Influence of cerium additions on high-temperature-impact ductility and fracture behavior of iridium alloys
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INTRODUCTION
RADIOISOTOPE thermoelectric generators (RTGs), used for supplying electric power to interplanetary space missions, utilize the energy liberated due to decay of the radioisotope fuel. The material used for cladding the fuel pellets is an iridium-based alloy developed at Oak Ridge National Laboratory, which contains nominally 0.3 wt pct W, 60 wppm Th, and 50 wppm Al, generally known as DOP-26. Thorium is added to this alloy to improve its hightemperature impact ductility.[1] The beneficial effect of thorium is derived from two factors: improved grain boundary cohesion due to its enrichment at the grain boundaries[2] and refinement of grain size due to pinning of grain boundaries by second-phase precipitates.[1] The highest ductility is obtained when Th is present at a level of about 200 wppm.[1] Unfortunately, Th at this level reduces the weldability of the alloy, as it promotes hot cracking by the formation of a low-melting Ir-Ir5Th eutectic.[3] Hence, the current generation DOP-26 alloy contains nominally only 60 wppm Th. This article reports on part of an ongoing research program aimed at finding substitutes for Th that are not radioactive and that improve high-temperature impact ductility without reducing weldability. Early in the program, George and Ohriner[4] selected Ce as a promising substitute based on the following reasoning. To be beneficial, the selected dopant should segregate to the grain boundaries, improve grain boundary cohesion, and refine the grain size of iridium. A driving force for grain boundary segregation is the large atomic size misfit between the dopant and the matrix.[5] This size mismatch also causes low solid solubility of the dopant, which results in A.N. GUBBI, formerly Graduate Student, Materials Engineering Program, Auburn University, is currently Senior R&D Engineer, with DENTSPLY International, 1301 Smile Way, York, PA 17404. E.P. GEORGE, Senior Research Staff Member, and E.K. OHRINER, Senior Development Staff Member, are with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6093. R.H. ZEE, Professor, is with the Materials Engineering Program, Auburn University, Auburn, AL 36849-5351. Manuscript submitted June 4, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
second-phase precipitation. The second-phase particles, if they are stable at elevated temperatures, are expected to refine the grain size by pinning the grain boundaries. Once a dopant segregates, it should enhance the grain boundary cohesion if it is to be beneficial. It is difficult to predict which elements would improve the grain boundary cohesion in iridium. Cerium was chosen based on its expected chemical similarity to thorium and its large atomic size mismatch relative to iridium. In earlier studies,[6,7] it was found that Ce segregated strongly to the grain boundaries in iridium. An experimental investigation of resistance to weld hot cracking[8] in an iridium alloy doped with 50 appm cerium revealed that this alloy had better weldability than currently used DOP
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