1 Effects of Aging in High Temperature Helium Environments on Room Temperature Tensile Properties of Nickel-Base Superalloys Daejong Kim 1,2 , Injin Sah 1 , and Changheui Jang 1 * 1 Department of Nuclear and Quantum Engineering, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea 2 Nuclear Materials Technology Development Division, KAERI, 150 Deogjin-dong, Yuseong-gu, Daejeon 305- 600, Republic of Korea *Corresponding author, Tel.: +82-42-350-3824, Fax: +82-42-350-3810, E-mail: chjang@kaist.ac.kr Abstract The influence of high temperature aging treatment on room temperature tensile properties of wrought nickel-base superalloys Alloy 617 and Haynes 230 was investigated. A significant decrease in elongation was observed for Alloy 617 exposed to a heavily oxidizing and decarburizing condition because of coarsening of grain boundary carbides and extensive inter-granular oxidation. On the other hand, Haynes 230 showed much lower ductility when exposed to a heavily carburizing condition, especially at 1000 o C because extensive carburization occurred due to a reaction with tungsten. Considerable loss of ductility for Alloy 617 and Haynes 230 was also observed in He-H 2 -H 2 O-CO- CO 2 -CH 4 and He-H 2 O-CO-CO 2 environments, which were the slightly oxidizing and decarburizing conditions. Loss of ductility was predominantly associated with brittle inter-granular cracking, while the extent of loss of ductility decreased depending on the decarburization depth. Decarburization was observed more extensively in helium with H 2 -H 2 O-CO-CO 2 -CH 4 than helium with H 2 O-CO-CO 2 , and for Alloy 617 than for Haynes 230. Finally, the role of H 2 in accelerating decarburization is discussed. Keywords: nickel-base superalloys; decarburization; carburization; inter-granular fracture