THE EFFECT OF CREEP AND OXIDATION ON HIGH-TEMPERATURE FATIGUE-CRACK PROPAGATION IN [001]-LOADED CMSX-2 SUPERALLOY SINGLE-CRYSTALS

Fatigue crack propagation (FCP) behaviour of [001] axially oriented si ngle-edge notch tension specimens (R=0.05) of CMSX-2 single-crystal su peralloy has been examined in the 650-950 degrees C temperature range. Secondary orientations of the specimens, i.e. the crack propagation d irections, are mainly [100] or [110]. The increase of the test tempera ture from 650 degrees C to 750 degrees C leads, at 4 Hz, to an increas e of the FCP rates. A further increase of the temperature up to 950 de grees C causes a FCP rate decrease. Time-dependent FCP mechanisms are explored at 950 degrees C performing triangular (4 Hz) and trapezoidal (0.19 Hz) load wave-shape tests. Tests in air reveal much slower FCP rates than tests in vacuum, especially at low Delta K and when 5 s hol d time is added. Oxide closure at the crack tip can explain the drasti c environmental effect on FCP at low Delta K values while creep mechan isms can explain the acceleration of FCP at high Delta K when hold tim e is added in vacuum.