treatment:early-stage incubating → med-stage abundantly forming → later-stage de-twinning,corresponding to strain-hardening → structure-softening → re-hardening of mechanical properties,respectively.(3)The continuous accumulation of stress concentration with RHSL treatment leads to shear strain against martensite laths that not only causes the twining in early stage of RHSL treatment but also leads to the de-twining in later stage.Based on HRTEM observation on twin boundaries and sub-grain boundaries,the crystal shear displacing models to illustrate the behaviors of twining and de-twining into sub-grans are constructed.(4)The yield strength is highly correlated to the dislocation density while the elongation highly to the fraction of twin boundaries.Based on the remarkable variations of the dislocation density and the fraction of twin boundaries in martensitic stainless steel during the RHSL treatment,new strengthening model and new toughening model of martensite stainless steel are constructed,which can be used to determine the contributions of dislocation density and twins’ fraction in strengthening and toughening.