Rapid phase transformation kinetics on a nanoscale: Studies of the α → β transformation in pure, nanocrystalline Ti using the nanosecond dynamic transmission electron microscope

Using the unique capabilities and high time resolution of dynamic transmission electron microscopy (DTEM), the fast kinetics of α → β-phase transformation in nanocrystalline Ti films were investigated using single-shot electron diffraction and bright-field TEM images. From quantitative analysis of the diffraction patterns, the transformation rates were determined for temperatures between transition start (1155 K) and melt temperature (1943 K). The experimental data were summarized in a time–temperature-transformation (TTT) curve with nanosecond time resolution. Theoretical TTT curves were calculated using analytical models for isothermal martensite and available thermodynamic data. Above 1300 K, there is excellent agreement between the experiment and the discrete–obstacle interaction model, suggesting that the nucleation rate and thermally assisted motion of the martensite interface are controlled by interface–solute atom interactions. However, theory predicts much slower transformation rates near the transition temperature than experiment. Experimental data fits using the Pati–Cohen model suggests that an increase in autocatalytic nucleation may partially account for the fast transformation rates at lower temperatures.