Title

Investigation of the effects of the surface nanocrystallization and hardening (SNH) process on bulk metallic components

Date of Completion

January 2005

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Bulk metallic components with a grain size gradient ranging from nanocrystalline at the surface to coarse grained in the bulk and superior hardness properties have been obtained with the Surface Nanocrystallization and Hardening (SNH) process. The evolution of surface topography and microstructural characteristics, as well as mechanical properties, have been studied as a function of processing parameters employing a variety of analytical techniques. Connections between microstructural features, observed hardness behavior and potentially active hardening mechanisms have been made, arriving at the conclusion that deformation-twin-dominated work hardening is responsible for the 140% increase in hardness displayed by SNH-processed specimens. A grain-refinement mechanism led by multiple deformation micro- and nano-scale twinning and complemented by dislocation activity has been put forth in order to explain the nanocrystallization of the coarse-grained material employed in this investigation. Potential harm from contamination at the processed surface has been assessed and its detrimental effects on the performance of treated components have been discussed. Finally, a few lines of work aimed at further exploring, understanding and exploiting the benefits of the Surface Nanocrystallization and Hardening process have been suggested. ^