| Increasing interests have been focused on nanocystalline(nc)materials during the past decade with the anticipation that their properties will be different from and often superior to those of conventional coarse-grained materials.However,difficulties exist in obtaining "ideal"(i.e.,flaw-free, contamination-free,residual stress-free and with a sufficiently large sample size)nc samples by using the present preparation techniques.With increasing evidences of novel properties in nc materials,it is reasonable to achieve surface modification by the generation of a nanostructured surface layer so that the overall properties and behavior of the material are significantly improved.A nanostructured surface layer has been successfully obtained in many material systems.Nevertheless,up to now,a clear scenery of microstuctures and properties of the nanostructured surface layer is still lacking.Therefore,understanding of the underlying mechanism for grain refinement by plastic straining is becoming more and more crucial.In this work,microstructural characteristic at different depths induced by surface mechanical attrition treatment(SMAT)was investigated by means of X-ray diffraction(XRD),transmission electron microscope(TEM);Hardness of the surface layer was measured;During surface mechanical attrition treatment,the grain refinement mechanism of Pure Aluminum sample was proposed.The compressive behavior of Pure aluminum under high strain rate (1000～2300s-1)had been studied by split Hopkinson pressure bar(SHPB)at room temperature.By comparativing compressive stress-strain curve,the influence of strain rate on flow stress and yield strength was analysed;the influence of microstructure on dynamic compression of pure aluminum was studied by using transmission electron microscope(TEM).The main results are given as follows:1.Equiaxed nanocrystallites with random crystallographic orientations were obtained in the surface layer of pure aluminum by means of SMAT.The average grain size of nanostructured regime is about 6nm.Depending on the gradient variation of microstructures,the SMATed surface layer can be subdivided into two sections along depth from the top surface,(1) nanostructured regime(0～20pro);(2)deformation regime(20～65μm).With the depth increasing,the size of grains or cell blocks increases,and the micro-strain decreases.2.There are two kinds of dislocation structures in the plastic deformatoin regime,i.e.,dislocation wall and dislocation tangling.The dislocation wall is formed by dislocation slipping and accumulating.The dislocation tangling is confused arrangement of high density dislocation.With the strain increasing, the dislocation wall and tangling transformed into dislocation cells,subgrains, Dense dislocation walls-Micbands(DDWs-MBs)and Lamellar Boundaries (LBs).With the strain increasing,grains are subdivided on a smaller scale and finally Equiaxed nanocrystallites with random crystallographic orientations were formed.3.The high strains with a high strain rate are necessary for the formation of nanocrystallites during plastic deformation of metals. 4.The microhardness of the nanostructured surface layer is found to be obviously increased,and is twice more than that of the original sample.The microhardness decreases gradually to that of the matrix along the depth of the SMAT sample.5.The polarization curves show that the nanostructured surface layer can improve the corrosion resistance behavior of the samples.6.Flow stress and yield strength of the pure aluminum obviously increases with the increasing strain rate and the stress is sensitive to strain rate positively.7.In the view of the characteristics of the Stress-strain curve there are obvious dynamic recovery and recrystallization features;so we calculate the temperature rising under the condition of approximate adiabatic temperature and from the TEM the dynamic recovery and recrystallization features also is found.So the dynamic recovery and recrystallization of pure aluminum is completely possible under the conditions of high strain rate...
|
PDF Full Text Request