Crack initiation and propagation during thermal-mechanical fatigue of IN792: Effects of dwell time

Thermal-mechanical fatigue (TMF) fatigue crack growth measurements are performed on the polycrystalline alloy IN792, in mechanical strain control and in the temperature range 100-750 °C. The effect of both in-phase (IP) and out-of-phase (OP) TMF cycles on the crack growth rate are investigated and the impact of prolonged dwell times at the highest investigated temperature is emphasised. Scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) techniques are used to characterise the cracking phenomena; cross-correlation-based analysis of EBSD patterns is used to obtain high resolution maps of lattice rotation and elastic strains near second phases such as carbides. The six hours dwell time at maximum temperature increases crack growth rate for both types of TMF; higher rates are observed in the case of IP. MC-type carbides are found to play an important role in crack initiation performance of IN792 by decreasing the crack initiation life, particularly in the IP-tests. Finally, a transition from transgranular to intergranular crack propagation was observed for the IP-test as dwell time increased, the transgranular character of the cracking in the OP-tests was not affected by the dwell time.