“…planar slip characteristics (also induced by ordered o' precipitation), which enhances slip-reversibility (and hence creates less "damage") at the crack tip (5,6,9), and the tortuous nature of the crack path, which promotes crack tip shielding via crack deflection and closure mechanisms (7,(9)(10)(11)(12). Whereas all three mechanisms undoubtedly act in concert to various degrees, in view of the high measured levels of closure and deflection, the large load ratio dependence of near-threshold growth rates, and the anomalously high small crack effects (15,23), it would now appear that enhanced shielding is the prominent factor for the excellent fatigue crack growth properties of these alloys (see also refs.…”