Experimental characterization and physical modeling of strain path dependent microstructure evolution: A case study of primary hot working of titanium alloy

Lamellar globularization, a multi-stage and significant microstructure transformation behavior during primary hot working of titanium alloys, directly affects the microstructure and performance of the product. However, the lamellae dynamic globularization represents a significant dependence on the strain path, which is hard to be predicted only by experimental characterization. To deepen the scientific insight into the effect of the strain path on the globularization behavior, systematic non-monotonic loading experiments (including multi-Axial compression and forward-reverse torsion) were extensively performed to characterize the structure features. Further, an internal-state-variable microstructure model is developed to track the globularization process. Experimental results show that there is a short-Term stage where the formed α/α substructures annihilate due to the restoration of ideal BOR during the early deformation after the loading direction changes, which leads to the reduction in globularized fraction. However, on the other hand, the globularization would be improved due to the adjustment in lamellae orientation and accelerated lamellae rotation caused by the change of loading direction. The microstructure model considering the strain path effect is developed by introducing the description of the strain path and coupling it into the physical model incorporating dislocation density and α/α substructures evolution. The proposed model is suitable for predicting the lamellar globularization under monotonic and non-monotonic loading of titanium alloys with the α-colony structure, which has been validated from the comparative analysis of the simulated and experimental results. Compared with the existing microstructure models, it broadens the scope of application and improves prediction efficiency. This work throws some light on the correlation between the strain path and microstructural development, which can be utilized to optimize the primary hot working of titanium alloy aiming at improving microstructural refinement and heterogeneity of forging pieces.