Interaction of Microsegregation and Coarsening During Dendritic Nickel-Base Alloy Monocrystal Growth

摘要

Dendritic monocrystals of various Ni-Al-Ta and Ni-Al-Cr alloys, some of which were microalloyed with C,B,Zr or Hf were processed at various growth rates up to 200 cm/h, or cooling rates up to 4.44 K/s under a thermal gradient in the mushy zone of 80 K/cm and in some cases 180 K/cm. The solidification microstructure was substantially refined with increasing growth rate and dendrite arm spacings as fine as 1 to 2 micrometer were observed in laser or electron beam surface glazed monocrystals, corresponding to local cooling rates in excess of 100,000 K/s. Microsegregation of Al, Ta and Cr was found to depend on growth conditions. With increasing cooling rate, within the range of those encountered in monocrystal processing the volume fraction of nonequilibrium interdendritic upsilo minus upsilo prime eutectic or secondary upsilo prime increases. At the high growth rates achieved in surfae glazing this volume fraction is greatly reduced. Very high growth rates are required for complete elimination of interdendritic upsilo prime through solute trapping, and formation of a completely homogeneous surface layer. Such high rates could be reached during glazing. With the present chilling techniques available growth of homogeneous monocrystals at such high rates, which would enhance productivity, is not possible.