The convergence-fault mechanism for low-angle boundary formation in single-crystal castings

A set of nickel-based superalloy single-crystal investment castings was evaluated for crystal perfection. Defect structures near the seed emergence point and other geometric features were examined using X-ray topography. Topographic images were compared with metallographically observed dendritic structures to establish a convergence-fault criterion for the location of low-angle boundaries. To support the phenomenological description of low-angle boundary formation through convergence-faulting, a tip-growth model for simulation of the growth of the dendritic network was developed. This model predicts the local primary growth direction from among three possible [001] directions and identifies regions of convergent growth. A connectivity parameter is defined and used to map the locations where convergence-fault formation is most likely. Predictions are compared with experimental observation.