In situ TEM observations of thickness effect on grain growth in pure titanium thin films

Highlights: • In-situ TEM heating reveal that a gradient structure was formed in the TEM samples. • An accurate and in situ method to measure the thicknesses of sample based on CBED. • Recrystallization behavior of thin film and bulk material were investigated. • Ultrafine-grained pure Ti exhibited superior thermostability at nanoscale thickness. • The mechanisms that caused the stagnation in the growth of grain were discussed. Ultrafine-grained materials have a strong tendency to transform into coarse-grained materials due to the high density of grain boundaries at elevated temperature. In this study, pure titanium was processed by high-pressure torsion for 10 turns to give an ultrafine-grained (UFG) structure with an average grain size of ~96 nm. The recrystallization behavior of the UFG Ti was investigated by in-situ transmission electron microscopy (TEM). It is found that the gradient microstructures with average grain size ranging from ~129 nm to ~655 nm are formed under in-situ TEM heating up to 800 °C for 30 min. The Kossel-Möllenstedt (K-M) fringes in a convergent-beam electron diffraction (CBED) pattern were used to provide an accurate measure of the sample thicknesses. The results demonstrate that grain growth is significantly suppressed in the UFG pure Ti thin film compared to bulk material. Mechanism analysis shows the combined effects of driving force and drag force on grain boundary migration is the primary cause of the grain growth inhibition in the UFG pure Ti thin film.