Static globularization mechanism of Ti-17 alloy during heat treatment

The mechanisms of static globularization of Ti-17 alloy during heat treatment are investigated at 820 °C and 840 °C. Static globularization fraction increases with the increasing time and temperature during heat treatment. The quantitative analysis indicates that the process of static globularization can be divided into the two distinct stages. The first is a fast globularization stage and occurs at the beginning of heat treatment, and the second is a slow globularization stage and includes microstructure change during prolonged heat treatment. The first stage consist of the globularization of alpha lamellae via boundary splitting, whereas the second stage is controlled by microstructure coarsening including termination migration and Ostwald ripening. Boundary splitting is a process in which the distortion energy is rapidly released, and this process is finished via shearing surfaces caused by shear deformation and substructure generated by recovery. The duration is about 0.5–1 h. Termination migration and Ostwald ripening are the diffusion process of elements, and they are influenced by temperature and geometrical shapes of microstructure. The driving power comes from the energy gradient among the different positions in the same alpha particle or the different alpha particles. Termination migration leads to the separation and coarsening of the lamellar alpha and Ostwald ripening causes the decreasing of amount of particles and microstructure coarsening. Boundary splitting, termination migration and Ostwald ripening jointly promote static globularization process of Ti-17 alloy.