Dynamic globularization kinetics of a powder metallurgy Ti-22Al-25Nb alloy with initial lamellar microstructure during hot compression

The flow behavior of a powder metallurgy (P/M) Ti-22Al-25Nb alloy was evaluated during hot compression at temperature range of 950-1070 °C and strain rate range of 0.001-1 s^-1. The dynamic globularization kinetics at elevated temperature was quantitatively characterized and investigated. The results showed that the dynamic globularization kinetics and kinetics rate were sensitive to deformation conditions. The variation of globularization fraction with strain approximately followed an Avrami type equation. It can be found that the process of dynamic globularization was promoted by decreasing strain rate and increasing deformation temperature. Moreover, the critical strain (ε_c) for the onset of dynamic globularization and the completed strain (ε_f) for the full dynamic globularization were predicted to be 0.094-0.198 and 1.082-2.113, respectively. The kinetics rate of dynamic globularization firstly increased severely to a peak value at a strain corresponded to 12.7-26.7% globularization fraction, and then decreased sharply with increasing strain. It was revealed that the peak value of kinetics rate increased with increasing temperature and decreasing strain rate. Furthermore, the microstructure examination was conducted by applying optical microscope (OM), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) techniques. The results exhibited a good agreement with the predicted dynamic globularization kinetics model.