Convection-induced peritectic macro-segregation proceeding at the directional solidification of Ti–46Al–8Nb intermetallic alloy

Abstract

Authors extended the research on TiAl–Nb solidification/modeling reported in Kartavykh et al. (2010) [1]. Unidirectional counter-gravity (upward) melt solidification of Ti–46Al–8Nb (at.%) intermetallic alloy was performed in the three-zone resistive tube electro-furnace TEM 01-3M with power-down thermal profile operation. The laboratory refinement of this cylindrical ingot growth technique was developed in course of terrestrial preparation experiments in the furnace facility specially designed for a sounding rocket flight. Although Ti–46Al–8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically transformed microstructure was firstly observed in solidified ingots by SEM-BSE and EDX analyses, where Al content locally exceeds 47 at.%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat-mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform as-solidified microstructure of alloy produced confirms the modeling findings. Valuable data are obtained for the optimization of cast processes of TiAl-intermetallics.