Low cycle fatigue properties and fatigue mechanism of DD6 single crystal superalloy under asymmetrical cyclic loading

The low cycle fatigue（LCF） properties of DD6 single crystal superalloy were investigated at 700 ℃ and R of 0.05. SEM was used to study the fracture surface and fracture microstructure. The results show that the LCF life of the alloy decreases with the increase of strain amplitude. LCF properties of the alloy are excellent under asymmetrical cyclic loading. The alloy has no transition fatigue life during LCF tests at all total strain amplitudes. LCF fatigue damage can be dominantly contributed to elastic damage and the plastic deformation is very minimal. The plastic damage increases with the increase of total strain amplitude. The crack initiation site, the fatigue crack propagation area and the final fracture zone can be observed in the fracture surface and all specimens is similar to quasi-cleavage fracture. The fatigue cracks are initiated from the micro-pores on the surface, sub-surface or far from the surface. Far from the surface crack fractures have fish-eye feature. The fatigue crack propagates perpendicularly to main stress at first and then along {111} plane. Typical fatigue striation, cleavage steps and river pattern characteristic are formed on fatigue crack propagation zone. The cleavage plane, slip band and tearing ridge are seen in the final fracture zone. Fracture microstructure analysis shows that the γ′ phase morphology far from the fracture surface still maintains cubic shape， and the slip bands are visible seen near the fracture surface， and secondary cracks are formed along slip bands