| 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 propose 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, especially formation of nano sized crystallites from coarse polycrystals during plastic deformation, 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 and thermal stability of the nanostructured surface layer in Al-Zn-Mg alloy sample induced by surface mechanical attrition treatment (SMAT) was investigatedby means of X-ray diffraction (XRD), transmission electron microscope (TEM), high-resolution electron microscope (HREM) and optical microscope. Hardness of the surface layer were measured, and the properties of original and surface mechanical attrition treatment Al-Zn-Mg alloy samples under compression and bending conditions were tested. Based on the evolution of their microstructures during surface mechanical attrition treatment, the grain refinement mechanism of Al-Zn-Mg alloy sample was proposed.The micro-region structure of junction boundary and properties of 00Crl8Ni5Mo3Si2/16Mn combined steel plate produced by explosive welding technology were investigated by means of scanning electron microscopy (SEM), TEM, HRTEM and optical microscope. Based on the microstructural characteristics of junction boundary during explosive welding , the grain refinement mechanism of 00Crl8Ni5Mo3Si2/16Mn junction boundary was proposed. The main results are given as follows:1. Equiaxed nanocrystallites with random crystallographic orientations were obtained in the surface layer of Al-Zn-Mg alloy sample by means of SMAT. The average grain size of nanostructured regime is about 20nm. Depending on the gradient variation of microstructures, the SMATed surface layer can be subdivided into four sections along depth from the top surface, i.e., nanostructured regime (0~20^im), submicro-grained regime (20~50|j.m), transition regime (dislocation cell and tangling regime 50~100um) and matrix. 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 deformation regime, i.e., dislocation wall and dislocation tangling. The dislocation wall is formed by dislocation slipping and accumulating. The dislocation tangling isconfused arrangement of high density dislocation. With the strain increasing, the dislocation wall and tangling transformed into dislocation cells and subgrains further.3. The following physical processes are involved in the grain refined process of AL-Zn-Mg alloy by SMAT:(l)Development of dislocation walls and dislocation tanglings in original grains and in the refined grains and subgrains (under further straining) as well;(2) Transformation of dislocation walls and dislocation tanglings into subboundary with small misorientation separating individual cells or subgrains;(3) Evolution of subboundary to highly-misoriented grain boundary. Grains are subdivided during straining on a smaller and smaller scale.Formation of dislocation walls and subgrain boundary in refined grains or subgrains further subdivide them into smaller grains.4. The high strains with a high strain rate are necessary for the formation of nanocrystallites during plastic deformation of metals.5. The hardness of the surface layer in the surface mechanical attrition treatment Al-Zn-Mg alloy sample was evidently enhanced due to the grain refinement, and the hardness decreases gradually with the increase in depth. The nanostructured surface layer can increase yield strength of Al-Zn-Mg alloy.6. The morphology of junction region of combined steel plate is of sine-wave form and the fine grain region was formed nearby the junction interface. The white-bright layer consists of nanocrystalline and amorphousstructure formed due to rapid cooling under adiabatic condition and strong plastic deformation.7. Nanocrystalline and amorphous film may be fabricated by the explosive welding.
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