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

Abstract

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 -Ni 3 (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.

HRTEM analysis of the high-temperature phases of the newly developed high-temperature Ni-base superalloy VDM 780 Premium

“…The development of new alloys stable at even higher temperatures, with better and long term performance under operation conditions, is challenging for obtaining higher efficiency turbines including reduced emissions. With the aim to fulfill these requirements, alloy VDM 780 Premium has been recently developed [4,5]. This Ni-base superalloy is based on the g matrix (Ni-based cubic Fm-3m structure) and g' hardening precipitates (Ni 3 Al-based cubic Pm-3m structure).…”

In Situ Characterization at Elevated Temperatures of a New Ni-Based Superalloy VDM-780 Premium

“…WASPALOY* has a high amount of c¢ phase, which allows its use at higher temperatures; on the other hand, the high c¢ solvus temperature produces poor hot formability. [10] The development of the alloy 718Plus, in which half of the Fe of the alloy 718 is replaced by Co, improves the performance at high temperatures and is expected to be easier to process and to weld. [11] With improved composition, new, VDM-780 Premium (Ni, Co, Cr, Nb, Mo, Al, Fe, and Ti) was developed for higher service temperature and its structure and performance are under investigation.…”

“…[11] With improved composition, new, VDM-780 Premium (Ni, Co, Cr, Nb, Mo, Al, Fe, and Ti) was developed for higher service temperature and its structure and performance are under investigation. [10,12] This work presents the structural characterization of the new VDM-780 Premium after different aging conditions by means of neutron diffraction (ND) and X-ray diffraction (XRD) at room temperature (RT) and elevated temperatures. The morphology of the different phases is studied by scanning electron microscopy (SEM).…”

Invention of a New 718-Type Ni-Co Superalloy Family for High Temperature Applications at 750°C

“…Microstructure of (a-b) alloy L4 (billet, pilot plant scale) after standard precipitation heat treatment, (c) alloy L4 and (d) Alloy 718 (both laboratory scale) after heat treatment: 700°C/500h[16] Microstructure of alloy L4 (laboratory scale) after heat treatment: 800°C/500h…”