The microstructures of Ni-31Al-32Cr-6Mo-xGd hypereutectic alloy were investigated at the withdrawal rates of 10 μm/s, 30 μm/s, and 90 μm/s. For the Gd-free hypereutectic alloy, the Cr(Mo) primary dendrites appear at the beginning of solidification when the withdrawal rate is 10 μm/s. As the solidification proceeds, the Cr(Mo) primary dendrite is eliminated, and the fully eutectic structure can be obtained in the steady-state zone. With increasing the withdrawal rate, the Cr(Mo) primary dendrites decrease gradually, and vanish at 90 μm/s. In addition, at a moderate withdrawal rate (30 μm/s), an optimum addition of Gd content (0.1 wt.%) results in the refinement of the microstructure, including the refinement of the eutectic cells and the intercellular region. Meanwhile, the new white phase ((AlxGd1−x)2O3) appears in the boundary of the eutectic cells when the Gd content is not less than 0.1 wt.%.
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The work is supported by the National Natural Science Foundation of China (No. 51501147), the doctoral starting fund of Xi’an University of Technology (101-451115004), the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP201509) and National Nature Science Foundation of China (No. 51074128).