Advanced Characterization of Cu- and Si-Containing Intermetallic Phases in a Novel Ti-Based Superalloy for Additive Manufacturing

Near-α Ti alloys are widely employed in high-temperature applications within the aerospace and automotive industries due to their high strength, low density as well as excellent resistance against oxidation and creep deformation. While conventional manufacturing processes remain prevalent, additive manufacturing (AM) has gained a lot of interest in recent years due to its ability to produce components exhibiting complex geometries with a high level of efficiency. Current research has shown that conventional alloys like Ti-6Al-4V (wt.%) or Ti-6Al-2Sn-4Zr-2Mo (wt.%) present significant challenges in AM due to their response to the unique process conditions. Especially, high cooling rates, high temperature gradients and the layer-wise build-up yield a complicated, position-dependent thermal history and, eventually, lead to complex and unique microstructures. In Ti alloys, the formation of large columnar β grains oriented along the build direction results in anisotropic mechanical properties. To mitigate these microstructure-related challenges in AM, the development of new alloying concepts is essential. Recent research has shown that eutectoid-forming elements, such as Cu, can have a beneficial effect on the solidification behavior in Ti alloys. Together with the possible formation of intermetallic phases, leading to an increased number of nucleation sites, this can induce a columnar to equiaxed transition (CET) during solidification in the AM process.<br/>This study presents a fundamental investigation of a novel Ti-based superalloy with additions of Cu and Si suitable for high-temperature service and optimized for the wire-arc direct energy deposition (waDED) process. This work primarily focuses on the detailed analysis of the precipitation of intermetallic phases at the nm-scale, employing advanced characterization techniques such as in-situ small-angle (SAXS) and wide-angle X-ray scattering (WAXS), transmission electron microscopy (TEM), and atom probe tomography (APT). While WAXS provides insights into crystallographic information of the constituent phases as a function of temperature, the morphology and size distribution of nm-sized intermetallic precipitates can be observed simultaneously by SAXS. In particular, continuous heating experiments revealed the formation of a Cu-enriched phase between 580-760°C and a silicide phase between 660-930°C, which correspond to temperatures below this alloy’s β-transus temperature. The results of complementary high-resolution TEM measurements classified these intermetallic phases as Ti_2-3Cu and Ti₆Si₃ (S2-type) with slightly off stoichiometric chemical compositions. Furthermore, the combination of TEM and APT investigations revealed the morphology and chemical composition of these two phases. These results allowed a detailed interpretation of the SAXS data in a complex multiphase Ti-based system. Thus, this study includes the comprehensive characterization of phase fraction, size distribution and shape of the forming precipitates with high statistics in a time-resolved manner.<br/>By employing advanced complementary characterization techniques, this research enhances the fundamental understanding of a novel near-α Ti-based superalloy optimized for AM processes. A profound knowledge of phase transformations and microstructure control is crucial for the practical application of these alloys. These findings not only provide a solid foundation for future research but also highlight the critical role played by innovative alloying concepts in overcoming the limitations of conventional Ti-alloys and advancing their use in manufacturing technologies such as waDED.