MicroMech_gallery17

**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.

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 doi:10.1016/j.ultramic.2005.10.001 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 doi:10.1016/j.scriptamat.2010.01.010 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 doi:10.1243/03093247JSA587 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 doi:10.1016/j.actamat.2011.07.016

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.