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 Nbss+ Nb5Si3eutectics and Ti-rich Nbss+ Nb5Si3+ Cr2Nb eutectics. With the increase of withdrawal rate, the primary Nb5Si3is eliminated, Nbss+ Nb5Si3eutectic cells turn round and connected with the microstructure refinement and Nbss+ Nb5Si3+ Cr2Nb eutectics turn to a river-like morphology. After heat treatment,Nbss+ Nb5Si3+ Cr2Nb eutectics disappeared and petaloid Nbss+ Nb5Si3eutectics 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 Nbssof Nbss+ Nb5Si3eutectics change regularly with the increase of withdrawal rate and heat treatment at 1450 C for 10 h.

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℃ 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+Nb₅Si₃+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+Nb₅Si₃+Cr₂Nb eutectics turn to a river-like morphology. After heat treatment, Nb_ss+Nb₅Si₃+Cr₂Nb eutectics disappeared and petaloid Nb_ss+Nb₅Si₃ 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℃ for 10 h.