Alloys for application at ultra-high temperatures: Nb-silicide in situ composites
P. Tsakiropoulos
Abstract
The paper reflects on the development of Nb-silicide based alloys vis-à-vis property goals and compares them with refractory-complex-concentrated-alloys (RCCAs) for their density, oxidation and mechanical properties. The Nb-silicide based alloys are separated in two groups, namely those with and without Ti addition. It is discussed how alloying has supressed pest oxidation and scale spallation in Nb-silicide based alloys. The processing of the latter and RCCAs is discussed, similarities are highlighted, and the significance of macrosegregation is stressed. The importance of alloying additions, Ti/Si ratio, solidification processing, growth rate in directional solidification, and thermomechanical processing for meeting the fracture toughness goal is highlighted. Discussion about the creep of Nb-silicide based alloys emphasises the role of microstructure type, the importance of the Ti/Si, Ti/Hf, Nb/(Ti + Hf) ratios and alloying additions. Nb-silicide based alloys and RCCAs have same phases in their microstructures, use essentially the same alloying additions, and their alloying behaviour is described by the parameters δ, Δχ, VEC, ΔHmix, ΔSmix and Ω. It is shown that RCCAs with Nb and other RM additions studied to-date can be represented together with Nb-silicide based alloys in the same maps of the aforementioned parameters. Methods of development, future directions of research and development are discussed.