ELEMENTAL PARTITIONING IN DIRECTIONALLY SOLIDIFIED NIAL SINGLE-CRYSTALS

Elemental partitioning effects caused by crystal growth processes in binary Ni-Al alloys are revealed. The directional solidification process is analysed in terms of a solidification model of binary Ni-Al alloys having regard to a composition dependent partition coefficient. The predictions are compared with electron probe microanalysis results of elemental distributions within the crystal and related to microhardness determinations. Deviations of the melt composition from stoichiometry of the NiAl intermetallic compound give rise to continually increasing stoichiometry deviations (excess of Al or Ni) along the rod axis and also to radial segregation effects resulting in considerable microhardness fluctuations within the single crystal. These effects, which are particularly important for large deviations of the initial melt composition from stoichiometry, are ascribed to solute accumulation in the residual melt (macrosegregation in growth direction) and melt convection (radial inhomogeneities).