複数環状切欠きを有するNi基超合金IN738LCのクリープ損傷過程と結晶方位差測定

抄録

Ni based superalloy IN 738 LC is used for the first stage blades 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 under multiaxial with stress gradient on keeping 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 IN 738 LC. Crystal misorientation parameters of damaged specimens were measured by an EBSD method. Creep rupture times of the notch specimens were longer than those of the smooth specimens showing notch strengthen effect. Voids were observed at the grain boundaries vertical to stress direction in the smooth specimen, whereas voids were also observed at the grain boundaries with a small inclination from the axial direction in the notch specimens. The maximum void length took the maximum value around the notch root in R0.5, and decreased toward center of the specimen. Those results correspond to higher axial tensile stress and axial creep strain yielding at around the notch root in R0.5. From the crystal misorientation measurement, significant difference of GRODave was not observed at the same creep damage level between the smooth and around notch root of the R0.5 specimens. Accumulated axial creep strain around notch root of the R0.5 specimen is almost equal to that of the smooth specimen at the same damage level. This suggested that GRODave corresponds to accumulated axial creep strain regardless of stress states.