Strain ratio effect on microstructural evolution during low cycle fatigue exploitation of nickel base superalloy IN 740H

Strain ratio (Re) is varied during isothermal strain-controlled fatigue tests of nickel-base superalloy IN740H at 760 degrees C; R-epsilon = -1, 0, 0.5. Unlike most alloys, highest fatigue life is observed under R-epsilon = 0 test condition compare to R-epsilon = -0.5 and -1. From extensive electron microscopy, i.e. energy-filtered TEM (EF-TEM), WDS and EDS studies, it is confirmed that the creep effect due to tensile mean stress under R-epsilon = 0 condition was offset due to segregation of Nb at dislocation and formation of Nb(CN) at grain boundaries, which proved to be fatigue strengthening. Debonding of Nb(CN) and. matrix provided preferential fatigue crack initiation sites and resulted in lowering the fatigue life for R-epsilon = 0.5 test condition.