Manager, General Electric Company, Cincinnati, OH
Pages: 18 Published: January 1988
This research was conducted to identify, understand, and model the interactions of oxidation and low cycle fatigue (LCF) in a single crystal nickel-base superalloy, René N4. The LCF testing was conducted at 1093°C (2000°F) in three test environments: purified helium (<1 ppm oxygen), laboratory air, and enriched oxygen (80% oxygen at 2 atm pressure). Two cycle frequencies, 20 and ½ cpm, were investigated.
The fatigue life and behavior of René N4 at 1093°C was found to depend on both oxygen concentration and cyclic frequency. These parameters influenced the portions of fatigue life required for crack initiation and crack propagation to different extents, although both portions were decreased under conditions which were environmentally more severe. A relatively small oxygen pressure dependence of fatigue life was seen (Nfα PO2−0.23). The fatigue process in René N4 showed a mixed time and cycle dependency leading to reduced fatigue life for longer cycles. Crack initiation was distinctly more time dependent than crack propagation and varied from ½ to 1/10 or less of the total fatigue life over the conditions investigated.
Crack initiation in René N4 occurred by fatigue assisted cracking and spalling of oxide products to produce a roughened and pitted surface. This process also accelerated the observed oxidation rate. With further cycling, these pits developed into oxide spikes and then into sharp fatigue cracks. A modification of an oxidation-fatigue model by Antolovich was satisfactory in describing this process. The crack propagation process was also influenced by oxidation. However, the precise mechanism could not be clearly defined.
low cycle fatigue, environmental effects, single crystals, high temperature fatigue, oxidation, crack growth