Crystallographic orientation and temperature effects on the fatigue crack growth rate and resulting fracture surface morphology in PWA1484 single crystal superalloy

This research looked at the effect of crystallographic orientation and temperature on the fatigue crack growth rate and the resulting fracture surface morphology in PWA1484 single crystal superalloy. Two groups of single edge notch tension specimens, one group with controlled secondary orientations and one group with uncontrolled secondary orientation, were tested at temperatures from 649 degrees C to 982 degrees C at R-ratios of 0.1 and 0.7. It was found that the effect of temperature on the crack growth rate becomes more pronounced as the crack driving force increases while the secondary orientation and R-ratio effects on the crack growth rate increase with increasing temperature. Two types of crack surface morphology were seen during fractography. The first was a precipitate avoidance (gamma' avoidance) morphology that was rolling but still predominantly flat when observed on a larger scale. In <001> primary oriented specimens, this fracture mode tended to follow the precipitate/matrix faces (microscopically cubic) while macroscopically staying essentially normal to the applied loads. The second mode was a form of cleavage (gamma' shearing) and occurred predominantly on octahedral crystallographic planes.