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| COMPETITIVE GROWTH IN BI-CRYSTAL OF NI-BASED SUPERALLOYS DURING DIRECTIONAL SOLIDIFICATION |
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Cite this article:
. COMPETITIVE GROWTH IN BI-CRYSTAL OF NI-BASED SUPERALLOYS DURING DIRECTIONAL SOLIDIFICATION. Acta Metall Sin, 2011, 47(6): 641-648.
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Abstract Single crystal nickel-based superalloys, which offer improved creep-rupture, fatigue, oxidation and coating properties, have been widely applied in turbine blades and vanes. In general, the single crystal structure is produced through competitive grain growth by directional solidification technology. To understand the competitive grain growth mechanism during directional solidification, the bi-crystal experiment has been employed by many researchers, and several models have been proposed. Up to now, the competitive grain growth mechanism is still not clear yet, which is reflected in two areas, as shown in the follows. First, the influence of the growth rate on the competitive grain growth is not clear yet; second, the illustration of competitive grain growth are based on the two-dimensional simplified models. However, in actual directional solidification process, the orientations of the primary and secondary dendrites are random due to the randomness of the grain nucleation. Therefore, the actual competitive grain growth during directional solidification is a three-dimensional process. However the related literatures are fewer. In the present study, an experimental study has been conducted to investigate the three-dimensional competitive grain growth behavior under different withdrawal velocities by the bi-crystal samples with special orientation relationships. The results show that the whole sample with bi-crystal microstructure can be obtained when the two seeds are with the same primary dendrites orientation paralleled to the heat flow direction but different orientations of the secondary dendrites. And the withdrawal velocity has little influence on the competitive grain growth in this disposition relationship. However, only the favorably oriented grain can be obtained when the two seeds are with the non-coplanar primary dendrites and a group of secondary dendrites with the same orientation. The favorably oriented grain overgrows the misaligned grain in a heliciform way. And the overgrowth velocity is in direct proportion to the withdrawal velocity.
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Received: 24 December 2010
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| Fund: ; National Basic Research Program (973 Program) of China |
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