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

Mikael Segersäll,Daniel Leidermark,Sten Johansson,Johan J Moverare,J.Y Guédou,J Choné

doi:10.1051/matecconf/20141419003

Mikael Segersäll, Daniel Leidermark + Show 4 more

Open Access

https://doi.org/10.1051/matecconf/20141419003

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Abstract

In this study, the difference between in-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) cycling from 100 to 750 °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 γ / γ ′-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.

Highlights

Han et al [3] studied the difference between IP and OP thermomechanical fatigue (TMF) and found that OP fatigue lives were shorter than IP fatigue lives
Other research has found that the difference between IP and OP TMF life seems to be strain range dependent [4]
For the 001 direction, it can be seen that for a high strain range, IP TMF cycling leads to a higher number of cycles to failure compared to OP cycling

Summary

Introduction

Han et al [3] studied the difference between IP and OP TMF and found that OP fatigue lives were shorter than IP fatigue lives. Kovarik et al [8, 9] performed extensive research regarding the formation of microtwins during deformation of superalloys at intermediate temperatures (660–700 ◦C). It seems that the twinning partial dislocations propagate through the L12-ordered γ -precipitates on {111} planes. The aim is to investigate the difference in TMF behaviour and to study if there are any differences in deformation and damage mechanisms between the two loading conditions and crystal orientations To study both the 001 and 011 orientations is of importance since crack propagation on the blade platform sometimes is dependent on properties in different crystal orientations, where the 011 direction is one of the most important

Experimental

The TMF behaviour

Fracture appearance

Scanning electron microscopy

Conclusions