Static globularization kinetics for Ti-17 alloy with initial lamellar microstructure

The kinetics of static globularization of the hot worked Ti-17 alloy with initial lamellar microstructure was investigated. For this purpose, Ti-17 alloy was isothermally forged to different height reductions at 820 °C or 860 °C and subsequently heat treated for times ranging from 10 min to 8 h at 820 °C or 860 °C. Microstructure evolution during static heat treatment was quantitatively analyzed. Microstructure observations indicate that both boundary splitting and microstructure coarsening promote static globularization process. Boundary splitting plays an important role in the initial stage of static heat treatment. Microstructure coarsening occurs throughout heat treatment process, but it is more significant during prolonged static heat treatment. The static globularization kinetics is sensitive to the amount of strain prior to heat treatment, heat treatment temperature and time. Globularization process is accelerated with the amount of strain and heat treatment temperature increase. Globularization fraction increases fast at first and then slow down with the increase of heat treatment time. The amount of strain prior to heat treatment mainly influences the initial stage of heat treatment and plays a supporting role in the latter stage. Heat treatment temperature has an important role during the whole heat treatment process. Static globularization kinetics is found to be less dependent on deformation temperature. Higher deformation temperature is conducive to static globularization when strain prior to heat treatment is lower and heat treatment time is shorter. However, difference caused by deformation temperature disappears with the increase of strain and heat treatment time. To model the static globularization behavior, the theoretical models of boundary splitting and termination migration were proposed. A comparison of experimental observations and model predictions indicate that the theoretical models can provide a reasonable prediction of time for finishing static globularization.