Hydrogen effects on low-cycle fatigue of the single-crystal nickel-base superalloy CMSX-2

The effects of hydrogen on the low-cycle fatigue behavior of CMSX-2 [001]-oriented single crystals were examined. Fatigue tests were conducted under constant plastic strain amplitude control. Cyclic stress-strain curves and fatigue life data at different plastic strain amplitudes were determined for hydrogen-free and hydrogen-charged specimens. Two charging procedures, leading to different hydrogen concentrations, were applied. Hydrogen was found to decrease significantly the number of cycles to failure under the various experimental conditions. The increasing hydrogen concentration and ratio of the hydrogen to nonhydrogen-containing volume were found to shorten fatigue life in hydrogen-charged specimens. Based on the analysis of cyclic stress-strain curves and optical and transmission electron microscopy (TEM), it was established that hydrogen enhanced strain localization and promoted crystallographic, stage I cracking, leading to embrittlement. The overall fracture mechanism is discussed in conjunction with Duquette and Gell’s stage I fracture model.[16]