GND Distributions in deformed Ti-6Al-4V

We have been developing ways of using the higher angular resolution that cross-correlation-based analysis of EBSD patterns provides to determine the dislocation distribution in deformed metals. Misorientation gradients can be linked to the density of geometrically necessary dislocations (GNDs) needed to bend and twist the lattice in the way that is observed. Unfortunately there are many many ways that a set of dislocations can be used to generate a given lattice curvature but we can find a possible and energetically favourable arrangement. This gives a lower bound estimate of the dislocation content.

We have made a reasonably extensive study of Ti-6Al-4V in the annealed state, and after tensile, fatigue and dwell fatigue loading. The material had been processed to give a microstructure composed primarily of globular alpha grains with small amounts of beta predominantly at the grain boundaries. In the annealed state the GND density was mostly low (mode of ~1014 m-2) but there was the occasional grain with a much higher density often with some alpha phase adjoining it. Tensile deformation by 6% gives a marked increase in the GND density as was obviously expected (mode of ~2x1016 m-2) and various structures from cells to quite planar slip bands could be seen. The type dislocations were in greater abundance by ~20x compared to the type. Using these GND densities within a Taylor model allowed the intragranular patterning of the flow stress to mapped out.

Ti64 GND annealed

Ti64 GND tension
flow stress

Contact: Philip Littlewood and Benjamin Britton and Angus Wilkinson

High Resolution Elastic Strain Measurement from Electron Backscatter Diffraction Patterns :-  New Levels of Sensitivity
Wilkinson A. J., Meaden G. and Dingley D. J.
Ultramicroscopy (2006), vol. 106, 307-313

Electron backscatter diffraction study of dislocation content of a macrozone in hot-rolled Ti–6Al–4V alloy
TB Britton, S Birosca, M Preuss and AJ Wilkinson
Scripta Materialia, (2010), vol. 62, 639-642

High-resolution electron backscatter diffraction: an emerging tool for studying local deformation
AJ Wilkinson, EE Clarke, TB Britton, P Littlewood, PS Karamched
Journal of Strain Analysis for Engineering Design, (2010), vol. 45, 365-376

Geometrically necessary dislocation density distributions in Ti–6Al–4V deformed in tension
PD Littlewood, TB Britton, AJ Wilkinson
Acta Materialia, (2011), vol. 59, 6489-6500