Mechanisms of macrozone elimination and grain refinement of near α Ti alloy via the spheroidization of the Widmannstätten structure

Regions with sharp local textures, i.e., macrozones, are common microstructural inhomogeneities in titanium (Ti) alloys that significantly reduce the fatigue resistance of Ti alloy parts. In this study, a two-stage thermomechanical operation was developed to eliminate macrozones and optimize microstructure in a near α Ti alloy. Multiscale microstructure characterization and full-field crystal plasticity (CP) modeling were conducted to uncover the underlying mechanisms. Spheroidizing through dynamic recrystallization (DRX) and superplasticity are indispensable ingredients. The alloy exhibited multiple DRX phenomena, which are strongly associated with grain morphologies. The discontinuous DRX and the main spheroidizing mechanism of the equiaxed primary α phase failed to eliminate macrozones but exacerbated microstructure heterogeneities due to the strong anisotropy of the hexagonal close-packed α phase. The Widmannstätten structure (WS), fabricated through the β heat treatment that generated abundant nano-defects and serrated grain boundaries, significantly promoted the occurrence of geometric dynamic recrystallization (GDRX). A complete GDRX of lamellar α grains can generate a sufficiently fine-grain structure to achieve superplastic deformation in the subsequent hot compression at a moderate deformation. The full-field CP simulations considering slip only versus slip + superplasticity confirmed that the dislocation slip dominated deformation aggravated deformation heterogeneity and the formation of macrozones. In contrast, the superplastic deformation suppressed the heterogeneous deformation and randomized the grain rotation, which in turn homogenized the microstructure and eliminated macrozones. Based on these observations, the β heat treatment followed by α hot compression is an effective method to achieve grain refinement and macrozone elimination of Ti alloys.