THE INFLUENCE OF CRYSTALLOGRAPHIC ORIENTATION AND STRAIN-RATE ON THE HIGH-TEMPERATURE LOW-CYCLIC FATIGUE PROPERTY OF A NICKEL-BASE SINGLE-CRYSTAL SUPERALLOY

Fully reversed low-cyclic fatigue (LCF) tests were conducted on [001], [012], [(1) over bar 12], [011], and [(1) over bar 14] oriented singl e crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950 degrees C. The cyclic strain rates were chosen as 1.0 X 10(-2), 1.33 X 10(-3), and 0.33 X 10(-3) s(-1). The octahedral slip sy stems were confirmed to be activated on all the specimens. The experim ental result shows that the fatigue behavior depends on the crystallog raphic orientation and cyclic strain rate. Except [001] orientation sp ecimens, it is found from the scanning electron microscopy (SEM) exami nation that there are typical fatigue striations on the fracture surfa ces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and var ies with the total strain range. A simple linear relationship exists b etween the width and total shear strain range modified by an orientati on and strain rate parameter. The nonconformity to the Schmid law of t ensile/compressive flow stress and plastic behavior existed at 950 deg rees C, and an orientation and strain rate modified Lall-Chin-Pope (LC P) model was derived for the nonconformity. The influence of crystallo graphic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied in this article.