ELEMENTAL PARTITIONING IN DIRECTIONALLY SOLIDIFIED NIAL SINGLE-CRYSTALS

Elemental partitioning effects caused by crystal growth processes in b inary Ni-Al alloys are revealed. The directional solidification proces s is analysed in terms of a solidification model of binary Ni-Al alloy s 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 microhardnes s determinations. Deviations of the melt composition from stoichiometr y of the NiAl intermetallic compound give rise to continually increasi ng stoichiometry deviations (excess of Al or Ni) along the rod axis an d also to radial segregation effects resulting in considerable microha rdness fluctuations within the single crystal. These effects, which ar e particularly important for large deviations of the initial melt comp osition from stoichiometry, are ascribed to solute accumulation in the residual melt (macrosegregation in growth direction) and melt convect ion (radial inhomogeneities).