On the Plastic Deformation of Ti6al4v in Shot Peening

Abstract

The plastic deformation behavior within the surface deformed layer of Ti6Al4V alloy samples subjected to shot peening at room temperature was evaluated thoroughly for its depth of impact, strain localization, grain rotation and active slip system. A complex hardening–softening phenomenon is elucidated as severe shot peened condition induced grain fragmentation, lattice strain, dislocation activation/annihilation behavior. The plastically deformed layer thickness and strain localization were analyzed from local misorientation data using the electron back scattered diffraction (EBSD) technique selected for its crystallographic orientation and lattice distortion characterization. A severely deformed outer surface un-indexed black layer with depths of 8, 16, and 26 μm at three different shot peening intensities, respectively, were characterized. . Further beneath the layer is defined as the moderately deformed layer with a misorientation angle of up to 2° and locally to10° for severe deformed location with depth of about 28, 34, and 46 μm. The total plastic deformation layer is characterized at 37, 50, and 72 μm, respectively for the lower, medium and high shot peening conditions. Strong strain localization within the moderately deformed layer is observed with a majority local misorientation angle of up to 2°, indicating a large amount of dislocation accumulation. The strain localization concentrated along small acicular (2 - 5 μm) grain boundaries, triple points, β grain boundaries, as well as phase boundaries (between α & β phase), indicating their blockage effect to dislocation movement as well as its easy nucleation at those strain localized areas. Grain fragmentation into nano-sized crystallites was observed within the moderately deformed layer with grain kinking and rotation. The Schmid factor analysis shows possible activation of Basal, Prismatic, and Pyramidal slip systems working in multiple slippery modes for complex slip interactions and Prismatic dominated type within the hCP Ti6Al4V lattice structure in the way of dislocation gliding as no twinning was observed within the deformed plastic layer in all the samples. Excessive grain rotation is observed at the severely deformed location within the moderately deformed layer, which may surpass dislocation mode. A more difficult Pyramidal type of slip could be activated under critical conditions.

Keywords: plastic deformation, Shot peening, Strain Localization, dislocation, grain rotation