Orientation anisotropy and strain localization during elevated temperature tensile deformation of single crystal and bi-crystal Ni-based GTD444 superalloy

The aspects of grain boundary character as well as orientation of constituent grains are important for high temperature applications of directionally solidified (DS) blades as they are expected to influence the overall mechanical behavior; however, there exist limited studies at critical test temperature. Single crystals (SX) with orientations [001] and [101] and their equivalent bi-crystals (BX) are used to understand the effects of crystal orientation and grain boundary misorientation on the deformation of Ni-based (GTD444) superalloy at 650 degrees C. The high temperature tensile tests are coupled with digital image correlation (DIC) to study plastic heterogeneity as a function of crystal orientations. Among the two SX samples, the [001] orientation had slightly higher yield stress than [101] orientation. On the other hand, the ductility of [101] orientation is 1.5 times of [001] orien-tation. DIC strain field maps and orientation imaging microscopy (OIM) showed extensive flow localization in one of the grains in BX samples. However, flow localization appeared to be insensitive to grain boundary misorientation. The measured lattice rotation by OIM is shown to be correlated with localized strain for all the samples. Selected transmission electron microscopy and detailed fractography analyses of the deformed samples were performed to understand the deformation mechanisms active in each of the orientations. It is found that there is greater strain localization within & gamma; channels in [001] which leads to higher hardening. In contrast, shearing of & gamma;' precipitates followed by increased dislocation activity in conjunction with higher lattice rotation lead to higher ductility in [101] orientation.