Formation of Accumulated Misorientation During Directional Solidification of Ni-Based Single-Crystal Superalloys
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ingle-crystal superalloys have become the best option for modern turbine blades because of the elimination of grain boundary and imposition of the elastically soft h001i orientation. In fact, the formation of misorientation during directional solidification makes it difficult to obtain perfect single crystal with exactly the same orientation everywhere.[1–8] Bellet et al. observed the mosaic spread of superalloys.[9] Newell et al. examined the effects of dendrite branching and growth kinetics on the formation of misorientation, and proposed that it originated from plastic deformation of the dendrite stems within the mushy zone.[10] Using in situ synchrotron X-ray radiography, dendrite deformation in Ni-based superalloys was observed and suggested that the dendrite deformation was significant at converging
SONGSONG HU, LIN LIU, WENCHAO YANG, DEJIAN SUN, MIAO HUO, TAIWEN HUANG, JUN ZHANG, HAIJUN SU, and HENGZHI FU are with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China. Contact e-mails: [email protected]; [email protected] Manuscript submitted October 1, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
side with respect to the mold wall.[11,12] However, the dendrite deformation process was unclear and little attention was given to the impact of withdrawal rate on the formation of misorientation.[13] Therefore, Ni-based single-crystal superalloys in the h001i direction that deviated from cylinder axis by 20 deg were prepared using a seeding method at different withdrawal rates in order to characterize the accumulated misorientation and analyze the influence of withdrawal rates. Furthermore, the mechanism of misorientation formation was discussed. A third-generation Ni-based single-crystal superalloy was used, with nominal composition (wt pct) of Ni-2.5Cr-9Co-1.5Mo-7W-6Al-8Ta-0.1Hf-4Re-0.2Ti and < 0.02 C. A modified Bridgman directional casting furnace with liquid metal cooling (LMC) was used in this work, and the thermal gradient was approximately 170 C/cm. The cylinder-shaped seeds (F4 mm 9 20 mm) in the h001i direction that deviated from cylinder axis by 20 deg were determined by an X-ray diffractometer, and then the projection of the plane consisting of the cylinder axis and the h001i direction on the end face was marked. A pure alumina tube was mounted on a steel chilling plate, which was used as a mold for preparing single-crystal samples. The alumina tube has diameter of 4/6 mm (inside/outside diameter) and a length of 150 mm. The seed and master alloy (machined into a cylinder with diameter of 4 mm and length of 120 mm) were placed inside the alumina tube and the position of the alumina tube was adjusted to ensure complete melting of the master alloy and the top portion of the seed during the heating stage. The tube was heated to 1650 C and held for 30 minutes, and then withdrawn at a predetermined rate. The direction of solidification was parallel to the cylinder axis of the samples. Following directional solidification, the casting was macro-etc
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