Mechanisms of globularization of Ti-6Al-4V during static heat treatment

Abstract

The mechanisms controlling static globularization of Ti-6Al-4V after deformation and annealing at 900 °C and 955 °C were established. Microstructural observations suggested that the process of globularization can be divided into two stages. The first includes microstructural changes during deformation and the initial stages of static heat treatment; the second occurs during prolonged static annealing. The initial stage consists of segmentation of the lamellae via boundary splitting, whereas microstructural coarsening characterizes the latter stage. Thus, the process of static globularization is only moderately dependent on the formation and evolution of dislocation substructure; the additional driving force is provided by the reduction in interface energy. The duration of the initial stage of static globularization was calculated by estimating the time required for the completion of the boundary-splitting process. The calculations were in excellent agreement with microstructural observations and showed that the duration of the initial stage at 900 °C and 955 °C lasted approximately 10 hours and 1 hour, respectively.