Creep Strengthening of Iron-Nickel-Base Superalloy for High-Stress Condition by Cubo-Octahedral Nanoparticles Precipitat
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Creep Strengthening of Iron-Nickel-Base Superalloy for High-Stress Condition by CuboOctahedral Nanoparticles Precipitation Ming-Yen Li1, Shih-Ming Kuo1, Yeong-Tsuen Pan1 1 New Materials Research & Development Department, China Steel Corporation, 1 Chung Kang Road, Hsiao Kang, Kaohsiung 81233, Taiwan, R.O.C. ABSTRACT Under a creep condition of relatively intense stress, dispersive precipitation of TiC nanoparticles could be promptly brought out within the austenite grain when the iron-nickel-base superalloy was fabricated through the specific production of the ingenious alloy design, applicable electro-slag remelting and high-temperature solution-anneal developed in this study. Both the TiC and coexistent M23C6 precipitates are cubo-octahedral in shape, remaining the cube-to-cube orientation relationship and coherent {111}carbide/γ and {100}carbide/γ interphase interface with the austenite, which bring about the dispersion-strengthening effect so that the time to rupture for the present alloy could be increased by 1.8 times of magnitude relative to the commercial creep-resistant product of the same grade. The improved creep property can be attributed to the mechanism that the nanometer-scale intragranular TiC and submicron intergranular M23C6 can act as pinning points for individual dislocations and grain boundaries, respectively, resulting in the raised Orowan strengthening and suppressed grain boundary sliding. Since the strengthening media found in this study are thermally stable and able to emerge readily prior to the formation of γ’ phase at the temperature range studied, this work suggested an improved grade of heat resistant alloy for applications in a relatively harsh environment that the creep lifetime might be shorter than the incubation period of the common strengthening phase for superalloys. INTRODUCTION Iron-nickel-base superalloy is one of the most widely used alloys in the energy and petrochemical industries. Heat-resistant Alloy 800H is the representative grade, which provides a favorable combination of excellent creep properties, good resistance to high temperature oxidation, corrosion and carburization, and good structural stability at high temperatures [1]. The material hence was recently recommended as a candidate material for the generation IV nuclear power plants [2], which may bring about more challenging conditions for its high-temperature application. Since the creep resistance is the crucial property for its high-temperature applications, Alloy 800H, with a minimum 0.05 wt% C content, must be solution-annealed at the minimum temperature of 1121 oC to maintain a stable austenitic structure for use at temperatures above 593 oC. Both the high carbon content and the high solution-anneal temperature promote a large stable grain size (ASTM No. 5 or coarser), which provides better creep resistance. Beside, after a long serving time at temperature higher than 550 oC, γ’ precipitates tend to form due to Al + Ti content of 0.6-1.2 wt% in Alloy 800H. These precipitates can further increase the creep r
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