High temperature fatigue of the nickel-base single-crystal superalloy CMSX-10

Push–pull fatigue tests were conducted under a sinusoidal stress waveform with a frequency of 1 Hz and a trapezoidal one with a hold time in both tension and compression at 300 MPa-amplitude. Tests were conducted at a temperature of 1273 K using smooth bar specimens of the nickel-base single-crystal superalloy CMSX-10. Small cracks were observed on the surface of the interrupted specimens by means of optical and scanning electron microscopes and their number and length were measured. The fatigue behaviour was characterized as follows: (1) A number of small cracks were initiated at a relatively early stage on the grain boundaries of the surface oxide which were perpendicularly to the tensile stress axis direction. (2) Some of these cracks grew inside and reached the base metal. Their growth brought about final fracture of the specimen. (3) The creep strain during the stress hold period accelerated the growth rate of the small cracks and shortened the fatigue life.