Microscale stray grains formation in single-crystal turbine blades of Ni-based superalloys

Abstract

Microscale stray grains (MSGs), which are composed of misoriented and fragmented high-ordered dendrite arms, small-sized equiaxed grains, and columnar grains, are a novel grain defect that occurs within the shrouds of single-crystal (SC) blades made from Ni-based superalloy. State-of-the-art non-destructive testing methods are incapable of detecting this defect, resulting in a high-risk application of SC blades. This study aims to control this defect by systematically investigating directional solidification processing parameters, shroud dimensions, and rhenium (Re) content in the formation of MSG defects. The results reveal that the defect forms exclusively at the overhanging extremities of the upper shrouds on the back side of the blades that face the heater. Increasing the withdrawal rate reduces the occurrence of MSG defects. As the shroud dimensions increase, MSG defects appear in the transition area between the downward suspended extremity of the blade shroud and the side of the blade body, as well as near the corner of the downward suspended extremity of the shroud. The occurrence of MSG defects increases with increasing shroud dimensions. Varying Re content sharply decreases the formation of MSG defects. A concentration-attached Rayleigh-Taylor instability (RTI) superheating (CARTISH) model, considering solidification shrinkage, was proposed to comprehend the formation of MSG defects. Simulation results based on this model are consistent with the experimentally observed distribution and degree of MSG defects under different conditions. Effective control of the CARTISH is critical to managing MSG defects.