A crystallographic model for the orientation dependence of low cyclic fatigue property of a nickel-base single crystal superalloy

Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [(1) over bar 1 2], [0 1 1] and [(1) over bar 1 4] oriented single c rystals of nickel-bared superalloy DD3 with different cyclic strain rates a t 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 systems were confirme d to be activated on all the specimens. The experimental result shows that the fatigue behavior depends an the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the sc anning electron microscopy(SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up o f cracks. The width of the fatigue striations depends on the crystallograph ic orientation and varies with the total strain range. A simple linear rela tionship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid l aw of tensile/compressive flaw stress and plastic behavior existed at 95 de grees C, and an orientation and strain rate modified Lall-Chin-Pope ( LCP) model was derived for the nonconformity. The influence of crysrallographic orientation and cyclic strain rate on the LCF behavior can be predicted sat isfactorily by the model. In terms of an orientation and strain rate modifi ed total strain range, a model for fatigue life was proposed and used succe ssfully to correlate the fatigue lives studied.