Nanocrystallization of zirconium subjected to surface mechanical attrition treatment

Abstract

A nanostructured surface layer with thickness of about20 µm was formed on commercially pure zirconium using surface mechanical attrition treatment(SMAT). The microstructural features of the surface layer were systematically investigatedusing optical microscopy (OM), x-ray diffraction (XRD), transmission electronmicroscopy (TEM) and high-resolution transmission electron microscopy (HRTEM),respectively. Based on the results obtained, a grain refinement mechanism induced byplastic deformation during SMAT of Zr is proposed. At the initial stage of SMAT,twinning dominates the plastic deformation of Zr and divides the coarse grains of Zrinto finer twin plates. With increasing strain, intersection of twins occurs, anddislocation slips are activated, becoming the predominant deformation mode insteadof twinning. As a result of the dislocation slips, high-density dislocation arraysare formed, which further subdivide the twin plates into subgrains of size about200–400 nm. With a further increase of strain, the dislocations accumulate andrearrange to minimize the energy state of the high-strain-energy subgrains, the densedislocation walls convert to grain boundaries, and the submicronic grains aresubdivided, leading to the formation of nanosized grains at the top of the treatedsurface.