Crystal plasticity modelling of the effect of microtextured regions on the strain localisation of forged Ti-6Al-4V.
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Abnormally coarse β-grains (ACG) result in large α-colonies approximately 5mm in diameter, that form in forged β-annealed Ti-6Al-4V alloys. They are expected to have poor mechanical performance but, standard experimental fatigue testing cannot reliably quantify their impact due to the unpredictability of ACG formation and difficult non-destructive detection. Crystal plasticity (CP) offers a non-destructive method of predicting strain localisation and crack initiation. Many studies use CP to predict strain localisation but only validate the global macroscopic response [1], while the resulting local microscopic response can be non-unique. In this work CP is used to predict crack initiation and is validated by in-situ high resolution digital image correction at the local scale, to provide a full 3D description of strain localisation and develop a more statistically-representative estimation of fatigue life. The CP framework DAMASK was used to simulate monotonic and cyclic loading in microstructures representative of conventional β-annealed forged Ti-6Al-4V. Simulation workflow package MatFlow was used to handle interaction between DAMASK and other software such as Paraview and MTEX making modelling easier and more reproducible. Once validated, the simulation framework was used to simulate ACGs under different loading conditions, finding the load condition which produces the highest level of strain localisation, which is not possible experimentally. When loaded along soft crystallographic orientations, strain localisation was greatest and the region of highest strain localisation is a likely position of crack initiation. Greater understanding of crack initiation mechanisms in Ti-products will improve life estimation, as approximately 80% of fatigue life is accounted for by crack initiation. The results contribute to our understanding of the performance of these materials, and help to use them more efficiently, by designing lighter components, thus lowering the environmental impact of flying. [1] M. Zhang, J. Zhang, and D. L. McDowell, “Microstructure-based crystal plasticity modeling of cyclic deformation of Ti-6Al-4V,” Int. J. Plast., vol. 23, no. 8, pp. 1328–1348, 2007, doi: 10.1016/j.ijplas.2006.11.009.
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