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

Abstract

Fully reversed low cyclic fatigue (LCF) tests were conducted on [0 0 1], [0 1 2], [1 1 2], [0 1 1] and [1 1 4] oriented single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950°C. The cyclic strain rates were chosen as 1.0×10⁻², 1.33×10⁻³ and 0.33×10⁻³s⁻¹. The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [0 0 1] orientation specimens, it is found from the scanning electron microscopy (SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950°C, and an orientation and strain rate modified Lall-Chin-Pope (LCP) model was derived for the nonconformity. The influence of crystallographic 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.