COMPETITIVE GROWTH IN BI-CRYSTAL OF NI-BASED SUPERALLOYS DURING DIRECTIONAL SOLIDIFICATION

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.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 had 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 showed that the whole sample with bi-crystal microstructure could be obtained when the two seeds were with the same primary dendrites orientation paralleled to the heat flow direction but different orientations of the secondary dendrites.And the withdrawal velocity had little influence on the competitive grain growth in this disposition relationship.However,only the favorably oriented grain could be obtained when the two seeds were with the non-coplanar primary dendrites and a group of secondary dendrites with the same orientation.The favorably oriented grain overgrew the misaligned grain in a heliciform way.And the overgrowth velocity was in direct proportion to the withdrawal velocity.