Alloy Design for Refractory High Entropy Alloys with Better Balanced Mechanical Properties at Both Room Temperature and Elevated Temperatures

Licentiate thesis, 2023

Motivated by the desire to improve the energy efficiency of gas turbines by operating them at higher temperatures (HT), which will contribute to a more energy efficient and carbonless society, the quest for novel ultrahigh-temperature materials can never be overwhelming. High entropy alloys, the recently emerged multi-component alloys with equiatomic or close-to-equiatomic compositions, are considered highly promising as next-generation ultrahigh-temperature materials. In particular, refractory high entropy alloys (RHEAs), one category of HEAs comprising refractory elements with high melting points, are thought to hold the greatest potential to surpass the current state-of-the-art HT materials, Ni-based superalloys, whose upper bound of service temperature has been limited by the melting point of Ni.

The alloy design of RHEAs for HT applications is highly challenging though. Specifically, how to balance HT strength, room-temperature (RT) ductility and oxidation resistance is a formidable materials challenge. For instance, the solid solution hardening (SSH) strategy has been proved to work nicely to enable excellent HT strength for singe-phase bcc structured RHEAs, however, at the cost of losing tensile ductility at RT. Another example is that adding Al, Cr or Si into RHEAs could improve their oxidation resistance, which however harms their RT ductility due to the easy formation of undesirable intermetallics. Innovative strategies to design RHEAs that can meet these demanding materials requirements, i.e., simultaneously possessing excellent HT strength, acceptable RT ductility and excellent oxidation resistance, are desperately in need and constitute the main topic of this licentiate thesis.

Here in this work, the solid solution softening (SSS) strategy was utilized to soften selected RHEAs to achieve RT ductility without compromising HT strength. Minor additions of substitutional transition metals, Mn, Al and Cu, were confirmed to soften a Hf20Nb31Ta31Ti18 RHEA from RT to 1000oC. Further, with the solo Mn additions into a (HfNbTi)85Mo15 RHEA, a concurrent SSS at RT and SSH at intermediate temperatures was achieved, which led to better-balanced mechanical properties at both RT and elevated temperatures. Combining SSS at low temperatures and SSH at intermediate temperatures holds the potential to induce non-zero tensile ductility for those RHEAs with decent HT strength, hence, to deliver desirable mechanical properties required by ultrahigh-temperature materials, and contributes to accelerate the alloy development and engineering applications of RHEAs.