Influence of wall thickness on microstructure of the directionally solidified turbine vane with high withdrawal rate

In order to investigate the influence of the specimen thickness on the microstructure of directionally solidified (DS) hollow gas turbine vanes prepared with constant representative casting parameters, characteristic morphologies of dendrite and MC carbides in the transverse section were studied systematically. Optical microscope (OM) images show that the prime dendrite arm spacing falling within the range of 360-480 μm decreases with decreasing the wall thickness and secondary dendrite arms are abnormally prosperous in some regions, indicating that the morphology of dendrite and the prime dendrite arm spacing are correlated not only with thermal gradient and withdrawal rate but also with heat conduction condition. Scanning electron microscope (SEM) results exhibit that the morphology of MC carbides transforms gradually from blocky and rode-like appearance to script type and nodular morphology, finally to small blocks with distance away from the center of the vane wall; blocky carbides make up the greatest fraction of MC carbides present in this microstructure and are preferentially located in the interdendritic regions surround by a γ-γ' matrix and some of these carbides uniformly distribute parallel along one direction. It is interesting to note that some small Chinese-script type carbides occasionally locate within secondary dendrite trunks, which is different from current experimental and theoretical results. This may be related to large amounts of metallic element Ta in this material.