Computational Polarised Light Microscopy: Rapid Mapping of Micro-Texture Regions in Ti

Mapping of grain structure and crystallographic texture in Ti alloys is of huge technological importance.  The processing of these alloys leads to ‘macro-zones’ or 'micro-texture regions' (MTRs) in which volumes of material much larger than the grain size are all close to a common underlying crystal orientation.  These MTRs have significant influence on mechanical properties including fatigue and cold dwell fatigue that have been implicated in component failures (see for example investigation reports on Air France flight 66 https://www.bea.aero/uploads/tx_elydbrapports/BEA2017-0568.en.pdf and United Airlines flight 1175 https://t.co/ixzxH9kgWx?amp=1).

Studying the statistics of the size, shape and crystallographic orientation distribution of MTRs is difficult as even the most rapid EBSD measurements struggle to map large enough areas.  The hexagonal crystal structure of Ti leads to anisotropy in optical properties that make the grain structure visible under polarised light.  Systematically varying the illumination conditions and quantifying the reflected light intensity should allow the orientation of the c-axis to be determined.  There are multiple potential strategies that could be used to extract information allowing the c-axis orientation to be mapped.  There is potential for new low cost instrumentation for rapid quantitative crystallographic mapping.

zz cross polars

Deformation twins in zirconium - rotating crossed polarisers

The project will explore the best routes for computational polarised light microscopy in terms of speed and ease of operation, reliability, accuracy and precision of the results and construct the instrumentation to deliver this.  The optimised method will then be applied to explore how deformation process conditions alter MTR distributions in a common Ti alloy.  There is also scope to explore multiple other application eg more local analysis of deformation structures, mapping non-cubic inclusions in Al alloys, geological materials.  The project will involve collaboration with Rolls Royce failure investigation team.