In- and out-of-phase thermomechanical fatigue of a Ni-based single-crystal superalloy

In this study, the difference between in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) cycling from 100 to 750 degrees C has been investigated for the Ni-based single-crystal superalloy MD2. In addition, two different crystal orientations were studied, the < 001 > and < 011 > orientations respectively. When comparing IP and OP TMF lives, a strain range dependency is found for the < 001 > direction. For high strain ranges, IP cycling leads to a higher number of cycles to failure compared to OP. However at lower strain ranges, OP cycling leads to a higher number of cycles to failure compared to IP. Microstructure investigation shows that for the < 001 > direction, deformation twinning within the gamma/gamma'-microstructure is much more pronounced during OP conditions compared to IP. However for the < 011 > direction, the opposite is observed; twinning is more pronounced during IP TMF. From the microstructure investigation it is also visible that intersections between twins seems to trigger formation of TCP phases and recrystallization. These intersections also work as initiation points for TMF damage.