The evolution of abnormally coarse grain structures in beta-annealed Ti-6Al%-4V% rolled plates, observed by in-situ investigation

The factors controlling the evolution of abnormally coarse grain (ACG) structures during standard ,B an-nealing of titanium-6Al%-4V% (Ti64) hot-rolled plates, have been investigated in more detail than previ-ously, using both ex-situ through-thickness large area, and in-situ EBSD micro-texture observation. Start-ing with a typical, through thickness, alpha + ,B deformation texture distribution that gives rise to ACGs at the plate mid-section, each stage of the annealing process has been sequentially characterised and phe-nomenologically linked to the spatial texture evolution that occurs during transient heating through the alpha -> ,B phase transformation and the subsequent isothermal hold at the ,B-annealing temperature. It was found that during the ramp heating phase, the rotated cube texture component greatly expands from just below the ,B transus by broad front strain induced boundary migration (BF-SIBM), driven by a disparity in stored energy with neighbouring texture bands from the alpha and gamma fibres. This subsequently sets up the necessary conditions for unstable coarsening of surviving grain clusters with predominately near alpha fibre orientations during the super-transus hold, in a process with similar characteristics to discontinu-ous recrystallisation. Humphreys' mean field model of the stability of cellular microstructures, effectively explains the process by which these grain clusters, which have a lower misorientation range relative to the rotated cube component matrix, have a higher probability of entering a discontinuous growth regime, compared to other components from the alpha and gamma fibres. Furthermore, the conditions that give rise to the development of ACG structures could be linked to the high strain rate and temperature experienced at the central mid-thickness of rolled plates. (c) 2021 Published by Elsevier Ltd on behalf of Acta Materialia Inc.