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| STUDY ON THE TENDENCY OF STRAY GRAIN FORMATION OF Ni–BASED SINGLE CRYSTAL SUPERALLOYS |
| ZHANG Xiaoli, ZHOU Yizhou, JIN Tao, SUN Xiaofeng |
| Institute of Metal Research, Chinese Academy of Science, Shenyang 110016 |
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Cite this article:
ZHANG Xiaoli ZHOU Yizhou JIN Tao SUN Xiaofeng. STUDY ON THE TENDENCY OF STRAY GRAIN FORMATION OF Ni–BASED SINGLE CRYSTAL SUPERALLOYS. Acta Metall Sin, 2012, 48(10): 1229-1236.
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Abstract Ni–based single crystal (SC) superalloys are preferential materials for manufacturing blades and vanes of gas turbines, due to their superior mechanical properties resulting from the absence of grain boundaries. However, as the component geometry becomes more complex and the content of refractory elements increases gradually, the forming frequency of stray grains increases significantly leading to the component rejection during directional solidification (DS) of Ni–based SC superalloys. This becomes now one of the major problems encountered during DS and single crystal growth. In the interest of saving the actual manufacturing cost, therefore, the alloys with weak tendency of stray grain formation should be first applied. However, there are still no effective method to quantitatively evaluate the stray grain formation ability of a certain Ni–based SC superalloy. Thus, it is quite necessary to design a new method to do so. In this study, the two model samples with different platform geometries are first designed to investigate quantitatively the tendency of stray grain formation, used to summarize the ability of stray grain formation of different alloys and reveal the mechanism of stray grain formation within platforms. The first model sample with the same platform height but length increasing by degrees along the solidification direction, is used to quantitatively characterize the stray grain formation tendency of different superalloys by using its platform length of stray formation occurring first in time. The second model sample with the same platform length but height decreasing by degrees along the solidification direction, is used to quantitatively characterize this tendency by using the platform height of stray formation occurring first in time. The experimental results show that it is easier for stray grains to nucleate and grow within the platform region with either platform length increasing or platform height reducing. The stray grain formation within outside platform is prior to that within inside platform. This tendencies for the first, second and third generation SC superalloys, however, are different: the first is the weakest, following by the second, and the third is the strongest. Furthermore, the formation of stray grains is dominated by undercooling. The melt undercooling at platform edges is larger than within platform insides and increases gradually with either platform length lengthening or platform height reducing. When the undercooling at platform edges exceeds the critical nucleation undercooling, the stray grain would be able to nucleate and overgrow quickly into the platform insides.
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Received: 09 May 2012
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| Fund: Supported by National Natural Science Foundation of China (Nos.U1037601 and 50931004), National Basic Research Program of China (No.2010CB631206) and the Program of "One Hundred Talented People"of the Chinese Academy of Sciences |
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