Effect of solidification parameters on the segregation and γ/γ' eutectic evolution in Ni-based single crystal superalloy DD6

The microstructure, the segregation and the evolution of γ/γ' eutectic during the solidification of Ni-base superalloy DD6 prepared by dual heating zone melting and liquid-metal cooling (LMC) direction solidification technique were investigated over a range of cooling rates. The microsegregation behavior during solidification was also quantitatively examined by EPMA to clarify the influence of elemental segregation on the evolution of γ/γ' eutectic. The effect of dendrite arm spacing was also taken into consideration. The results demonstrate that the γ/γ' eutectic fraction first increases and then decreases with the increasing of cooling rate. The back-diffusion in solid phase would reduce the microsegregation during dendrite growth. Back-diffusion occurs during the solidification of Ni-based single crystal superalloy DD6, but its influence on decreasing of the volume fraction of γ/γ' eutectic would be negligible depending on the diffusivity of each solute element when the cooling rate is up to a certain limit. As the cooling rate increases, the dilution effect of microsegregation owing to dendrite arm spacing decrease plays a leading role during solidification.