Effect of heat treatment on microstructure and mechanical properties of VDM alloy 780 premium

Abstract

VDM Alloy 780 Premium is a new 718-type superalloy recently developed in a cooperation between Technical University Braunschweig and VDM Metals GmbH [1, 2]. It contains the γ'-phase for strengthening in addition to a second phase, in the following referred to as δ-phase (Note that determination of the crystal structure is pending. It may not have the same DOastructure as the δ-phase in Alloy 718.). As in Alloy 718, the δ-phase may be used for grain refinement and strengthening of grain boundaries at elevated temperatures. Most important differences in chemical composition compared to Alloy 718 are the essential replacement of Fe by about 25% Co and a higher Al-content (about 2%) in combination with a lower Ti-content (about 0.2%). Firstly, these measures stabilize the strengthening phase and allow for application temperatures of up to 750 °C. Secondly, the δ-phase is preserved despite the relatively high aluminum content. To make best use of Alloy 780, the precipitation kinetics of both phases along with the concurrent evolution of microstructure and mechanical properties must be understood. Consequently, this article deals with the influence of heat treatments on the phase kinetics, microstructure evolution and mechanical properties. Firstly, it will be demonstrated that proper control of the heat treatment parameters allows for a distribution of the δ-phase as in Alloy 718. Secondly, Vickers hardness testing along with creep rupture experiments will be presented, revealing interdependencies between heat treatment strategies and mechanical properties. At ambient temperature, the strength of the new alloy meets or exceeds that of Alloy 718 despite absence of the γ"-phase, while its creep strength is far superior.