Unexpected dynamic recrystallization behavior of Ti-7Cu alloy in semi-solid state

Titanium alloy obtained from semi-solid processing has received increasing attentions recently for its excellent formability and mechanical properties. This work intends to unveil the puzzling deformation process of the alloy during semi-solid processing. Through the compressive tests of semi-solid Ti-7Cu under various conditions (temperatures or strain rates), the deformation is found to be dominated by the plastic deformation of solid particles. Strikingly, the dynamic recrystallization (DRX), as commonly occur during hot deformation, is also observed. Based on the Poliak's theory and Avrami equation, the critical strain that triggers the DRX and the DRX kinetics are analyzed. It is found that the kinetics of DRX during semi-solid deformation is highly sensitive to temperature and strain rate. More specifically, higher percentages of DRX grains present at lower temperatures or higher strain rates, which is in contrary with that in solid state. Such phenomenon is considered as originated from the lubricating effect caused by the flow of the liquid phase during semi-solid deformation. Further, a modified DRX kinetic model is developed based on the Zener–Hollomon parameter Z, the predicted grain size agrees well with the experimental measurements.