Damage Process and Measurement of Crystal Misorientation by EBSD under Creep and Creep Fatigue Conditions on Ni Base Alloy HR6W

Abstract

Nickel based alloy HR6W is one of candidate materials for high-temperature components in an advanced ultra-super critical power plant in Japan. In application to piping, it is necessary to clarify progress of damage and life property under creep and creep-fatigue loading conditions. In this study, fatigue, creep and creep-fatigue with up to 24 hours strain hold time per cycle tests were performed on HR6W at 750°C, and damaged specimens were produced by interrupting the tests. Cyclic failure life with 1hour hold time reduced to about 1/5 of the fatigue life, and reduction of the failure life increases with increasing the hold time due to accumulation of intergranular creep damage. Failure lives of the creep-fatigue tests were predicted by using the ductility exhaustion method. Creep voids were observed at lower damaged specimens both under creep and creep-fatigue loading, and lengths of the voids increase with increasing the damage resulting in formation of micro cracks at higher damage. For the creep and creep-fatigue damaged specimens, crystal misorientation represented by such as KAM (Kernel Average Misorientation) and GROD (Grain Reference Orientation Deviation) values were measured by using EBSD (Electron Backscatter Diffraction) analysis. KAM average values increase with the damage monotonously, in which the difference between the creep and the creep-fatigue tests was not clearly seen. On the other hand, GROD average values of the creep damage specimens is larger than those of the creep-fatigue damage specimens at the same damage indicating that creep and creep-damage of the HR6W may be detected by measuring GROD average.