WEB Unravelling the grain boundary structure in Ti thin films using aberration corrected (S)TEM
Grain boundaries (GB) are interfaces separating two crystallites inside a material. They can be considered as a quasi-two-dimensional ‘phase’ that can undergo phase transitions. Understanding their structure and chemistry, under certain thermodynamic conditions hence becomes necessary to understand the mechanical, chemical and electrical behaviour of materials. Much of such studies have been performed in cubic crystal structure systems. In Titanium (Ti), which has hexagonal close packed crystal structure, there is so far no understanding of any GB transitions. Ti is an important engineering material owing to its high strength-to-weight ratio and biocompatibility. Body-centered close packed to hexagonal close packed allotropic transition in Ti makes it and its GBs especially interesting to be studied.
Thin films were grown for producing pure tilt GBs since in this method a myriad of GBs can be obtained on the same film. It also ensures that the GBs have negligible curvature along the viewing direction. In this study, several thin films of Ti were deposited by varying substrates, deposition methods and parameters. Preliminary screening of films was done using X-ray diffraction (XRD) and scanning electron microscopy (SEM) imaging. Thin films with microstructures varying from nano-crystalline (nc) to single crystalline were obtained. The misorientation angle and GB planes were analysed using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM). In a nc-film deposited on Si (100), low angle GBs with 13⁰ misorientation consisted of an array of 1/3 [112 ̅0] dislocations, arranged periodically with a spacing of 13 nm. In thin films deposited on MgO (100) substrate, Ʃ13 high angle GBs were observed to be the most prominent interface present. The asymmetric Ʃ13  (11 ̅00)/ (2 ̅110) GB was seen to consist of periodic structural units. The globally curved GB was observed in TEM to facet by 30⁰ at periodic intervals. This faceting led to the low energy Ʃ13  (11 ̅00)/ (2 ̅110) GB plane being uniquely consistent throughout the curvature of the GB, as can be predicted by a Wulff-plot. Apart from this, resistivity was found to be independent of film thickness and residual stress in all single crystalline films. Dependence of residual stress with varying substrates was also studied.
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|Extended Abstract||Figure 1: HAADF image (with FFT in the inset) of periodic dislocation arrangement in low angle grain boundary in Ti thin film||This is the figure discussed in the abstract.||1 MB||Download|