Dislocation structure, phase stability, and yield stress behavior of L1 2 intermetallics: Ir 3 X (X = Ti, Zr, Hf, V, Nb,
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NTRODUCTION
MODERN two-phase g/g nickel-based superalloys, designed to be used under extreme conditions (high temperatures, aggressive environment, and high operating stress), have found a wide range of high-temperature applications such as jet turbines and power generators.[1,2] At present, however, the potential for a further increase of the temperature capability of these materials has been exhausted; the current g/g alloys operate at temperatures up to T 1100 °C, which is about 85 pct of their melting temperature. A new approach for the development of high–temperature superalloys is based on the use of platinum group metals (PGMs) with their higher melting temperatures and superior environmental properties: it was demonstrated that Pt-Zr alloys can sustain high strength properties up to 1300 °C[3] and Ir-Nb alloys even up to 1800 °C,[4] which is the highest temperatureresistant alloy of all the platinum group alloys (PGAs) studied so far. Although there are no doubts as to the potential of the PGA for ultra-high-temperature applications, further progress in their development is hindered due to an insufficient understanding of the fundamental factors that control their mechanical properties. In particular, there is no reliable information about dislocation structure and mobility in Ir3X alloys, except for a single observation of the superdislocation in Ir3Nb.[5] As a result, the basic mechanisms determining strength and plasticity of the Ir3X alloys are still unclear. O.Y. KONTSEVOI, Research Associate, and A.J. FREEMAN, Professor, are with the Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208-3112. Y.N. GORNOSTYREV, Senior Research Associate, Department of Physics and Astronomy, Northwestern University, is Principal Researcher, Institute of Metal Physics, Ekaterinburg 620219, Russia. A.F. MAKSYUTOV, Graduate Student, and K.Y. KHROMOV, Researcher, are with the Russian Science Center “Kurchatov Institute,” 123182 Moscow, Russia. 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
One important characteristic affecting the potential of alloys for high-temperature applications is the temperature dependence of the yield stress, sy(T ). It is now well established that the yield stress temperature anomaly (YSA), or the increase of sy(T ) with temperature, is an intrinsic property of many L12 alloys resulting from features of their superdislocations, which can lose mobility due to thermally activated transformations into a non-coplanar configuration (so-called Kear–Wilsdorf locks).[2,6] There are only a few L12 systems (for example, Pt3X alloys) in which the YSA has not been observed; for Ir3X, existing
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