DEVELOPMENT OF HIGH TEMPERATURE CAPABILITY P/M DISK SUPERALLOYS E. S. Huron, K. R. Bain, D. P. Mourer 1 and T. Gabb 2 1 GE Aviation, Cincinnati, OH. 2 NASA Glenn Center, Cleveland, OH. Keywords: Superalloy, powder metallurgy, Boron, Carbon, Hafnium, Magnesium, Zirconium, Tantalum Abstract A study was conducted to optimize the major element chemistry of powder metallurgy (PM) alloys for the challenging goals of a High Speed Civil Transport (HSCT) application. Two iterations were performed. Subscale heats of experimental powders were atomized, consolidated by extrusion, isothermally forged, and supersolvus heat treated. Key relationships were identified between alloying elements resulting in the identification of an optimized alloy composition. The final alloy showed significant improvements in creep and in hold time crack growth compared to state-of-the art commercial alloys. Introduction In the mid-1990’s, NASA began a program aimed at developing an engine for the High Speed Civil Transport (HSCT). This program included a materials-targeted effort called the Enabling Propulsion Materials (EPM) program and resulted in a joint contract effort between GE Aircraft Engines, Pratt & Whitney, and NASA Glenn Research Center (formerly the NASA Lewis Center). The High Speed Civil Transport (HSCT) mission represented a unique durability challenge for compressor and turbine nickel-base superalloy disk materials. In a commercial subsonic transport engine, combined high operating temperature and stress conditions are encountered only during takeoff and thrust reverse after landing. The cumulative duration for both of these cycle points is typically only 3 to 5 minutes per mission cycle. Conversely, the highest operating temperature in the HSCT mission was to occur during the cruise portion of the mission cycle, with a duration of hours instead of minutes. As a consequence, the total hot time exposure over the life of an HSCT engine was to be many thousands of hours, as opposed to the 300 or so hot hours typically accumulated during the life of a subsonic transport engine. This imposed the need for a substantial improvement in the creep durability, surface integrity, and environmental resistance of disk alloys, and raised concerns regarding the possibility of highly deleterious dwell- fatigue interactions resulting from the long hold times. To develop superalloys to meet the challenge of the HSCT mission, the team reviewed prior published and unpublished superalloy data and immediately realized that successfully meeting the project goals would require alloys beyond the then- current commercial alloys (such as IN100 and Rene’ 88DT), with improvements required in both bulk element composition and in grain boundary chemistry and structure. The team first explored grain boundary element composition and microstructural optimization (References 1 and 2) and then moved on to bulk alloy composition. This paper describes the results of the bulk alloy study, which was performed in two iterations. First Iteration Alloy Matrix Design and Test Matrix Screening tests on experimental alloys from earlier programs provided initial guidance on key factors that affect nickel-based superalloys. These key factors can be categorized into chemistry and microstructural effects. Following the completion of the initial grain boundary element Designed Experiment (DOE) (Reference 1), a series of strong chemistry trends on behavior were observed. Based on the earlier program screening results and the outcome of the grain boundary studies, the variation in chemistry, particularly tantalum, was found to be important. These trends were incorporated into a new designed experiment. The goal was to maintain good creep and dwell fatigue crack growth while simultaneously reducing solution temperature and enhancing process window. The key features were balancing tantalum, niobium and tungsten to increase creep resistance and lowering dwell fatigue crack growth resistance while controlling percent gamma prime, cobalt and Al/Ti ratio to reduce the gamma prime solvus temperature. The selection of alloys for the first major element matrix (ME#1) involved the variation of elements and of γ' volume fraction. This was achieved by varying the ratio of particular elements, such as Al:Ti, W:Mo, and Nb:Ta while controlling the volume fraction of γ', which was varied from 48% to 55%. These ratios are shown in Table 1. Table 1: First Iteration Refinement Study Variables Ratio/ Variable Minimum Maximum Al:Ti 1:1 1.6:1 Mo:W 0 2:1 Ta:Nb 0 2:1 Four additional alloys were concurrently processed with the ME#1 material: CH98, A3, HK97, and HK81. These alloys were promising compositions from prior internal studies. The full experimental design is shown in Table 2. In addition, several experimental alloys developed prior to the program were produced for evaluation. These alloys allowed exploration of the effects of lower chromium and cobalt along with higher niobium. The test plan (Table 3) concentrated on tensile, creep, LCF and low temperature cyclic crack growth as well as high temperature time dependent crack growth. The effect of long time exposure was evaluated using an exposed creep specimen. Table 2: First Iteration Alloy Development Matrix (a) DOE Parameter Design Study of Nb/Ta, Al/Ti ratio, γ’ Content, Co, Ta, and W/Mo. 181 Superalloys 2008 Roger C. Reed, Kenneth A. Green, Pierre Caron, Timothy P. Gabb, Michael G. Fahrmann, Eric S. Huron and Shiela A. Woodard TMS (The Minerals, Metals & Materials Society), 2008