Effect of Al addition on oxidation behavior of Nb-based refractory alloys at elevated temperatures
Ladda ner
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Materials engineering (MPAEM), MSc
Publicerad
2024
Författare
Zhao, Yufei
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Improving gas turbine efficiency is closely linked to reducing global carbon emissions, which requires the
development of materials that can operate at ultra-high temperatures. In this thesis, the oxidation behavior
of three quaternary Nb-based refractory alloys, Nb-15Hf-5.5W-4Al, Nb-15Hf-5.5W-8Al, Nb-15Hf-5.5W-
12Al (nominal compositions, in at.%) is reported. The alloys were produced by vacuum arc melting, and
the oxidation studies were performed at 800℃, 1000℃, and 1200℃ for 1h, 8h, and 24h, respectively, while
the phase constitution, microstructure, and composition of oxidation products were analyzed by XRD,
SEM, and EDS. Microstructures of three alloys all showed a single-phase bcc structure with a typical
dendritic structure, and their Vicker hardness monotonously increased with the increasing Al content. The
oxidation products contained different complex oxides including Nb2O5, Hf6Nb2O17, AlNbO4, WO3, and
Nb11AlO29. At 800℃, the oxidation behavior was linear, but at higher temperatures, the oxidation behavior
was more parabolic. Although oxide layers and transition layers were seen to form in samples after
oxidation at 1000℃ and 1200℃, all formed oxide layers were not protective, and consequently, pesting was
observed almost in all cases, except the condition of oxidation at 800℃ for 1h. The results showed that
adding Al up to 12 at.% helped to improve the oxidation resistance of these Nb-based refractory alloys, but
the improvement was still limited and importantly it was not capable of forming a dense and protective
oxide layer yet.
Beskrivning
Ämne/nyckelord
Refractory alloys , Nb-based alloys , Oxidation resistance , Pesting , Microstructure