Nanostructure formation mechanism of α-titanium using SMAT

A nanostructured surface layer up to 50 mum thick was produced on commercially pure titanium using surface mechanical attrition treatment (SMAT). The microstructural features of the surface layer produced by SMAT were systematically characterized by cross-sectional optical microscopy observations, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) investigations. The grain refinement process, accompanied by an increase in strain in the surface layer, involves: (1) the onset of twins and the intersection of twin systems, (2) the formation of dislocation walls, (3) the nucleation of microbands associated with the splitting of dislocation walls, (4) the subdivision of microbands into low angle disoriented blocks and then highly disoriented polygonal submicronic grains, and (5) further breakdown of submicronic polygonal grains into randomly oriented nanograins. The final grain refinement step to form nanograins has been discussed on the basis of a recrystallization process. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.