Origin of morphological variation of grain boundary precipitates in titanium alloys

Morphological variations of grain boundary (GB) allotriomorphs and widmanstatten sideplates (WS) of α precipitate in a β titanium alloy, have been systematically investigated by combining three-dimensional (3D) phase-field simulations, 3D experimental characterization and crystallographic orientation analysis. The inclination angle, θ, between the habit plane of the GBα and the hosting GB plane is found to dictate the morphologies of GB precipitates. For the first time, three distinct regimes of α morphology at a β GB, separated by two critical angles (θ_c¹, θ_c²) are observed: θ≤θ_c¹, GBα alone decorates the GB as a continuous layer; θ_c¹<θ≤θ_c², α precipitate appears as a mixture of GBα and a WS emanating from it; θ>θ_c², WS alone grows directly from bare β GBs. The dramatic morphological variation owes its origin to the dynamic interplay between the spreading along GBP and the intragranular growth of the GB precipitate, and its dependence on θ.