Static coarsening behavior of the lamellar alpha in Ti-17 alloy

Static coarsening behavior of Ti-17 alloy with the lamellar alpha structure has been investigated as a function of time at temperatures of 820°C and 860°C. The resultant microstructures have been characterized using scanning electron microscopy. Microstructure prior to heat treatment contained a fully lamellar alpha structure. Heat treatment at 820°C and 860°C resulted in the continuous coarsening of alpha lamellae. The coarsening rate decreases with heat treatment time. Static coarsening of Ti-17 alloy with the lamellar alpha was mainly controlled by termination migration mechanism. Solute atoms migrated from the termination or local defect positions of lamellae to the flat interface, leading to the coarsening or separation of the lamellar alpha. The coarsening behavior could be interpreted in terms of Lifshitz-Slyozov-Wagner (LSW) theory, and the kinetics equation could be established by the modified LSW theory model (d = Ktⁿ). The coarsening coefficients n of Ti-17 alloy with the lamellar structure were determined to 0.33-0.4 for 820°C and 0.45-0.5 for 860°C, respectively. Therefore, the coarsening was more likely influenced by bulk diffusion at 820°C, and controlled by interface reaction at 860°C. In addition, a mathematical model was developed to predict static coarsening rate of the lamellar alpha of Ti-17 alloy. The results showed that the model can provide a relatively reliable prediction for static coarsening behavior of the lamellar structure of Ti-17 alloy.