Nucleation kinetics of the alpha-gamma_M massive transformation in a Ti-47.5 at. percent Al alloy

摘要

Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements and quantitative stereological analysis of microstructures, coupled with calculations based on the classical theory of nucleation, were performed to study the nucleation kinetics of the alpha->gamma_M massive transformation in a Ti-47.5 at. percent Al alloy. Using previously determined thermodynamic data on the reaction temperature, time, undercooling and driving force for the massive transformation, the free energies for critical nucleus formation and the nucleation rates were computed for different nucleus shape models, including incoherent grain face, edge and corner nuclei and faceted grain face nuclei, and compared with experimentally determined nucleation rates. The results indicate that nucleation of singly faceted grain face nuclei and incoherent grain comer and grain edge nuclei are highly likely. Good agreement was also obtained between the calculated and experimental nucleation rates at these various sites. Based on these results, possible nucleation mechanisms during the massive transformation are discussed in light of current thinking on the nature of this transformation.