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Resolving the atomic structure and phase states of grain boundaries in titanium and its alloys

Grain boundaries (GB) are typically considered as 2-dimentional interfaces separating two differently oriented crystals inside a polycrystalline material. Understanding their structure and composition down to nano-scale regime is fundamental to explain their macroscopic properties. Discerning the contribution of GBs towards strength, corrosion resistance and high temperature properties is necessary to boost our efforts of making metals lighter, stronger and hence greener.

Titanium (Ti) is one of the most attractive materials for aerospace and bio-medical industries where high strength to weight ratio and chemical inertness play a vital role. Ti also makes an interesting case owing to its allotropic transition from the hexagonal close packed (HCP) to the body-centred cubic (BCC) phase at 882 ºC. Despite of its widespread industrial use very little is known about its GB structure and their possible transitions. Hence, the aim of this project is to look deeply into the atomic structure of GBs in Ti and to resolve the impact of alloying additions on their structure through advanced transmission electron microscopy (TEM) techniques. A challenging, yet intriguing task is to obtain defined tilt GBs in Ti and we found that epitaxial thin films are excellent candidates for generating thin films containing pure tilt boundaries in them.

In a first step, Ti films were deposited on various substrates like NaCl, MgO, Si, sapphire and mica to study their growth behaviour using deposition methods such as e-beam evaporation, Ar+-ion sputtering and high-power impulse magnetron sputtering (HPPMS), while also changing substrate temperature and deposition rate. The myriad of films hence grown were analysed using light optical microscope, X-ray diffraction and electron backscatter diffraction. Thin films with grain structure ranging from nanocrystalline on NaCl substrate to single crystalline deposited on sapphire were obtained. For the first time, the evolution of textured thin films containing pure tilt grain boundaries was found when depositing on MgO substrate, as shown in Fig 1(c) and (d). This deposition was performed using HPPMS technique in a collaboration with Dr. Marcus Hans from RWTH Aachen.

In a second step, site-specific lift-outs using focused ion beam preparation will be extracted from the thin films to study the GBs of interest in detail. Aberration-corrected scanning transmission electron microscopy (STEM) will be used to image low and high angle tilt grain boundaries down to atomic resolution. In a first attempt, a nanocrystalline film on Si (100) containing low angle grain boundaries with 13⁰ misorientation and [0001] tilt axis was observed to be composed of a dense network of [11-20] edge dislocations, as shown in Fig 1(b). Multi-slice STEM image simulations and atomistic simulations will be performed to link the atomic structure of the observed grain boundaries to their intrinsic properties. Furthermore, alloying elements will be added to explore the influence of segregation on the GB structure and to elucidate possible structural transformations or early stages of precipitation reactions. The combination of atomic resolution imaging and analytical techniques like electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) will be used to correlate the structure and chemistry of alloyed GBs and to resolve the influence of oxygen on GB structure.

Additionally, in situ heating experiments will be employed to observe temperature induced GB transformations as well as GB migration and grain growth in nanocrystalline films. The interaction of dislocations with GBs will be studied by in situ nanomechanical testing in the TEM to resolve the interaction of dislocations with different grain boundary types.

We gratefully acknowledge funding through the KSB Stiftung.

Figure 1 (a) Optical micrograph of a Ti thin film deposited on NaCl substrate, showing hillocks formed due to compressive residual stresses (b) HAADF-STEM image of a low angle grain boundary being composed of a dense network of [11-20] dislocations (c) Inverse pole figure map of a Ti thin film grown on MgO along with (d) the EBSD pole figure of film in (c) showing basal texture.

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