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.

high temperature strength

temperature dependence of yield stress

room temperature ductility

substitutional solid solutions, oxidation resistance.

solid solution softening and hardening

Refractory high-entropy alloys (RHEAs)

VDL/IMS
Opponent: Prof. Ehsan. Ghassemali, Jönköping University.

Author

Xiaolong Li

Chalmers, Industrial and Materials Science, Materials and manufacture

Xiaolong Li, Jin L, Huahai Mao, Hideyuki Murakami, Sheng Guo-Solid solution softening or hardening induced by minor substitutional additions in a Hf20Nb31Ta31Ti18 refractory high entropy alloy

Xiaolong Li, Mao Ding, Qiang Hu, Zhiyuan Liu, Huahai Mao, Sheng Guo-Solid solution softening at room temperature and hardening at elevated temperatures: A case by minor Mn addition in a (HfNbTi)85Mo15 refractory high entropy alloy

Ductile and Oxidation Resistant Ultrahigh-Temperature Materials

Swedish Research Council (VR) (2019-03559), 2020-01-01 -- 2024-12-31.

Driving Forces

Sustainable development

Roots

Basic sciences

Subject Categories

Metallurgy and Metallic Materials

Infrastructure

Chalmers Materials Analysis Laboratory

Areas of Advance

Materials Science

Publisher

Chalmers

VDL/IMS

Opponent: Prof. Ehsan. Ghassemali, Jönköping University.

More information

Latest update

1/24/2024