Mechanisms of macrozone elimination and grain refinement of near α Ti alloy via the spheroidization of the Widmannstatten 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 thermo-mechanical operation was developed to eliminate macrozones and optimize microstructure in a near alpha 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 alpha phase failed to eliminate macrozones but exacerbated microstructure heterogeneities due to the strong anisot-ropy of the hexagonal close-packed alpha phase. The Widmannsta center dot tten structure (WS), fabricated through the beta 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 alpha 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 random-ized the grain rotation, which in turn homogenized the microstructure and eliminated macrozones. Based on these observations, the beta heat treatment followed by alpha hot compression is an effective method to achieve grain refinement and macrozone elimination of Ti alloys.

Automated summary

Regions with sharp local textures, known as macrozones, have a negative impact on the fatigue resistance of titanium alloys. This study developed a two-stage thermo-mechanical operation to eliminate macrozones and optimize the microstructure. The results showed that spheroidizing through dynamic recrystallization and superplasticity can achieve balling and reduce heterogeneity. The beta heat treatment followed by alpha hot compression can result in fine-grain structure and elimination of macrozones.