Investigation On Surface Nanocrystallization Of Al-Zn-Mg Alloy And Its Properties

Surface nanocrystallization is a new surface modification technology that uses methods such as severe plastic deformation to generate a nanostructured surface layer. In industry, most of failures of machine parts begin from their surface, so it is important to explore new surface-strengthening technologies in order to improve their durability. Therefore, surface nanocrystallization has been regarded as one of the most prosperous surface strengthening technologies.In this work, an Al-Zn-Mg alloy was selected to be treated by means of the surface mechanical attrition treatment (SMAT) technique, and a nanostructured surface layer was obtained.The microstructure evolution, the forming mechanism of nanocrystallites, and the alloying phenomenon in the process of attrition were characterized using X-ray diffraction (XRD), optical microscope (OM), scanning electronic microscope (SEM) and transmission electronic microscope (TEM). The properties were also examined, including microhardness, thermal stability, corrosion resistance and mechanical property. The conclusions can be summarized as follows:(1) Equiaxed nanocrystallites with random crystallographic orientations were obtained in the surface layer of Al-Zn-Mg alloy samples by means of SMAT. The average grain size of nanostructured regime is about 20nm. In the process of severe plastic deformation, grains were subdivided during straining on a smaller and smaller scale. Dynamic recrystallization may be the final cause of the nanocrystallites formation. The high strains with a high strain rate are necessary for the formation of nanocrystallites during plastic deformation of metals.(2) In the process of surface nanocrystallization induced by SMAT, Fe, Cr and Ni atoms in the GCr15 shots transferred into aluminum surface under intensive mechanical force and formed an inhomogeneous alloying layer of about 30μm in thickness. This might be attributed to the existence of substantive nanocrystalline grain boundaries which made the alloy element atoms could diffuse fast and segregate. This provides a new way to achieve metal nanocrystalline alloying layer at room temperature.(3) Vacuum annealing was carried out for the SMATed samples at different temperatures. The results indicated that the nanocrystalline layer has satisfying thermal stability, i.e. when annealing below 150℃, no significant change can be found for the grain size and also the microhardness; when annealing at 250℃, substantive precipitated phases in nano-scale (about 20nm in size) brought up, and the grain size grew to about 300nm, the surface microhardness was above 400HV, which was still much higher than that of matrix.(4) The chemical stability of SMAT Al-Zn-Mg alloy was studied by using electrochemical corrosion in NaCl, NaOH and HC1 solutions, respectively. The corrosion-resistance of SMATed samples represents downtrends. Grain size has an important effect on the corrosion behavior of nanocrystallites. Reduction of grain size results in enhancing the activity of surface atoms and intergranular atoms, which accelerates the electrochemical reaction.(5) The nanostructured surface layer can increase yield strength of Al-Zn-Mg alloy. The main cause of material strengthening is that the nanostructured surface layer hindered dislocation slippering.