AFM, SEM AND TEM study of damage mechanisms in cyclically strained mar-M247 at room temperature and high temperatures

Cylindrical specimens of gamma precipitate strengthened superalloy MAR-M247 were cyclically strained under total strain control conditions using a symmetrical push-pull cycle at 23 degrees C, 700 degrees C and 800 degrees C in air. The fatigue hardening/softening curves and low cycle fatigue life curves are presented. Special attention is paid to the effect of temperature on plastic strain localisation and fatigue crack initiation at 23 degrees C, 700 degrees C and 800 degrees C. Plastic strain localisation into dislocation-rich slabs parallel to the {1 1 1} slip planes was found at 23 degrees C and 700 degrees C. Plastic strain localisation manifested itself by the formation of pronounced surface relief, known as persistent slip markings (PSMs), at locations where dislocation-rich bands emerged on the surface. PSMs consisted of extrusions accompanied by parallel intrusions, where the intrusions were identified as primary crack nucleation sites at 23 degrees C. With an increase in the testing temperature to 700 degrees C, the impact of PSMs on fatigue crack initiation was reduced. At 800 degrees C, the appearance of both PSMs and dislocation arrangement changed significantly. The short PSMs with a wavy character appeared and bands of superlattice stacking faults (SSFs) on {1 1 1} planes with Burgers vector b of < 1 1 2 > type were identified. At 800 degrees C, fatigue cracks were initiated at the defects and carbides.