Effect of Etching on Fatigue Properties of DD6 Single-Crystal Superalloy

The effects of etching on the surface integrities and the high cycle fatigue properties of DD6 single-crystal superalloy were investigated. The standard heat treated DD6 specimens were etched twice and four times using HCl/H₂O₂ solution, respectively, and the three-dimensional surface topography of specimens etched for different times was observed by Leica DCM8 confocal microscope and SEM. After that, the specimens with different surface states were subjected to high cycle fatigue testing at 760 °C and 980 °C in ambient atmosphere, respectively. The results showed that the surface of un-etched specimens had many longitudinal and parallel fine scratches which were caused by polishing, and the surface roughness was very low. Etching caused the occurrence of etching pits in the interdendritic areas, and with the increase in etching time, both the depth of the etching pits and the surface roughness increased. Moreover, the fatigue properties of the alloy were reduced after etching. The fatigue properties decreased with the increase in etching time. Etching was found to play a significant role on the high stress amplitude zone, while a smaller influence on the low stress amplitude zone. Compared with the results at 980 °C, the fatigue properties of the alloy at 760 °C were more affected by etching. The fatigue limits at 760 °C of the alloy after etching can be well predicted by the \( \sqrt {\text{area}} \) parameter model.