On understanding the role of V addition in design and development of high specific strength γ'(L12)-strengthened Co-base superalloys

Abstract

Ni-based superalloys with excellent mechanical, thermal and oxidation resistance properties have great demand in energy applications. Over the time, varied compositions of Ni-based superalloys have been utilized for the fabrication of aero-based and land-based turbines. Recently, these are also proposed as key alloys for construction of Advanced Ultra-supercritical (AUSC) plants.

Co has 40 ºC higher melting temperature than Ni, and offers superior sulfidation, hot corrosion and thermal fatigue resistance at elevated temperatures. Recent discoveries suggest that Co-based superalloys having similar microstructure to Ni-based superalloys can be synthesized with base compositions of Co-Al-W or Co-Al-Mo-Nb/Ta. Further developments of these demonstrated that Co-base superalloys can be a promising alternative to Ni-based superalloys. The major challenges with the Co-based superalloys development is to (a) obtain a high γʹ(L12)-solvus temperature, (b) low mass density, (c) high specific strength at elevated temperatures, (d) long-term microstructural stability (e) better creep properties and (f) optimum oxidation resistance.

In the current work, V is investigated as a potential additive in Co-Al based system to produce γʹ(L12)-strengthened Co-base superalloys. V having complet­­e solid solubility with Mo was systematically replaced in base composition of Co-10Al-5Mo-2Nb alloy, earning benefits in γʹ-solvus temperature and mass density. A detailed microscopic characterization and microanalysis studies on the developed Co-10Al-5V-2Nb alloy demonstrated the role of V and its interaction with other participating elements in γ- and γʹ- phases. This further guided development of a novel γʹ-strengthened Co-Al-V ternary system. In a further endeavour, Ni and Ti was added, which led to design a new class of low-density Co-based super alloys. In a parallel effort, oxidation behaviour of the developed Co-Ni-Al-Ti-Nb-V alloy was studied at 800 ºC and 900 ºC. Addition of Cr significantly inhibits the accelerated corrosion of the V containing alloys. These results can provide beneficial inputs for the development of new class of γʹ(L12)-strengthened Co-based superalloys.