Effects of low angle boundaries on the mechanical properties of single crystal superalloy DD6

Abstract

During the solidification/growth of single crystal blades and vanes, defects in the form of both high angle boundaries and low angle boundaries occasionally form. High angle boundaries (HAB) often result in rejection of a casting. Low angle boundaries (LAB) are tolerated with in production casting to a certain extent. LABs within a single crystal affect the mechanical response of the material and thus the quality of the blade or vane. This work assesses the effect of LABs on the mechanical properties of the second generation single crystal superalloy DD6. Bicrystal slabs with low angle tilt boundaries were cast using seed techniques. Specimens for tensile and creep testing were machined from these slabs with the LAB misorientations varying from 0° to 12°. The material characterization was carried out by optical microscopy (OM) and scanning electron microscopy (SEM). It is notable that the effects of LAB on the ultimate tensile strength are weaker than those on the tensile elongation. The results show that when the misorientation angle of the LAB is less than 9°, the LAB has a relatively minor affect on the ultimate tensile strength at a given temperature. The elongation of the alloy however shows a significant decrease at a given temperature with increasing LAB orientation angle. The creep rupture properties of die alloy with LAB were performed at 800°C, 850°C, 900°C and 980°C In general under a given set of test conditions, the creep rupture life decreases with increasing LAB misorientation. Features characteristic of both quasi-cleaveage and intergranular mechanisms were observed on the fracture surfaces of the specimens with LABs. Both the [001] fracture surfaces, as well as the fracture surfaces on the specimens with LABs had rectangular dimples. It is hypothesized that the rectangular dimples were created as fracture occurs on {111} planes. The specimens with LABs appear to fail initially in an intergranular manner, followed by mode that creates quasi-cleavage surface features.