Grain Refinement and Microstructure Formation by Inoculation

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The previous funding periods focused on the investigation of grain refinement of primary β(Ti) and of peritectic α(Ti) in Ti-45Al-xB alloys due to heterogeneous nucleation on TiB2 particles. These particles were added ex-situ for β(Ti) refinement or formed in-situ just before peritectic α(Ti). This third period shall focus on heterogeneous nucleation and grain refinement of the α(Ti) phase in alloys with higher Al (49 to 51 at.%) and adequately chosen B contents (0.0 to 1.5 at. %). For this composition range, the α(Ti) phase is no longer peritectic, but primary. Different to bcc-β(Ti), α(Ti) crystallizes in a hexagonal lattice structure, which eventually leads to different nucleation behavior and growth kinetics. The TUHH group will consider the heterogeneous nucleation of α(Ti) under casting conditions, with TiB2 inoculants being introduced ex-situ from a master-composite. They will analyze the grain refinement using the free growth model. These studies will be accomplished by the characterization of TiB2 morphology, of TiB2α(Ti) interfaces and crystal lattice orientation relationships, being performed by the HZG group in Geesthacht. Scanning and transmission electron microscopy as well as high energy x-ray diffraction with synchrotron radiation shall be applied. The ACCESS group will investigate α(Ti) growth using directional solidification experiments. Phase field simulations will be performed to gain insight into the details of microstructure formation and to bridge the scale between nucleation and early microstructure evolution. As a final aim, the project team seeks for a comprehensive description of the various operating grain refinement mechanisms in the composition range Ti-44…51Al-0…2B, based on thermodynamic and crystallographic data and a unique set of nucleation model parameters valid for the free growth model and the phase field simulations.