Understanding the deformation of polycrystalline structural materials is the key to improving performance and reliability of complex engineering components and systems. The interaction between grains in the metal and how they behave under stress is vital to determining the strength of a component and how it will deform. However capturing full details of polycrystalline deformation is a significant challenge, partly because of the sheer volume of information involved, and partly due to the complexity of interactions, and the interplay between global and local effects. Scientists from the University of Oxford have been using the Test Beamline B16 to study deformation behaviour using a novel technique called micro-beam Laue diffraction.
Deformation in polycrystalline materials does not occur uniformly in the constituent grains, but shows strong inter- and intra-granular variations. The local deformation response depends on lattice orientation, anisotropic elastic-plastic properties, hardening and damage behaviour and the microstructural neighbourhood. Understanding these complex interactions is vital for the construction and validation of crystal plasticity models and the fundamental understanding of the material’s deformation behaviour.
This experiment used a novel setup for micro-beam Laue diffraction on the B16 Test beamline at Diamond, with the aim of collecting high quality datasets characterising mesoscopic deformation behaviour within a polycrystal containing a small number of large grains. Waisted dogbone shaped samples were machined from a pure Nickel sheet and then heat treated to promote grain growth and furnace cooled to minimize cooling-induced residual elastic strains.
Micro-beam Laue diffraction provides an excellent tool for the study of inter- and intra-granular deformation, allowing, unlike other microscopy methods, measurements within the bulk of the material. A focused and collimated “pink” beam is used to probe the interior of individual grains within a polycrystalline sample. The diffracted radiation forms a pattern of Laue spots which is captured by an area detector. From the spot positions, lattice orientation and elastic strains can be deduced. The intensity distribution within individual spots can be interpreted in terms of the dislocation arrangement within the gauge volume.
The results show that individual grains can be easily identified and followed during the deformation process. Laue spots arising from a “hard”, a “soft” and a “streaking” grain confirm the grain “hardness” prediction based on Schmid factor and provide some clues to the underlying dislocation microstructure and deformation mechanism.
"Over the last two decades the capabilities for numerical simulation of polycrystalline deformation have advanced very considerably. It is now possible to model scale- and orientation-dependent deformation behaviour of polycrystalline metals. The outstanding challenges concern our ability to validate these models by comparison with experimental observations at the relevant length scales of a few microns or even nanometres. The existing advanced microscopy techniques allow imaging at those resolutions, but do not provide the capability for measuring the elastic strain and evaluating stress - the most important parameter for structural engineering. The developments in micro-beam Laue capabilities at Diamond undertaken by our Oxford-based team, in collaboration with Dr Igor Dolbnya on the B16 Test Beamline, open up the way for non-destructive intra-granular stress analysis in polycrystals. Our initial findings and the recent instrumental developments on B16 are sufficiently encouraging for us to hope that achieving micron-resolution mapping will become possible in the next series of experiments. This would open up the way for confirming or disproving some of the most interesting current models of micro-scale deformation."
Prof Alexander Korsunsky, University of Oxford
Work towards a more detailed analysis of these phenomena and comparison with crystal plasticity predictions is currently underway.
Felix Hofmann, Xu Song, Igor Dolbnya, Brian Abbey, Alexander M. Korsunsky, Probing intra-granular deformation by micro-beam Laue diffraction, Procedia Engineering, Volume 1, (2009) 193-196
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