Topologically close-packed phase precipitation and thermal stability in alloy 22
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. INTRODUCTION
ALLOY 22 has been proposed by the United States Department of Energy as the outer container material of the waste package to provide long-term corrosion resistance for containment of high-level radioactive waste in the potential geologic repository at Yucca Mountain, Nevada. The development of modern Ni-Cr-Mo alloys, such as Alloy 22, has significantly increased the corrosion resistance of this class of alloys by minimizing the formation of carbide precipitates, improving phase stability, and promoting passivity. However, these alloys undergo phase transformations when exposed to temperatures ranging from 600 °C to 900 °C that result in the formation of secondary topologically close-packed (TCP) phases, such as , , and .[1,2,3] The preferential precipitation of Mo-rich TCP phases at grain boundaries has been shown to decrease the localized corrosion resistance and affect the mechanical properties of thermally aged Alloy 22 with respect to the millannealed material.[3–6] Thus, phase instability in Alloy 22 has a direct bearing on its use as a waste package material. Potential fabrication processes used in the production of waste packages include longitudinal and circumferential welding of the hot-rolled cylinders and welding to attach the bottom lid and the trunnion collars.[7] After welding operations, the disposal containers are solution annealed at 1150 °C and water quenched to remove residual stresses and minimize phase instability in the weldments. In addition, dual closure lids are welded to the disposal containers after waste loading. The residual stresses in the outer closure lid weld are mitigated by laser peening or low plasticity burnishing. Elevated temY.-M. PAN, Senior Research Engineer, D.S. DUNN, Principal Engineer, and G.A. CRAGNOLINO, Institute Scientist, are with the Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute, San Antonio, TX 78238. This article is based on a presentation made in the symposium “Effect of Processing on Materials Properties for Nuclear Waste Disposition,” November 10–11, 2003, at the TMS Fall meeting in Chicago, Illinois, under the joint auspices of the TMS Corrosion and Environmental Effects and Nuclear Materials Committees. METALLURGICAL AND MATERIALS TRANSACTIONS A
perature exposures as a result of welding and solution-annealing processes may alter the microstructure and reduce the corrosion resistance of the Alloy 22 waste package outer barrier. Microstructural characterization of the welded Alloy 22 in the as-welded condition showed the formation of a dendritic structure and the presence of element segregation with TCP phases in the interdendritic regions.[8,9] The effects of postweld heat treatments on the corrosion and mechanical properties of Alloy 22 welds after long-term thermal aging have been reported by Summers et al.[9] and Rebak et al.[10] The amount and size of TCP precipitates in the welded and aged material increased with both aging time and temperature, degrading the mechanical properties and corrosion resistance of
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