Mechanical properties of Ir-Nb alloys containing Ni and Al
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CTION THE Ir-base alloys with an fcc/L12-Ir3Nb coherent structure have excellent high-temperature strength and oxidation.[1] The weak points of Ir-Nb alloys are their intergranular brittleness and their high density. Many attempts have been made in our research group to improve these weak points. In one attempt, Ir was replaced with Ni.[2] The recommended amount of Ni was 10 at. pct. In the other attempt, we prepared quaternary Ir-Nb-Ni-Al by combining Ir-Nb with Ni-Al binary alloys in different proportions.[3] We expected the two kinds of two-phase regions, fcc ⫹ L12-Ir3Nb and fcc ⫹ L12-Ni3Al, to connect with each other from the Irrich corner to the Ni-rich corner in the diagram of quaternary Ir-Nb-Ni-Al. However, the actual results were unexpected. The two kinds of two-phase regions were separated by a three-phase region, which consisted of fcc, Ir3Nb, and Ni3Al.[4] A two-phase region (fcc ⫹ Ni3Al) was indeed observed near the Ni-rich corner.[5] Thus, we wondered if the two-phase region, fcc ⫹ Ir3Nb, and an fcc/Ir3Nb coherent structure existed in the Ir-rich corner. Since Ir-base binary and ternary alloys with two-phase structures exhibit excellent high-temperature strength, we expected the quaternary Ir-base alloys also to have excellent high-temperature strength and an excellent balance of high-temperature strength and ductility. In the present study, Ir-Nb alloys mixed with Ni and Al were investigated to examine their microstructural evolution and high-temperature strength and ductility. II. EXPERIMENTAL PROCEDURE Ir-15Nb and Ni-18.9Al (at. pct) alloys were used to prepare the quaternary Ir-Nb-Ni-Al alloys. Table I shows the X.H. YU, Postdoctoral Researcher, is with the Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. Y. YAMABE-MITARAI, S. NAKAZAWA, and Y. RO, Senior Researchers, and H. HARADA, Senior Researcher and Project Director, are with the High-Temperature Materials 21 Project, National Research Institute for Metals, Ibaraki 305-0047, Japan. Manuscript submitted June 26, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
nominal composition and the proportion of two kinds of binary alloys. The ingots were prepared by arc melting in an Ar atomsphere. Each ingot was 30 g. Samples were cut from these ingots and heated at different temperatures, from 1500 ⬚C to 2200 ⬚C, for different times within a vacuum furnace in order to understand their microstructural evolution. Samples were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). X-ray diffraction was used for phase analysis and latticemisfit calculation. The TEM sample was prepared by the ion-milling method. The SEM sample was etched electrolytically by a 4 pct HCl ethyl alcohol solution at 10 V AC for 5 minutes. A cylindrical sample 2.5 mm in diameter and 5 mm in height was used for the compressive test. The compressive test temperatures ranged from room temperature to 1200 ⬚C in air. In an argon atmosphere, tests were conducted at 1500 ⬚C and 1800 ⬚C. The init
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