Microstructure evolution of a hypereutectic Nb-Ti-Si-Cr-Al-Hf alloy processed by directional solidification

In this work, the Nb-14Si-24Ti-10Cr-2Al-2Hf-0.1Y alloy (at.%) was processed by the liquid-metal-cooled directional solidification (DS) at 1750°C with withdrawal rates of 1.2, 6, 18 mm/min and post heat treatment (HT) at 1450°C for 10 h. The microstructures of the directionally solidified and heat treated samples were investigated. The results show that the microstructure of directionally solidified alloy mainly consists of petaloid Nb_ss + Nb₅Si₃ eutectics and Ti-rich Nb_ss + Nb ₅Si₃ + Cr₂Nb eutectics. With the increase of withdrawal rate, the primary Nb₅Si₃ is eliminated, Nb _ss + Nb₅Si₃ eutectic cells turn round and connected with the microstructure refinement and Nb_ss + Nb ₅Si₃ + Cr₂Nb eutectics turn to a river-like morphology. After heat treatment, Nb_ss + Nb₅Si₃ + Cr₂Nb eutectics disappeared and petaloid Nb_ss + Nb ₅Si₃ eutectics turn to a specific fiber-mesh structure gradually, which is promoted by higher withdrawal rates. Furthermore, both the volume fraction of Cr₂Nb and the content of Cr in Nb_ss of Nb_ss + Nb₅Si₃ eutectics change regularly with the increase of withdrawal rate and heat treatment at 1450°C for 10 h.