The Mechanical Behavior Of Gradient Structure IF Steel

Single stage, homogeneous nanometer metal materials at roomtemperature in the process of the quasi static stretching, showing thedefect of low plasticity is a major bottleneck of restricting itsdevelopment. And by changing the microstructure, producing strain，gradient and stress gradient in order to improve the nano metalcomprehensive mechanical properties, offers a new way for us toovercome the bottleneck. At the same time, the gradient structurematerials in good strength and plasticity after the match, also appearedsimilar to traditional yield reduction behavior of low carbon steel. Soabout gradient in nanometer metal yield behavior, different explanationfor its microscopic mechanism and its emergence and existence whatinfluence on the performance of the material mechanics, materialsresearch field is a gradient structure quite important scientific problems.Based on the gradient macro mechanical properties of nano metal, yieldbehavior and strain hardening behavior, surface local straindistribution.To deepen the understanding of the scientific problems, and expand the cognition to the phenomenon of gradient structure in the metalyield, eventually hopes to the optimization of nanostructures metalmechanical properties, the understanding of the gradient of nano structuremetal mechanical behavior and development to provide a little reference.This article selects the IF steel as experimental material, making useof Mechanical grinding Surface Treatment (SMGT) technology principle,processing the IF steel sheet by Surface nanocrystallization way, and byadopting different processing technology, prepared a variety of gradientstructure with different yield behavior IF steel. According to the quasistatic stretching experiment results, the micro-hardness test, cyclic stressrelaxation, three dimensional full strain testing, such as experimentalmethod, respectively of gradient structure in the IF steel yield behavior toexplain several different points.The main research results are as follows:(1) In the processing of SMRGT, the yield strength can be improvedto annealing state of2-3times, and keep the uniform elongation at90%of annealing stage. However, with the change of the processingtechnology, the GS-IF steel appeared three kinds yield behavior，just as“up turn, yield drop following up turn, yield drop”,plus the sampleannealed state yield phenomenon, a total of four kinds of different yieldbehavior.(2) From the perspective of the microhardness test of thesesamples,the surface layer hardness is higher than core layer, and the hardness of the core layer is higher than that of annealing state samples,this showing the sample in the process of machining, the both the surfacegrain size and core grain is received influence; At the same time,comparing the hardness between before and after stretching, account forthe main source of work hardening ability is rooted in core sample grain,but the surface layer grain is also involved in the work hardening process.(3) From the perspective of the results of cyclic stress relaxation, themovable dislocation density drops，leading to the phenomenon such as"up turn, yield drop".When movable dislocation is insufficient, leading tothe shortage of the work hardening rate, at the same time physicalactivation volume increasing, indicated dislocation motion mean free pathto across the barriers hinder increasing; And the reply of work hardeningability is due to the movable dislocation recovery. The physical activationvolume decreased, indicating dislocation movement through the obstaclesof the mean free path is reducing.(4) From the experimental results of DIC, the emergence of "upturn" phenomenon, sample local appear high strain zones, but the highstrain area will not develop into the necking area, it will continue withtensile gradually disappear;"Yield drop+up turn" phenomenon appears,sample local there will be two high strain zones, one of the high strainzone eventually evolved into the necking zone, and the other high strainarea also gradually disappear in the process of drawing; For a simple "yield drop" phenomenon, the samples will be at the beginning of thedrawing, also can appear a high strain zone, high strain zone appearscorresponds to the yield strain in effect at the time of the fall, and the highstrain area will soon be developed into the necking area.(5) The GS-IF steel put up different yield behavior is related to thetensile strain path: when the sample processing ball rolling directionparallel to the tensile loading direction, no matter how to change thepressuring depth, the samples are only a smooth and continuous yieldingbehaviour; When the ball rolling direction perpendicular to the directionof tensile loading, with the increase of pressuring depth, sample canappear in succession "up turn","yield drop+up turn","yield drop", suchas different yield phenomenon.