An Assessment and Estimation of the Damage Progression Behavior of IN738LC under Various Applied Stress Conditions Based
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IN738LC is a Ni-base superalloy that is precipitation strengthened by the c¢ phase in grains; it is used in applications requiring high-temperature strength, such as the gas turbine blades of high efficiency combinedcycle power plants. With extended operating periods at high temperatures, deterioration tendencies such as the coarsening of the c¢ phase and the hardness deterioration of the material are observed.[1] However, a complex inner cooling system of the blades complicates the distribution of temperatures and stresses in the blades.[2] Furthermore, the relationship between the change of microstructure and creep damage is not clear. Also, the Nibased superalloy has a pronounced brittle property even at high temperatures, and therefore creep degradations, such as deformation of components or initiation of microscopic creep voids detected by conventional observation techniques, only appear at the end of its available creep fracture life. Therefore, in order to accurately assess the integrity of a target component, it is important DAISUKE KOBAYASHI, Senior Researcher, MASAMICHI MIYABE, Senior Manager, and YUKIO KAGIYA, Principal Manager, are with the Electric Power Research & Development Center, Chubu Electric Power Co Inc, Nagoya 459-8522, Japan. Contact e-mail: [email protected] RYUJI SUGIURA, Associate Professor, and A. TOSHIMITSU YOKOBORI, JR., Professor, are with the Department of Nanomechanics, Tohoku University, Sendai 980-8579, Japan. Manuscript submitted July 12, 2012. Article published online March 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
to detect creep degradation or evidence of some kind of damage. The essence of damage progression is considered to be the growth of microscopic plastic strain,[3] and in the past, investigations into crystallographic aspects have been carried out as follows. It is known that plastic deformation throughout crystal grains is not homogeneous and that differences in misorientation development by lattice rotations in a grain will be found.[4] For many years, Laue X-ray photographs using a large-grained aluminum have demonstrated the inhomogeneity of deformation within grains, as well as from grain to grain.[5] Also, a shift of crystallographic orientation due to creep or plastic strain, measured by microbeam X-ray diffraction, will vary depending on the distance from grain boundaries.[6,7] With the current advancement in electron backscattered diffraction (EBSD) acquisition systems in conjunction with scanning electron microscopy (SEM),[8] it has become possible to make unprecedented submicron resolved measurements of the local crystal structure distribution at the millimeter scale. Making full use of these advances, the EBSD method is now widely used in characterization of materials.[9] Wilkinson et al.[10] found that the diffuseness of electron back scatter patterns (EBSPs) was found to increase with plastic strain. The degradation of EBSPs by cold work was investigated using the Al 6061 alloy. A method was presented for quantitative determination
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