Influence of Multi-Axial Stress on Damage Evolution and Change in Crystal Misorientation under Creep Loading of Hastelloy X

Ni based superalloy Hastelloy X is used for high temperature components of power generation gas turbines and aircraft jet engines. Creep damage occurs preferentially in stress concentration portions. Therefore, it is important to clarify creep damage evolution process and to develop creep damage detecting method under multiaxial with stress gradient for safety operation. In this study, creep tests have been conducted by using smooth and notch specimens which have multiple round notches with notch tip radius of 0.5 mm (R0.5) and 2.0 mm (R2.0) on a Hastelloy X. Crystal misorientation parameter GROD of damaged specimens was measured by an EBSD method. Creep rupture times of the notch specimens were longer than those of the smooth specimens showing notch strengthen effect. Larger amount of creep voids on grain boundaries was observed in the notch specimens than that in the smooth specimens. Void number density took the maximum value around the notch root in R0.5, and at the specimen center in R2.0 indicating that a distribution pattern depends on notch root shape. Those distribution patterns correspond to the maximum stress distribution thorough the notch root section. From the crystal misorientation measurement, GROD maps show that values of GROD increase with increasing creep damage, and that higher GROD values occurred at around notch root in the notch specimens. Change in GROD average at the notch root section corresponds to axial creep strain distribution obtained from creep analysis of the notch specimens. Eventually, an equation indicating relationship between axial creep strain and the normalized GROD average independent on temperature and stress states was derived. ©2023 The Society of Materials Science, Japan.