Anisotropic thermomechanical fatigue of a nickel-base single-crystal superalloy Part I: Effects of crystal orientations and damage mechanisms

Thermomechanical fatigue in distinct crystal orientations of nickel-base single-crystal superalloy was studied. Crack initiation modes and damage mechanisms were characterized on fracture surfaces and longitudinal sections by scanning electron microscope and optical microscope. Creep damage nucleated from casting pores in IP-TMF and propagated under mode I. The creep facets were controlled by the activated slip systems, whereas fatigue damage was orientation-dependent and developed by slipping along different crystallographic planes. Oxidation-assisted cracking in OP-TMF tests was non-crystallographic for all crystal orientations. Multi -layer structures near the crack tip revealed successive growth of oxide and the crack grew within the oxidized material.