DEVELOPMENT OF QUATERNARY Ir-Ta-Ni-Al REFRACTORY SUPERALLOYS Xihong Yu, Yoko Yamabe-Mitarai, Yoshikazu Ro, Yuefeng Gu and Hiroshi Harada National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan (Received January 7, 1999) (Accepted in revised form March 16, 1999) Keywords: Ir-Ta-Ni-Al; superalloys; microstructure; compressive strength Introduction The development and commercial application of a range of Nickel (Ni) based superalloys for use as single crystals has contributed significantly to the continued improvement of gas-turbine efficiency through increased operating temperature. But the melting temperature of Ni of 1455°C has prevented its application to higher temperatures. For applications at higher temperatures, Iridium (Ir) is one of six platinum group metals, and is characterized by excellent oxidation and corrosion resistance and a relatively high melting temperature of 2447°C (1). Ir-0.3%W alloys are used as the primary contain- ment in space-based power systems and for general-purpose heat sources (2). Iridium is also used to make solar thrusters, which is one of the most important items in Solar Thermal Propulsion (3). We have systematically investigated many Ir-based alloys (4) and named them “Refractory Super- alloys”(5) because of the existence of coherent structures, similar to those in Ni-based superalloys. Our preliminary results show that these refractory superalloys are of superior strength to Ni-based super- alloys at temperatures higher than 1400°C. However, intergrandar fracture in Ir-based binary alloys is a serious problem in commercial applications (6). Other problems are high cost and high density. High density is a problem because for a given part size, the weight of the part increases. For space applications weight must be minimized to keep payload size as large as possible, and in gas-turbine applications weight must be minimized to minimize rotational forces. One method for reducing density, cost, and intergrandar fracture is to add Ni (6). To improve the performance of Ir-based alloys, we propose alloys designed by combining Ir- and Ni-based alloys because Ni-based alloys are ductile, have a relatively low density (about 8.5g/cm 3 , compared to 22g/cm 3 for Ir), and low cost. The objective is to combine the high-temperature strength of Ir-based alloys with the good ductility, low density, and low cost of Ni-based alloys. If the fcc and L1 2 phases with coherent structure form in both alloys, we expect that the fcc and L1 2 two-phase regions of the Ir- and Ni-based alloys will connect with each other at the interface of the Ir-based and Ni-based alloys. The coherency of the fcc/L1 2 phase is very important for quaternary alloys to achieve high strength at high temperature. We chose the Ir-Ta alloy from the available Ir-based binary alloys because the strength of the Ir-Ta binary alloy is high at high temperature (over 700 MPa at 1200°C) and because Ta has a solid solution hardening effect in Ni-based alloys. For the Ni alloy, we chose the Ni-Al Pergamon Scripta Materialia, Vol. 41, No. 6, pp. 651– 657, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(99)00112-8 651