The Creep Fracture of Inconel X-750 at 700° C

The creep-fracture behaviour of Inconel alloy X-750 at 700° C has been studied using four grain sizes ranging from 9 to 200 μm over a range of stress from 207 to 414 N/mm². Cavity nucleation was continuous through the tests, starting during primary creep. Interlinkage of cavities to form cracks of a grain-boundary facet in length was completed towards the end of the secondary stage of creep and catastrophic failure resulted when these cracks interlinked to form a crack of critical size. From the angular distribution of cavities, it was deduced that grain-boundary sliding played a dominant role during the earlier stages of the growth of cavities, whereas vacancy condensation and/or mechanical tearing were the dominant growth processes during the later stages of the growth of cavities. Ductility increased significantly as the grain size was decreased. This can be related to the extended tertiary creep stage observed in the small grain sizes as a result of the presence of a larger number of triple junctions that limit crack propagation. Variation of rupture life with grain size was an inverse plot of the variation of minimum creep rate with grain size. However, the product of the two (έ _s × t _r ) was not a constant. It was observed that the stress-sensitivity parameter n(έ ∝ σ ⁿ ) increased with increasing grain size. The fracture criterion applicable fo creep in this alloy at 700° C is related to the ease of linkage and final propagation of cracks of a grain facet in length. A quantitative criterion of failure based on the product of stress and the square root of the average crack length = K _f , the creep-fracture toughness parameter, is reached.