Exploring Crystallization Phenomena in Al2TiO5-Based Chemical Vapor-Deposited Coatings by in Situ Transmission Electron Microscopy

Understanding crystallization is crucial to enable improved design approaches for inorganic materials. Despite this importance, a deepened understanding of the mechanisms controlling this phase transition is still lacking. Herein, we employ in situ heating in a transmission electron microscope (TEM) to unravel crystallization phenomena in an Al-Ti-O system. Specifically, annealing of amorphous chemical vapor deposited (CVD) Al2TiO5-based coatings has been carried out in the temperature interval 700-900 degrees C. Crystallization typically occurs through a two-step process, where numerous smaller (<10 nm) crystals are initially formed, followed by rapid crystal growth. Examinations of the initial crystallization stages indicate that the amorphous-to-crystalline transformation is polymorphic and that several phases may nucleate simultaneously, including Al6Ti2O13 and Al16Ti5O34. A transient (time-dependent) nucleation rate is discovered based on the displayed nucleation behavior. Subsequent evaluations of the growth stages also reveal a preferential growth into larger crystals, and the coherent findings of this study indicate that crystallization occurs through a diffusionless (displacive) mechanism.

Automated summary

Understanding crystallization and its controlling mechanisms is crucial for improving the design of inorganic materials. In this study, in situ heating in a transmission electron microscope (TEM) was used to investigate the crystallization phenomena in an Al-Ti-O system. The results indicate that crystallization occurs through a two-step process, with initial formation of numerous smaller crystals followed by rapid growth. The study also reveals the polymorphic nature of the amorphous-to-crystalline transformation and the preferential growth into larger crystals, suggesting a diffusionless mechanism.