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The FCG characteristics of SRR99 have been studied in air at 650°C and 850°C at 10 Hz and 1 Hz with an R ratio of 0.1, using CT specimens with the following crack plane2 (hkl) and propagation directions [UVW]: (001)[010], (110)[110), and (lll)[llO]. At 65O='C, the (110) and (111) crack plane specimens showed a frequencydependent FCG rate, not apparent in the (OOl)[OlO] specimens, due to an increased shear stress on the cuboidal cross-slip system giving increased cross-slip at the lower frequency and thereby reducing the reversibility of slip. Homogenization of slip at 85O'C prevented this effect. All specimens showed an improved threshold response at 850°C compared to that at 650°C due to enhanced oxide closure of the crack. Within the Paris regime, the (OOl)[OlO] specimens demonstrated a lower FCG rate at 850°C than at 650°C, due to an increased crack bifurcation at the higher temperature. Conversely, a higher FCG rate with increase in temperature occurs in the (110) and (111) specimens, since the crack path decreases in tortuosity as it changes from propagation along the y/y' interface to y'-precipitate cutting. The orientation-dependence of FCG rate and fracture path is influenced by the relationship between the crack front and the slip systems available for crack tip shear. At 650°C, those orientations most able to accommodate crack tip shear on primary slip planes that are near-parallel with the crack front provide the best FCG resistance, whilst at 850°C the best FCG response is found for the same orientations that have now developed large scale ridges as a result of out-of-plane cuboidal slip activity. Superalloys 1992 Edited by S.D. Antolovich, R.W. Stusrud, RA. MacKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstmm The Minerals, Metals & Materials Society, 1992

The Influence of Orientation, Temperature and Frequencey on Fatigue Crack Growth in a Nickel-Based Superalloy Single Crystal