Alloy Design for Refractory Alloys with Balancing Mechanical Properties and Oxidation Resistance

Doctoral thesis, 2025

Refractory alloys including refractory high entropy alloys (RHEAs) and conventional refractory alloys (CRAs) hold great potential as ultrahigh-temperature structural materials (≥1300oC), since they are equipped with high melting points enabling the high service temperature, metallic bonding sustaining the ductility, and the single body-centered cubic (bcc) structure ensuring the microstructural and thermal stability. However, alloy design of refractory alloys to simultaneously possess three critical materials requirements for ultrahigh-temperature applications, i.e., excellent high-temperature (HT) strength (Target 1), acceptable room temperature (RT) ductility (Target 2) and decent oxidation resistance (Target 3), is highly challenging which triggers the well-known ‘trade-off’ phenomenon in the alloy development. Thus, innovative strategies are desperately in need, and they constitute the topic of this doctoral thesis. The roadmap of alloy design in this doctoral thesis is to tackle two out of three challenges first, and eventually to tackle three challenges simultaneously.
Firstly, following this roadmap, solid solution softening (SSS) at RT and solid solution hardening (SSH) at HT, were employed to balance the strength at HT (Target 1) and the ductility at RT (Target 2). The softening effect at RT induced by minor substitutional additions of Mn, Al and Cu was identified in an originally ductile RHEA. Next, Mn as the most effective softener was added into an originally brittle RHEA, achieving a simultaneous SSS at RT and SSH at HT. Referring to the yield strength-temperature curve of bcc structured alloys characterized by a three-stage pattern: a temperature-dependent decrease at the low temperature regime, followed by a temperature-independent plateau at the intermediate temperature regime, and finally a temperature-dependent drop at the HT regime, our strategy enabled a reduced temperature dependence of yield strength in bcc-structured RHEAs.
Secondly, Nb alloys were alloyed with Al to balance the oxidation resistance (Target 3) and the RT ductility (Target 2). Although pesting was observed basically in all tested oxidation conditions suggesting the formation of non-protective oxide scales, the oxidation resistance of Nb alloys was significantly improved due to the beneficial effect of Al addition and was much superior to that of the benchmark WC3009 alloy.
Thirdly, an attempt was made to simultaneously meet the ultimate goal of this work: to balance strength at HT (Target 1), ductility at RT (Target 2) and oxidation resistance (Target 3) in Nb-based alloys. The alloying effect of Hf, W and Ti on the mechanical properties at both room-temperature and high-temperatures, the oxidation resistance, and more importantly their balance was explored. Novel Nb alloys were developed, with their oxidation resistance much superior compared to that of the benchmark C103 and WC3009 alloys, together with a reasonably high yield strength at HT.