Microstructure evolution of eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf alloy processed by directional solidification

In this work, the near-eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf (at%) alloy was directionally solidified at 1900 °C with withdrawal rates of 6, 18, 36, 50 mm·min⁻¹ and then heat-treated at 1450 °C for 12 h. The microstructure evolution was investigated. The results show that the microstructure of the directionally solidified (DS) alloy is composed of Nb_ss + Nb₅Si₃ eutectics within the whole withdrawal rate range, while the variation of rates makes a great difference on the solidification routes, the morphology and size of Nb_ss + Nb₅Si₃ eutectic cells. With the increase in withdrawal rates, the petaloid Nb_ss + Nb₅Si₃ eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nb_ss is formed gradually which isolates the Nb₅Si₃, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of Nb₅Si₃ exist in the heat-treated (HT) samples identified by crystal form and element composition, which are supposed as α-Nb₅Si₃ and γ-Nb₅Si₃, respectively. This study exhibits significant merits in guiding the optimization of Nb–Si-based alloys’ mechanical properties.