Grain structure development in directional solidification of nickel-base superalloys

The microstructures of three single crystal superalloys (CMSX4, CM186LC and CMSX10) that exhibit quite different casting properties have been characterised quantitatively following quenched directional solidification to identify variations in the grain selection mechanisms. No significant differences in the dendrite morphologies of the alloys were observed. Detailed observation of dendrites in directionally solidified bicrystals of CMSX4 with well-controlled misorientations showed significant modification of the growth of secondary dendrites at the developing seeded grain boundary. The generally accepted criterion of overgrowth by secondary dendrites cannot account for the present results. It is proposed that the diffuse textures that are frequently produced in second and third generation superalloys are a consequence of the inhibition of secondary dendrite development due to competing solute diffusion fields.