Evaluation of the crystallinity and the Strength of Grain Boundaries in Ni-Base Superalloy under Creep-Fatigue Loadings at Elevated Temperatures

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  • SUZUKI Wataru
    Department of Finemechanics, Graduate School of Engineering, Tohoku University
  • SUZUKI Ken
    Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University
  • MIURA Hideo
    Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University

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Other Title
  • 高温クリープ疲労負荷に伴うNi基超合金の結晶粒界強度および結晶品質評価

Abstract

<p>Advanced-Ultra Super Critical(A-USC) thermal power plant has been developed for improving the thermal efficiency and reducing the emission of CO2. Ni-base superalloy such as Alloy 617 is used for components in the A-USC because of its high strength at elevated temperatures. Since this alloy has higher coefficient of thermal expansion than conventional ferritic steels, however, the increase of thermal stress is concerned. In addition, frequent output change is demanded for thermal power plants because of the instability of the output of renewable energies. Therefore, it is necessary to consider the effect of creep-fatigue damage of this alloy on its lifetime. It was found, however, that the effective lifetime of this alloy decreased drastically under creep-fatigue loadings at elevated temperatures. The conventional linear damage rule could not explain this drastic decrease of the lifetime. In this study, Creep-fatigue test was conducted and the change of crystallinity of the broken specimen was observed by EBSD analysis. Then, the local strength of a grain and a grain boundary was measured by micro tensile test system. As a result, the fracture mode of the bicrystal sample around a grain boundary was changed from ductile fracture to brittle fracture. The main reason for the change was analyzed by EBSD method and it was found that the accumulation of dislocations around the fractured grain boundary caused the change of the fracture mode and the effective strength of the alloy.</p>

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