Enhanced weldability of next-generation refractory high entropy alloys (for Appendix I)

Metals and alloys joining through welding is a critical dimension to consider for high-performance structural applications such as aerospace, power generation, and military. Although the refractory metal and alloys have shown favorable benefits for high-temperature applications in aerospace and nuclear, the challenges associated with conventional welding limits its applications. The conventional refractory alloys, due to well-marked ductile to brittle transition temperature (DBT), and high chemical activity towards the impurity element, particularly oxygen, yield the physical and metallurgical defects, therefore, exhibit limited weldability. A novel alloy with reduced sensitivity for the oxygen and a favorable welding approach is required to enhance the weldability of the refractory alloys. The proposed research is focused on the manufacturing of refractory high entropy alloys (RHEAs) based on molybdenum (Mo), niobium (Nb), tantalum (Ta), and tungsten (W). The study focuses on achieving high yield strength for structural integrity at high temperatures, increasing the room temperature (RT) ductility for ease of manufacturing, and reducing the sensitivity for the oxygen to increase weldability. This study aims at designing the grain boundaries (GBs) characteristics to minimize the concentration of the interstitial element of RHEAs to increase the toughness and ductility. A study indicates that the addition of titanium (Ti) and vanadium (V) in equimolar composition in the base alloy (MoNbTaW) might increase the ductility. Ti has been widely used as a getter element for interstitial elements. It is expected that Ti in the RHEAs matrix reduces the interstitial elements in the alloys. The getter elements selectively form carbide, nitride, and oxide in the presence of C, N, and O interstitial elements. These compounds will be distributed in the RHEAs matrix, which will increase high-temperature strength, ductility, and weldability. Based on the equimolar composition of the Mo, Nb, Ta, W, V, and Ti, RHEAs will be developed. The RHEAs will be manufactured by powder metallurgy (PM) manufacturing technique. After achieving the bulk RHEAs with the desired composition, the bulk samples will be subjected to a range of characterization tools, including X-ray diffraction, scanning electron microscope, and electron backscattered diffraction, for the microstructure and phase analysis. Following the microstructure analysis, tensile testing will be conducted to determine the strength and ductility of the novel RHEAs. A novel solid-state low temperature friction stir welding (LTFSW) will be performed on the PM RHEAs in a controlled atmosphere to determine the weldability of the alloy. The low-temperature welding ensures that no deleterious phases are being formed during the welding process. The post-welding characterization and mechanical testings of the welded RHEAs will be done to determine the characteristics of the welding of RHEAs. Based on the results obtained, the PIs will provide a recommendation to NASA on the novel refractory alloy development with enhanced weldability.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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