Fabrication and characterization of bulk nanostructured materials of steel and aluminum alloys by air blast shot peening
In recent years, bulk nanostructured materials processed by methods of severe plastic deformation (SPD) have attracted the growing interest in academia and industry. This interest is caused not only by unique physical and mechanical properties inherent to various nanostructured materials, e.g., higher hardness and Young's modulus, etc, but also by the ability of SPD process to create bulk nanostructures. In this dissertation work, air blast shot peening (ABSP) a SPD process using very high shot velocities was used to achieve high strain rates of surface material causing bulk surface nanocrystallization. ABSP process parameters has been extensively investigated to understand their effects on bulk nanostructures formed on steel (AISI 1018, AISI 1045, AISI 1075) and aluminum (Al 6061, Al 7075) components. Finite element simulations have also been carried out to study the effect of peening parameters on stress/strains and temperatures in the peened component. The peened samples have been characterized using optical microscope and scanning electron microscope (SEM) and the grain size shows a gradual reduction from the subsurface to the surface. The grain size of the steel and aluminum peened samples based on the TEM images was less than 50 nm. The nanocrystalline (NC) zone showed a considerably higher hardness as compared to the unpeened material. Lastly a study of corrosion characteristics of the NC layers fabricated on steel and aluminum alloys indicated a higher corrosion potential of the NC layers as compared to the bulk material.