Microstructural transition of Nb-Si based alloy during directional solidification upon abruptly decreasing withdrawal rate

Multi-element Nb-Si based alloys are directionally solidified upon abruptly decreasing withdrawal rate from 100 to 10 μm/s and at the constant withdrawal rate of 10 μm/s respectively. With increasing solidification distance after decreasing withdrawal rate, the volume fraction of primary γ-(Nb, X)₅Si₃ increase from 6.7% to 13.9% firstly and finally decreases to 10.5%, which is similar with the variation in Si concentration of the alloy. On the other hand, the volume fraction of γ-(Nb, X)₅Si₃ in eutectic cells decreases from 36.7% to 28.5% gradually after abruptly deceasing withdrawal rate. After abruptly decreasing withdrawal rate, the average size of primary γ-(Nb, X)₅Si₃ increases from 31.1 to 58.0 μm, and the average size of eutectic cells increases from 42.5 μm to 128.2 μm, and the average interphase spacing increases from 1.9 μm to 4.4 μm. The phase proportion, microstructural sizes, hardness and elastic modulus of constituent phases in Nb-Si based alloys directionally solidified upon abruptly decreasing withdrawal rate are similar with that of the alloy at a constant withdrawal rate of 10 μm/s. The average hardness and elastic modulus for Nb_SS are about 6 and 150 GPa respectively, for γ-(Nb, X)₅Si₃ are 19 and 280 GPa respectively. However, the results of XRD and EBSD show that the coupling degree of eutectics obtained by abruptly decreasing withdrawal rate is better than that at the constant withdrawal rate of 10 μm/s, which indicates that the method of abruptly decreasing withdrawal rate can modify the microstructure.