Partial phase relationships in Ir-Nb-Ni-Al and Ir-Nb-Pt-Al quaternary systems and mechanical properties of their alloys
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I. INTRODUCTION
THE increasing demands for materials with higher temperature capability require a new generation of high-temperature alloys that perform better than Ni-base superalloys. As the coherent fcc/L12 two-phase structure results in excellent creep resistance for Ni-base superalloys, one approach is to develop a new superalloy with a microstructure analogous to that of the Ni-base superalloy but based on elements with higher melting points.[1,2] Iridium, which belongs to the platinum group metals, has a high melting temperature, an fcc crystal structure, and some properties that make it superior for high-temperature applications. The investigation on Ir-base binary and ternary alloys showed promising results, such as fcc/L12 two-phase structure formation and higher high-temperature strength.[3–7] However, the ductility of these alloys was not satisfactory, as they usually presented an intergranular fracture. The addition of alloying elements is one method for improving the ductility of alloys. Ir-Nb-Ni ternary alloys presented more ductility than the Ir-Nb binary alloy;[7] however, the improvement was insufficient for the type of application sought. Accordingly, quaternary alloys were developed. Because NiAl and Pt-Al alloys have good ductility, two quaternary systems, Ir-Nb-Ni-Al and Ir-Nb-Pt-Al, were successively investigated to assess their possible use in ultra-high-temperature applications. The idea for designing the quaternary alloys is to mix two types of binary alloys, both of which have an fcc/L12 twoC. HUANG, Postdoctor Researcher, Y. YAMABE-MITARAI and S. NAKAZAWA, Senior Researchers, and H. HARADA, Project Director, are with the National Institute for Materials Science (NIMS), Ibaraki 305-0047, Japan. Contact e-mail [email protected] X.H. YU, former Project Researcher, was formerly with the National Institute for Materials Science. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
phase structure. Accordingly, the fcc/L12 two-phase structure is expected to form in the quaternary alloys. The phase relationships concentrated on the fcc/L12 two-phase region in these two quaternary systems were primarily established, and mechanical properties were studied. The main results are summarized in this article. II. EXPERIMENTAL PROCEDURE Ingot samples were produced using an arc-melting method in an Ar atmosphere. After being arc melted, one sample from each quaternary system, Ir-Nb-Ni-Al and Ir-Nb-Pt-Al, was analyzed by the fluorescence X-ray method to determine its actual composition, which was found to be very close to the nominal value. This indicated the reliability of the composition of alloys obtained by arc melt
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