The effect of microstructure on the morphology of fatigue cracks in UDIMET® 720

The low cycle fatigue (LCF) behaviour of four variants of UDIMET(R) 720 was investigated. The materials comprised a fine grained (approximately 10 mu m), powder processed material with a fine bimodal distribution (similar to 20 and 80 nm) of secondary gamma'; the same material, but with enlarged secondary gamma' (similar to 480 nm); a coarse grained powder processed material (grain size similar to 62 mu m) and finally a cast and wrought material with a similar microstructural scale to the fine grained powder processed alloy, but with reduced interstitial element content. LCF testing was undertaken on corner notched square section specimens at 20, 300 and 600 degrees C with a frequency of 0.25 Hz, a cyclic stress range of 500 MPa and an R ratio of +0.1. At 20 and 600 degrees C fracture was found to be macroscopically flat for all materials. However, at 300 degrees C, significant shear fracture was observed in the two materials that had a fine grain size and a fine secondary gamma' size, leading to a characteristic 'tear-drop' appearance. Only minor shear fracture was observed in the coarse grained and enlarged secondary gamma' materials. Tensile tests indicated that weak dynamic strain ageing occurred in all materials at 300 degrees C. The fine grained powder processed U720 also exhibited dynamic strain ageing at 600 degrees C, but this was not the case for the coarse grained or cast and wrought materials. The origin of the shear fracture are discussed and related to the microstructure.