Thermal fatigue of a Ni-based superalloy single crystal

This paper presents a thermal-mechanical fatigue (TMF) test procedure and some results obtained in vacuum for a Ni-based superalloy single crystal. This procedure allows the monitoring of slip lines, fatigue-crack initiation and propagation until final failure. Single-crystal specimens with different crystallographic orientations were tested under different thermal-cyclic loading conditions. Tests were conducted on smooth specimens as well as on specimens with a central hole of 0.5 mm diameter to analyze the effects of stress concentration. Both crystallographic and non-crystallographic cracks were observed. An increase of the maximum temperature in the cycle (with the same amplitude of temperature change) causes a severe shortening of the service life, as well as stress concentration. It was observed that the first initiated microcrack arrests and the sample fracture takes place on newly nascent microcracks.