Microstructure And Grain Refinement Mechanism Of Surface Nanostructure Magnesium Alloys

Nanostructured metallic materials have been the subject of considerable research in recent years due to their unique microstructure and appealing mechanical property. Surface mechanical attrition treatment (SMAT) cannot only form the nanostructure of bulk density, porosity-free and contamination-free on materials surface layer, but also avoid bonding interface between the nanostructured surface layer and matrix. Mg alloys (HCP) have wide application in industry, especially, communication electron, aeronautics and astronautics, car manufacturing, due to low density, high specific strength and specific stiffness, also abundant resource. SNC Mg alloys possess exceptional mechanical properties. Hence, to study the mechanism of grain refinement and properties of surface layer is of great significance from both scientific and technical view.In this work, microstructural characteristic at different depths of the nanostructured surface layer in AZ91D and AZ31B Mg alloy samples induced by SMAT technique was investigated by means of optical microscope (OM), X-ray diffraction (XRD), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM) and so on. Micro-hardness and corrosion properties of the surface layer were measured. The plastic deformation and grain refinement mechanisms of AZ91D and AZ31B Mg alloy samples were analyzed and discussed. The main conclusions can be drawn as follows:1. Equiaxed nanocrystallites with random crystallographic orientations were obtained in the surface layer of AZ91D Mg alloy sample by means of SMAT. The average grain size is about 50nm at the top surface layer, gradually to more than 100nm at a depth of≈10μm. In the adjacent region of about 10～40μm in depth, the grain size increases from about 100nm to 400nm. With the increasing depth, the size of grains or sub-grains increases, and the micro-strain decreases.2. The microstructure change of SMATed AZ31B Mg alloy is similar to SMATed AZ91D. Depending on the gradient variation of microstructures, the grain size increases from about 40nm to 1～2μm from the top surface to 100pm.3. After the SMAT, the micro-hardness of the surface layer in the AZ91D and AZ31B Mg alloy samples was evidently enhanced, and the hardness decreases gradually with the increase in depth. The increasing hardness of AZ91D Mg alloy may be attributed to the refinement of grains, work-hardening and the re-dissolution ofβ-Mg₁₇Al₁₂. However, the increasing hardness of the single-phase AZ31B Mg alloy only may be attributed to the refinement of grains and work-hardening. Also, to certain extent, the corrosion properties of SMATed AZ91D and AZ31B Mg alloys were downgraded.4. The deformation mode of AZ91D Mg alloy is dislocation glide. There are three kinds of dislocation structures in the plastic deformation regime, i.e., dislocation walls (DWs), dislocation rings (DRs) and dislocation tanglings (DTs). The DWs and DRs are formed by dislocation slipping and accumulating. The DTs is confused arrangement of high density dislocation. With the increasing strain, they transformed into dislocation cells (DCs) and subgrains further. Finally, dynamic recrystallization (DRX) formed by increasing strain and temperature has induced refinement grains.5. During the SMAT, the plastic deformation of AZ31B Mg alloy was transformed from deformation twinning into dislocation slip with increasing of strain. DRX induced the formation of nanocrystalline with high strain levels.6. Besides percentage composition of Al,β-Mg₁₇Al₁₂ also plays an important role for the transformation of plastic deformation mechanism for Mg alloys from twinning into dislocation slip.