X-ray MicroBeam Characterization of the Near Surface Nanostructure Layer in Ti After Friction Stir Processing

Oleg Barabash 1Rozaliya I. Barabash 1,2G E. Ice 1Zhili Feng 1S. A. David 1

1. Oak Ridge National Laboratory (ORNL), One bethel Valley Road, Oak Ridge, TN 37932, United States
2. University of Tennessee (UTK), Knoxville, TN, United States

Abstract

Spatially resolved white beam Laue X-ray nano- and micro- diffraction at the synchrotron together with scanning electron and orientation imaging microscopy were used to characterize the structural changes in the Ti near surface region after Friction Stir Processing (FSP). It was established that after FSP a special surface layer with nanocrystalline structure is formed within the depth of 300 microns. Probing of this zone with a white microbeam (diameter ~0.5 microns) did not get any detectable signal. However probing of this zone with the white nanosize beam (diameter ~100nm) gave a distinct diffraction patter. Typically several grains were observed within each probing location. Most of the diffraction pattern consisted of long streaked Laue spots. Such streaking is indicating strong plastic deformation in this zone with the formation of strain gradients, geometrically necessary dislocations and boundaries, and resulting in the local lattice curvature in each grain.

Two specific zones are formed underneath the above nanocrystalline layer: thermal mechanical affected zone (TMAZ) and heat affected zone (HAZ). The size and structure of all zones is determined. The grain size increased sharply (by two orders of magnitude) from FSZ to TMAZ zones and reached micron size (5 - 30 microns) in the TMAZ. Intensive streaking of the Laue spots are observed with a microbeam in the TMAZ and HAZ zones. Large densities of geometrically necessary dislocations and strain gradients are found in the TMAZ based on Laue microdiffraction. Dislocation density gradually decreases with depth and reaches the value typical for base material. The geometrically necessary dislocations were inhomogeneously distributed within the TMAZ and HAZ. Inhomogeneity of geometrically necessary dislocations distribution was found at both scales: within the individual grains and between separate grains.

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Presentation: Oral at E-MRS Fall Meeting 2006, Symposium A, by Rozaliya I. Barabash
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-24 17:19
Revised:   2009-06-07 00:44
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