Development of a new 718-type Ni-Co superalloy family for high temperature applications at 750^∘C

¹ Technische Universität Braunschweig, Institute for Materials, 38106 Braunschweig, Germany
² Outokumpu VDM, 58791 Werdohl, Germany

^a Corresponding author: t.fedorova@tu-braunschweig.de

Abstract

Alloy718 has been used for many years in the aerospace industry due to its unique mechanical properties and good processing characteristics, especially its workability. However, the temperature limit of Alloy718 is about 650 ^∘C because of the thermal instability of the main strengthening phase γ^′′-Ni₃(Nb,Ti,Al). Numerous attempts have been made to develop a new wrought 718-type alloy for high temperature applications. The approach was to increase the stability, i.e. the solvus temperature of the γ^′-phase (T_γ^′,s). However, this affected workability as the solvus temperature of the δ-phase (T_δ,s) did not increase accordingly so that the window for fine grain forging T_δ,s-T_γ^′,s became smaller. In this paper the development of a new γ^′/γ^′-alloy on the basis of Alloy718 is presented, where the microstructure is stable at 800 ^∘C, mechanical properties are similar to Alloy718, yet do not deteriorate beyond 650 ^∘C, and the forging window is wider than the one of Alloy718, allowing for good workability. This was essentially achieved by the addition of about 17%–30% Co in combination with an Al/Ti-ratio of more than 5.0 and an Al-content of about 1.6%–2.2%. The key role of cobalt is to stabilize the δ-phase, allowing for solvus temperatures in excess of 1100 ^∘C. Consequently, the stability of the γ^′ -phase can be increased by further addition of aluminium. At the same time the Ti-content is reduced to prevent formation of the η-(Ni,Co)₃(Ti,Al,Nb) phase. Besides discussion of the alloy development concept, information on microstructure evolution and mechanical properties will be given.