Development of a Low-Density Superalloy Optimized for Components in Large Gas Turbines

Abstract In many aspects, the development of cast Ni-based superalloys is driven by demands from aerospace industry. Considering specific requirements for aero engine parts, alloy design often focuses on maximizing the creep strength at high temperatures up to 1100°C. Advances were mostly achieved by alloying concepts that contain higher concentrations of heavier elements, e.g. W, Ta, Re, Ru. In modern stationary turbomachinery, weight of parts is a key factor for mechanical loading of components and interfaces. For these components, the specific, which means density-compensated creep strength at elevated temperatures, i.e. between 700 and 850°C, is considered more important than high temperature creep strength alone and alloy density plays a major role in structural design to reduce stresses related to the weight of the component. In this case, adapted alloying concepts rather focus on reducing the alloy density while sustaining a sufficient level of mechanical strength and creep resistance. Thus, within this paper an alloy development will be introduced, which focuses on a significant reduction of density while keeping mechanical properties on a sufficient level for components operating in the above-mentioned temperature range. Targeting a density below 8 g/cm3, candidate alloy compositions have been selected using a CALPHAD-assisted selection process. As a major achievement, one selected alloy candidate revealed good castability and has been successfully cast into a prototype component geometry as used in modern stationary gas turbines using an industrial investment casting process. The parts showed no major defects or difference in casting quality in comparison to established alloys. A heat treatment cycle was defined to avoid incipient melting and secure sufficient gamma prime solutioning. In metallographic analysis, microstructure comparable to established Ni-base alloys was observed. The successfully cast alloy was then selected for testing of mechanical properties. First results suggest a performance potential competitive to industrially established Ni-based superalloys for the targeted application regime. Room temperature density was determined to be close to 7.9 g/cm3. In the targeted temperature range, the specific strength of the alloy lies on a competitive level with established Ni-based alloys for stationary gas turbine components. First creep tests reveal high creep elongations and thus indicate that long-term embrittlement is of minor relevance. Based on the promising results of the present study, application of the alloy for components in large gas turbines becomes considerable and further mechanical properties will be investigated. The overall objective is to extend the portfolio of Ni-base superalloys for gas turbine applications with an innovative new low-density material.