Effect of Ti addition on microstructure and crystalline orientations of directionally solidified Nb–Si based alloys

Multi-element Nb–Si based alloys with nominal compositions of Nb-xTi-15Si–5Cr-1.5Hf-1.5Zr (x = 0, 10, 20 and 25 at. %) were simultaneously directionally solidified at 2050 °C. When the Ti addition contents are 0 10 at. %, the microstructure is composed of primary α-(Nb, X)₅Si₃ and Nb_SS/α-(Nb, X)₅Si₃ eutectic. When the Ti addition content reaches to 20 at. %, primary γ-(Nb, X)₅Si₃ appears and Nb_SS/γ-(Nb, X)₅Si₃ eutectic is formed at Nb_SS/α-(Nb, X)₅Si₃ eutectic cellular boundary or around primary γ-(Nb, X)₅Si₃. When the Ti addition increases to 25 at. %, α-(Nb, X)₅Si₃ is completely substituted by γ-(Nb, X)₅Si₃. Besides, the increasing Ti addition content increases the area fraction of primary (Nb, X)₅Si₃ and promotes the refinement of Nb_SS/(Nb, X)₅Si₃ eutectics. The crystalline orientation relationship between Nb_SS and α-(Nb, X)₅Si₃ is <001>_Nb∥[001]_α and {110}_Nb∥{310}_α when the Ti addition contents are ranged from 0 to 20 at. %. When the Ti addition content is 20 at. %, there exists no crystalline orientation relationship between Nb_SS and γ-(Nb, X)₅Si₃. When the Ti addition content reaches to 25 at. %, the crystalline orientation relationship between Nb_SS and γ-(Nb, X)₅Si₃ is <111>_Nb//<0001>_γ and {110}_Nb//{101‾0}_γ. With increasing Ti addition, the orientation deviation degree between Nb_SS and (Nb, X)₅Si₃ decreases, which indicates that the coupling degree of the eutectic increases at higher Ti addition contents.