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

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

“…S2 and S3). Therefore, this diffraction pattern perfectly fits the B g' ¼ [1][2][3][4][5][6][7][8][9][10] zone axis orientation of the g' phase.…”

“…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).…”

“…The recently developed alloy VDM 780 Premium has been created with the aim of pushing further the limits of the currently used Nickel alloys by enabling the increase of the operating temperature for high-temperature applications, without losing the good workability of Alloy 718 [4,5]. It turns out that in order to be able to tailor the mechanical properties at high temperatures and to ensure the good performance within the final applications [17e21] of the newly developed alloy VDM 780 Premium, a special attention needs to be given to understanding the nature of the hightemperature phases and their growth mechanism.…”

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

“…[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).…”

On the Development Concept for a New 718-Type Superalloy with Improved Temperature Capability

“…Recently, a 718-type superalloy aiming at an even higher temperature capability than ATI 718Plus ® , approximately +100 • C compared to alloy 718, was developed collaboratively between the Technische Universität Braunschweig and VDM Metals GmbH [17][18][19]. Currently, this alloy is in the phase of market introduction under the trade name VDM Alloy 780.…”