Microstructure crack-growth relationships in Ti6Al4V

Titanium alloys are used extensively in the manufacture of aero-engine components, owing to their exceptional strength and relatively low density. These alloys owe their exceptional properties to their structure at the microscopic scale, the microstructure, which can be controlled during alloy processing and component manufacture.

One of their important properties of these alloys is their resistance to fatigue strength, that is, failure after repeated loading cycles, where the stress in the material is well below its yield strength. Although the stresses are very low, some of the crystals making up the alloy can deform and eventually introduce damage and eventually failure.

Through extensive testing, engineers have developed sound empirical relationships between fatigue crack growth rate and the microstructure of different Ti alloys. However, the actual physical mechanisms controlling this behaviour are not fully understood. This gap in our knowledge makes it difficult to account for material and/or operating conditions outside previous experience.

It also makes it difficult to understand how the alloys could be improved, so that less of it can be used, which would help build lighter, more efficient aero-engines.

The aim of this project is to better understand fatigue crack growth rates in Ti64 microstructures containing microtextured regions, including the effects of cold creep(dwell). The approach is to characterise the crack path, study crack tip plasticity and relate it to the material microstructure (including microtexture). This will be supported by crystal plasticity modelling of the deformation ahead of the crack tip.

The research program will include: - Study of the micromechanics ahead of crack tips in Ti64 using HRDIC - Characterisation of crack path using X-ray tomography - Characterisation of crack path using 3D EBSD - Correlative microscopy of crack path and microstructure - Crystal plasticity modelling of crack tip plasticity