Crack initiation and propagation behaviour under high-temperature very-high-cycle fatigue: Directionally solidified columnar-grained vs. single-crystal superalloys

Role of oxidation and recrystallisation on very-high-cycle fatigue of columnar-grained DZ125 and single-crystal DD6 superalloys are investigated. With the temperature increase from 850 to 1000 °C for DZ125 while from 1000 to 1100 °C for DD6, the Mode-I cracking tendency increases due to internal oxide penetration. Recrystallisation can occur for both the electrolytically and mechanically polished surface conditions, controlled primarily by temperature and test duration. Its presence can lead to strain localisation at subsurface, but the surface recrystallisation does not necessarily determine the oxide penetration. By comparison with the single-crystal DD6 that shows the pure Stage I at 1000 °C, the columnar-grained DZ125 exhibits the distinctive Stage I cracking followed by Mode I propagation. This fracture characteristics in DZ125 can be attributed to the combined effect of the high-angle grain boundaries and far-field stress. The microstructure-related factor plays a vital role when the crack length is small, whereas the role of stress becomes predominant when the crack length is comparable to a couple of grains, ultimately leading to the shift from the crystallographic Stage I to Mode I propagation.