Analysis of globularization modeling and mechanisms of alpha/beta titanium alloy

Abstract

Static globularization of titanium alloys is a complicated microstructure evolution which is influenced by multi-factor. The globularization fraction presents trend of increasing with the increasing heat treatment time, especially in the beginning of heat treatment. In this stage, the globularization process is controlled by boundary splitting. Boundary splitting is a transformation process of crystal orientation from low-angle boundary to high-angle boundary, finally to separation. The same particle may present the different crystal orientation, indicating that the formation of misorientation inside particle. Euler angles are used to calculated the changes of crystal orientation. The Schmid factor indicates that basal slip {0001}<11–20> or pyramidal slip {1–101}<11–20> is easy to activate in contrast to prismatic slip {1–100}<11–20>. In the mid and later stages of heat treatment, the globularization process is influenced by microstructure coarsening, and the globularization rate slows down. Microstructure coarsening, controlled by element diffusion, is a slow process influenced by temperature, time and microstructure morphology. In order to model the process of static globularization, the modified model coupling prestrain and heat treatment time is developed by introducing the parameter t0.5, i.e. the time required for completing 50% globularization fraction, in the framework of the JMAK model. In the obtained model, strain prior to heat treatment is first considered to establish kinetics equations of static globularization of titanium alloys. The linear relationship between ln(ln(1/(1-f))) and lnεt is found by a set of theoretical deduction, in which f, ε and t denote static globularization fraction, prestrain and heat treatment time, respectively. Static globularization fractions of Ti-17 and Ti-6Al-4V alloys at different temperature are predicted based on the established model. Predictions of static globularization fraction are in good agreement with the experimental results.