Sainan Yuan

Microstructure optimization of directionally solidified hypereutectic Nb–Si alloy

Abstract Nb–16Si–24Ti–10Cr–2Al–2Hf alloy was directionally solidified with withdrawal rates of 1.2, 6, 18, 36 and 50 mm/min and then heat treated at 1400, 1450 and 1500 °C with withdrawal rate of 50 mm/min for 10 h. The effects of withdrawal rate and heat treatment temperature on the microstructure were studied. The microstructure of directionally solidified alloy was composed of the primary Nb 5 Si 3 , Nbss/Nb 5 Si 3 eutectic cells and Cr 2 Nb, which distribute paralleled to the growth direction. The microstructure becomes more refined with the increasing withdrawal rate, accompany with the evolution of eutectic cells morphology. After heat treatment, Nbss phase connects and forms a continuous matrix, and the Cr 2 Nb phase becomes smaller and distributes more dispersedly. After heat treatment at 1450 °C for 10 h, the alloy achieves balance between the optimization of microstructure and alleviation of solute segregation.

Combined Free Wake / CFD Methodology for Predicting Transonic Rotor Flow in Hover

Abstract A new combined wake / CFD methodology is used to include realistic wake effects in CFD solutions for the rotor flow in hover. The analytical method for predicting the hovering rotor wake is described to investigate the motion of the helical tip vortex. Beginning with the generalized wake model, a Semi-Empirical Correction for the vortex core effect on the rotor wake is made. Then on the condition of circulation and wake geometry convergence, the free wake calculation is carried out. Finally, Euler equations are solved for transonic rotor flow employing Jamesons finite-volume explicit Runge-Kutta time-stepping scheme. The results are compared with the respective references and experimental data.

The Effects of the Withdrawal Rate and Heat Treatment on the Microstructure of Directionally Solidified Nb-14Si-24Ti Alloy

Abstract In this work, an Nb-14Si-24Ti alloy (at.%) was prepared by liquid-metal-cooled directional solidification (DS) and then heat treated (HT) at 1450°C for 100 h. The effects of the withdrawal rate and heat treatment on the microstructure were investigated. The results show that the DS samples consisted of NbSS dendrites, Nb3Si blocks and eutectic NbSS/Nb3Si. As the withdrawal rate increased, the microstructure became finer, the volume fraction of eutectic NbSS/Nb3Si increased and the morphology of eutectic cell transformed from petaloid to regularly arranged granular morphology. The microstructure of the HT samples consisted of NbSS and Nb3Si. The metastable phase of Nb3Si did not discompose through the eutectoid reaction. The dendritic NbSSconnected through diffraction and the secondary dendrite arm got coarser. The boundary of eutectic NbSS/Nb3Si became blurry and the black Ti-rich phase of the DS samples in this region disappeared. The Nb3Si connected to form the continuous matrix. The granular NbSS of eutectics spheroidized and the lamellar NbSS of eutectic interconnected and became more bending distributed uniformly in the Nb3Si matrix.

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.