Variation of fiber morphology and crystallographic orientation relationship in directionally solidified NiAlMo alloys

Abstract

In directional solidification of NiAlMo alloys solidifying along the monovariant eutectic trough L→γ + α, the morphology of α-Mo fibers and their crystallographic orientation relationship with the γ-NiAlCr matrix depend on growth rate and misorientation between the crystallographic [001] growth axis of the matrix and the local heat flow direction. At high growth rates and misorientations of more than 6 degrees, fibers can adopt a ribbon like shape but fibers and ribbons generally remain faceted, preserving the standard orientation relationship with the matrix. At low growth rates (< 2 cm/ h) and misorientations by more than about 10 degrees fibers become non-faceted and follow the Pitsch orientation relationship. Spacing of faceted fibers increases with increasing misorientation. If it is assumed that interface undercooling in eutectic growth is associated with maximum interface curvature and that eutectic growth occurs at minimum interface undercooling, spacing and morphological variations in faceted growth can be explained in terms of misorientation and interface energy. Simple structural criteria do not allow to identify the boundaries between matrix and fibers as well developed low-energy interfaces. There is evidence, however, that eutectic α fibers crystallize at the limit between faceted and non-faceted growth, and that fiber morphology is controlled by nucleation of new growth layers at the triple points between the γ-, α- and liquid phases. Solid state transformations change the volume fraction of fibers without much influencing fiber morphology. Concerning the phase diagram reported by Wakashima et al. [ Acta metall. 31, 1937 (1983)], displacement of phase boundaries in the vicinity of the ternary γ + γ' + α eutectic is proposed.