INFLUENCE OF PRESTRAINING ON HIGH-TEMPERATURE, LOW-FREQUENCY FATIGUE CRACK-GROWTH IN SUPERALLOYS

The objective of this paper is to investigate the influence of prestraining on the low frequency crack growth behavior of wrought alloy 718 at 650-degrees-C. A series of crack growth experiments were carried out on specimens with two levels of prestraining, 1% and 2%, in addition to specimens with no prior deformation. Work in these experiments included continuous measurements of crack length and near field crack tip displacements, fractographic analysis of fracture surface facets, qualitative determination of the slip density in the crack tip region and Auger spectroscopy analysis for the objective of determining the thickness of surface oxide layers formed during the fracture process. It is concluded from this work that prestraining leads to a reduction in the crack growth rate. This conclusion has been examined on the basis of the notion that the deformation-associated slip line density is the rate controlling element of the chromium oxide build up taking place at the slip line-grain boundary intersection nodes within the crack tip region.