Microstructure evolution upon directional solidification process of Nb-Si based ultrahigh temperature alloy

The microstructure evolution was investigated in the directionally solidified (DS) Nb-22Ti-14.8Si-3Cr (at%) alloy. The directional solidification zone of the DS bar prepared by the crucible integral directional solidification can be divided into initial transient zone, steady state solidification zone and final transient zone from re-melted interface to the top of the DS bar, according to the microstructure characteristics including phase constituent, crystallographic orientation, microstructure morphology, quantitative metallography and composition with distance away from the bottom of the DS bar. In the initial transient zone, the increase of growth rate leads to the decrease of the average size of eutectic cells, average interlamellar spacing and average size of primary γ-Nb₅Si₃. The decrease of the area fraction of primary γ-Nb₅Si₃ in the transverse section is attributed to preferential growth among the primary γ-Nb₅Si₃ blocks. In the steady state solidification zone, the microstructures are well aligned and the halo Nb_SS (Nb solid solution) mainly exists. In the final transient zone, the area fraction and size of primary γ-Nb₅Si₃ increase as the hypereutectic degree increases with the accumulation of Ti, Cr, and Si contents. The contents of Ti and Cr in the DS bar increase in the initial transient zone, remain at a stable level in the steady state solidification zone, and then increase in the final transient zone with the distance away from the bottom of the DS bar.