Part I Nucleation kinetics of the β → αm transformation in Ti-ag and Ti-au alloys

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Continuous cooling experiments were used to study the nucleation kinetics of the β → α m transformation in Ti-Ag and Ti-Au eutectoid systems. Combining the thermal arrest duration and temperature with stereological measurements of α m and β microstructures, nucleation rates were obtained as a function of temperature and were successively assumed to be a result from grain face, grain edge or grain corner nucleation. Two categories of nucleus models were tested for their ability to predict the experimentally determined rates at these sites: (1) The Clemm-Fisher spherical cap models, wherein all interphase boundaries are disordered and have high, orientation-independent interfacial energies, and (2) coherent models for which all or most of the interphase boundary consists of partially or fully coherent interfaces. Calculations based on classical nucleation theory and available ancillary data demonstrated that the disordered boundary nucleus models predict nucleation rates that are literally thousands of orders of magnitude lower than the experimental rates regardless of the nucleation site considered. Conversely, similar calculations suggest that the coherent critical nuclei are viable even at grain faces. These results show the necessity for the presence of low energy interphase boundaries during the nucleation process in massive transformations, and thus for the existence of specific and special lattice orientation relationships between parent and massive phases.