Nanostructures in the Direct Energy Deposited Metastable β Ti-5Al-5Mo-5V-3Cr Alloy for Aerospace Applications

There is an increasing demand for lightweight, high-strength metals and alloys in the aerospace industry, aiming to enhance fuel efficiency and reduce CO₂ emissions. Metastable β titanium (β-Ti) alloys have become critical structural materials in this field, such as Ti-5Al-5Mo-5V-3Cr (wt.%, Ti-5553), an alloy used for landing gears in aircraft, due to its exceptional properties including high specific strength, excellent corrosion resistance, and great fracture toughness. However, the relatively high production costs associated with conventional extraction and processing techniques significantly limit the broad application of Ti alloys. Direct energy deposition (DED), an additive manufacturing technique, offers a promising alternative in which components can be built layer by layer to a near net shape at an economically advantageous cost and reduce product waste. However, due to the complex heating and cooling cycles in the DED process, it is challenging to control the microstructure in DEDed metastable β-Ti alloys and thus their corresponding mechanical properties. For example, in conventionally processed metastable β-Ti alloys, nanostructures, such as nanoscale ω and O’ precipitates, can alter the local structure and composition in the parent β phase matrix, serving as effective heterogeneous nucleation sites of α precipitates, promoting refined α precipitate microstructure, and enhanced strength. Thus, to optimize the mechanical performance of DEDed metastable β-Ti alloys by tuning the microstructure, there is a critical need to understand the nanostructures formed in these alloys during the DED process and their impacts on the microstructure evolution during both the deposition and subsequent post-heat treatment (PHT) processes.

In this work, we investigated the nanostructures in the DEDed metastable β Ti-5553 alloy and their influence on the microstructure evolution during different PHTs using advanced characterization techniques. The mechanical properties of DEDed Ti-5553 and DEDed + PHT Ti-5553 were evaluated using tensile tests and nanoindentation. First, the microstructure and nanostructure in DEDed Ti-5553 were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and 3D atom probe tomography (APT). The grain structures and texture were analyzed via SEM electron backscatter diffraction (EBSD). Nanostructures, including nanoscale ω and O’ phases, were characterized in DEDed Ti-5553 for the first time using diffraction-contrast TEM analysis. The composition in these nanostructures was determined using 3D APT, revealing the formation of isothermal ω during the DED process. The number density and the distribution of these nanostructures were further analyzed using TEM dark field imaging and MIPAR^TM image analysis. Additionally, the microstructure evolution in the DEDed Ti-5553 during PHT was further studied using SEM. Two sets of PHTs were performed, considering the influences of both isothermal aging temperature and heating rate. Results have shown that during various PHTs, α precipitates of different sizes form in DEDed Ti-5553 due to the varying influence of nanostructures. Notably, a super-refined α microstructure with a number density of ~30 ppts/μm² was formed in DEDed Ti-5553 heated at 5^oC/min, which was attributed to the direct assistance of nanoscale ω precipitates. The phase transformation pathway in different PHTs will be introduced. Finally, the strength and ductility of DEDed Ti-5553 after different PHTs were evaluated using tensile tests and nanoindentation. The influence of nanostructures and various fine-scaled α precipitates on the mechanical behavior, including strength and ductility, in DEDed Ti-5553 will be discussed. This study provides insights into the microstructure control and mechanical performance in the DEDed Ti-5553 alloy for aerospace applications.