Effect of substrate orientations on microstructure evolution and stability for single crystal superalloys in rapid solidification process

摘要

Recast layer (RL), a main by-product in electrical discharge machining (EDM) drilling, has been criticized for high oxidation and poor thermal stability in advanced engine supercharging systems. In this study, it is found that the substrate crystallographic orientation has a significant effect on rapid solidification microstructure, including cellular-dendrite distribution characteristic of stack structure, and the cellular-dendrite microstructure characteristic around holes. By means of EBSD technique, and with the structural feature, it is proved that RL-substrate has imperfect single crystal (SX) structure. The low-angle grain boundaries are distributed at RL-substrate interface and second discharge region interface. The former is only 1-3 degrees while the latter is about 7-10 degrees. Precisely because of the different property characteristics, three types of boundaries almost take on various thermal stability that correspond to different fault mechanisms. Among these, the cellular-dendrite boundary plays an important role in structural damage in primary discharge region. The experiments show that the (100) inner RL has better thermal stability than (110) and (111). The result is in accord with the research purpose approximately that the probabilistic damage of structures could be deduced quantitatively on different substrates.