Subgrain boundaries in single crystal blade airfoil of aircraft engine

The single crystal turbine blades of aircraft engine were obtained by directional solidification of Ni-based superalloy via Bridgman technique at withdrawal rate of 5 mm/min. The CMSX-4 industrial alloy were used for crystallization. After crystallization the blades were subjected to heat treatment. The Laue diffraction, X-ray diffraction topography and SEM methods were used to investigate the subgrain structure of blade airfoils. It was stated that subgrain boundaries with the misorientation angle ranging from over a dozen to a few dozen angular minutes occurred in the airfoil. The number of low-angle boundaries in the cross-section near the tip of the blade airfoil is higher than that in the cross-sections located near the root area. It is caused by formation during crystallization of additional low-angle boundaries in thin-walled airfoil area and their propagation toward the tip of airfoil. The number of low-angle boundaries in cross-section located near the tip of airfoil can be determined by three mechanisms: dendrites deflection on surface of mould; creation of internal stress regions in the thin-walled area of blade airfoil and propagation of two additional boundaries from root to the airfoil.