Crystallography and morphology of as-grown and coarsened Al-Al₃Ni directionally solidified eutectic

Abstract

The structure of the rodlike Al-Al3Ni eutectic directionally solidified vertically at rates ranging from 0.8 to 36.5 cm/h was studied in as-grown and coarsened conditions. The observed crystallographic relations are growth direction ∥ [010]_A13_Ni ∥ [011]_A1or [21l]_A1 and (111)_A1 ∥ (102)_A13_Ni in the zone of the growth direction. The facet structure of as-grown rods is most pronounced at low growth rates. Elevated temperature exposure of the composite produces fiber coarsening and improved definition of the facets, both effects becoming more rapid with increasing growth rate. The facets themselves are, in many cases, of such crystallographic orientations that nearly equal atomic densities occur on both sides of the boundary. We conclude that, although faceting occurs to a marked degree, the related shape anisotropy is probably of second-order importance in calculations of coarsening rates. However, the presence of facets does preclude fiber foreshortening during coarsening.