Beyond Nickel Based Superalloys

Abstract

In the most advanced gas turbine engines, the gas temperature exceeds the melting temperature of Ni based superalloys. Further improvements in the performance of gas turbines require operation of metallic materials at higher temperatures. Research programs on Nb and Mo silicide based alloys have demonstrated that these materials can offer attractive mechanical properties that justify their consideration as suitable candidate materials to replace Ni based superalloys in specific structural applications in gas turbine engines. The research has also demonstrated that oxidation behavior of developmental alloys can be significantly improved via selection of specific alloy elements. The ongoing research to develop new alloys is outlined with the emphasis on new metallic materials based on the refractory metals Nb and Mo. For the Nb based alloys, alloy development research based on the Nb–Si–Ti system with transition/refractory and free electron metal additions is reviewed. For the Mo based alloys, the review concentrates on alloys based on the Mo–Si–B and Mo–Si–Al systems.

Ab Initio Study of Binary and Ternary Nb3(X,Y) A15 Intermetallic Phases (X,Y = Al, Ge, Si, Sn)

“…Additions of Al, Ge and Sn in Nb-18Si (at.%) alloys have been shown to benefit their oxidation behaviour [5,[7][8][9] and to influence the transformation of βNb 5 Si 3 to αNb 5 Si 3 [4]. These elements suppress the formation of the tetragonal (stable) Nb 3 Si [10][11][12] and Sn plays a key role in the elimination of pest oxidation [5,9].…”

“…Their microstructures consist of an Nb solid solution phase in which are embedded silicide(s) and other intermetallic phase (s) or vice versa, depending on alloy composition [3][4][5]. The choice of the latter is influenced by considerations about achieving a balance of high and ambient temperature properties [1, 3,4].…”

“…The Nb 3 Si and Nb 5 Si 3 silicides can be present in the microstructure of Nb silicide based alloys [3,4]. Both silicides can form as stable and metastable compounds [6].…”

“…Niobium silicide based alloys have attracted much interest as the next generation ultra-high temperature alloys owing to their creep, high temperature strength and ambient temperature fracture toughness [3,4]. Their microstructures consist of an Nb solid solution phase in which are embedded silicide(s) and other intermetallic phase (s) or vice versa, depending on alloy composition [3][4][5].…”

Experimental and thermodynamic assessment of the Ge-Nb-Si ternary phase diagram

“…They have desirable mechanical properties at both low and high temperatures and low density [1]. Introduction of germanium into these systems is particularly interesting as it can improve their oxidation resistance [2].…”