Microstructural characteristics of integrally directionally solidified Nb-Ti-Si base ultrahigh temperature alloy with crucibles

The integrally directional solidification of an advanced ultrahigh temperature Nb-Ti-Si base alloy was carried out with the use of ceramic crucibles at melt temperature of 2000°C. The directionally solidified microstructure is revealed. It has been found that the directionally solidified microstructure is composed of couple grown lamellar ((Nb, Ti)_ss + (Nb, X)₅Si₃) eutectic colonies and a few hexagonally cross-sectioned (Nb, X)₅Si₃ columns (where X represents Ti and Hf elements). The directionally solidified microstructure is straightly aligned along the longitudinal axis of the specimens, and is evenly distributed, which showed much superior directionality over those prepared by the floating zone melting directional solidification or Czochralski methods. Compared to both the EBFZM (electron beam floating zone melting) DS process and Czochralski DS process in a water cooled copper crucible, the axial thermal gradient is relatively higher, and the convection in the melt just ahead of the solid/liquid interface and 'Kelvin effect' can be alleviated during the integrally directional solidification process, thus the better directionally solidified microstructure was obtained.