Linfen Su

Microstructure evolution of directionally solidified Nb–Si alloy

Abstract By taking the method of liquid–metal cooled directional solidification, alloys with a nominal composition of Nb–14Si–24Ti–10Cr–2Al–2Hf (at-%) were prepared under different conditions. Alloys were initially directional solidified with different withdrawal rates (R = 1·2, 6, 18 mm min−1) at 1750°C and subsequently heat treated at 1450°C for 10 h. These processes aimed to investigate the microstructure of directionally solidified (DS) and heat treated (HT) alloys by XRD, SEM, and EDS. The microstructure of DS alloy was composed of (Nb,Ti)SS, (Nb,Ti)5Si3, and Laves phase Cr2Nb, and the former two components formed (Nb,Ti)SS+(Nb,Ti)5Si3 eutectics. In addition, (Nb,Ti)5Si3 laths only presented in DS1·2 alloy. With the increasing withdrawal rates, the microstructure of alloy altered from hypereutectic into pseudo-eutectic, accompanied with the eutectic morphology transformation from petaloid into coupled. Also, the dimension of constituent phases reduced. However, after heat treatment, the constituent phases did not change. The petaloid morphology of eutectics in DS specimens disappeared and coupled eutectic transferred into network. The block or needle-like Cr2Nb gathered along the boundary between (Nb,Ti)5Si3 and (Nb,Ti)SS, and the overall alloy composition became homogenisation.

Room temperature mechanical strength and fracture behaviour of Nb–Ti–Si–Cr based alloy

Abstract The effect of heat treatment on the mechanical strength of a three-phase NbSS/Nb5Si3/Cr2Nb alloy with composition Nb–22Ti–12Si–14Cr–2Al–2Hf was investigated. This included the evaluation of microstructure, room temperature mechanical strength and the fracture behaviours. The microstructures of heat treated samples were composed of NbSS, Nb5Si3 and Cr2Nb, and the phase size changed under different heat treatment schemes. The results showed that the microstructure composed of large NbSS and fine Nb5Si3 exhibited best mechanical strength due to the deformation of NbSS in these samples. Moreover, fractographic analysis indicated that the Nb5Si3 and Cr2Nb fractured in cleavage manner in all heat treated samples.

Microstructure Evolution in Nb-12Si-22Ti-14Cr-2Al2Hf Alloy Fabricated by Directional Solidification

Abstract In this work, the microstructure evolution of the Nb-12Si-22Ti-14Cr-2Al-2Hf alloy (at.%) fabricated by liquidmetal-cooled directional solidification (LMC) was investigated and the solidification behaviors were discussed. The results revealed that the growth rate was of crucial in controlling the microstructure of this alloy and the higher growth rate was beneficial to the formation of metastable phase Nb3Si. The microstructure of non-melted region consisted of NbSS dendrites, NbSS/Nb5Si3 eutectics and Ti-rich NbSS/Cr2Nb eutectics while the microstructure of quasi-steady-state growth region of this alloy consisted of NbSS dendrites, Nb3Si laths and Ti-rich NbSS/Cr2Nb eutectic. Nb3Si had the advantage of nucleation at medium/high growth rate. Therefore, the microstructure evolution occurred in this study was owing to the increase of growth rate.