Abstract
The crack growth properties of various microstructures developed in one heat of Inconel 718 alloy were investigated at 650 °C under air and vacuum environments. The microstructures included fine-grained material (ASTM grain sizes 6–8), coarse-grained material (ASTM grain sizes 3–4) and material of a necklace structure (ASTM grain sizes 3–4 and 8–10). The effect of grain boundary β (Ni 3Nb) phase precipitation was also studied. Continuous fatigue, creep and creep-fatigue conditions were examined. For continuous fatigue the influence of frequency was investigated over the range between 5 × 10 −2 and 20 Hz. For creep-fatigue conditions, hold times of 10 and 300 s were superimposed on a 5 × 10 −2 Hz triangular wave shape signal. It was shown that the grain boundary microstructure had a very strong effect when the fatigue crack propagation behaviour was essentially time dependent. This effect is associated with the occurrence of brittle intergranular fracture and dramatic increases in crack growth rate. The microstructure had no effect under vacuum testing.
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