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Nb–Si-based alloys show great potential to surpass the widely used Ni-based superalloys. The element Sr is widely applied in aluminum and magnesium alloys, but reports about the effects of Sr on Nb–Si-based alloys are quite rare. So, Nb–Si-based alloys with nominal compositions of Nb–15Si–24Ti–4Cr–2Al–2Hf–0/0.05/0.15Sr (at%) were prepared by directional solidification and heat treatment. The microstructural characterization and room temperature fracture toughness of Nb–Si-based alloy were systematically investigated. Results show that all these alloys consist of Nb5Si3 phase embedded within Nb solid solution (Nbss) matrix. The Nb5Si3 phase becomes refined and more discontinuous after adding minor Sr. As for the fracture toughness, 0.05 at% and 0.15 at% Sr additions do not cause significant change. The discontinuous and refining mechanism of Sr element was studied, and the analysis of toughness decreasing with Sr addition reveals that the size of Nbss phase plays a crucial role in determining the fracture toughness. [ABSTRACT FROM AUTHOR]

High-temperature oxidation behavior of directionally solidified (DS) Nb–Si-based alloys with Re additions was investigated at 1200 and 1250 °C, respectively. Microstructures and high-temperature oxidation behavior of the alloys were characterized. Results show that the microstructures in vertical section of Nb–24Ti–15Si–4Cr–2Al–2Hf–xRe (x = 0, 1, 3; at%) alloys grow parallel to the withdrawal direction and the cross section exhibits bud-like structures. The bud-like structures become finer with more Re additions. The weight gain of the Nb–24Ti–15Si–4Cr–2Al–2Hf–3Re alloy after oxidation at 1200 °C for 100 h is 198.1 mg·cm−2, and it is a bit higher at 1250 °C. The other two alloys perform somewhat worse. The influence of Re addition on the oxidation resistance at 1250 °C is more significant than that at 1200 °C. Although Re addition does not benefit obviously the high-temperature oxidation resistance of the DS samples, it does not compromise the oxidation resistance with a certain amount of Re additions in contrast with the alloy without Re addition. [ABSTRACT FROM AUTHOR]

The microstructure and oxidation behavior of directionally solidified (DS) Nb–15Si–24Ti–4Cr–2Al–2Hf alloys with separate vanadium, tantalum, tungsten and zirconium additions were investigated by X-ray diffraction (XRD), electron probe microanalyzer (EPMA) equipped with wave-dispersive spectroscopy (WDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with an energy-dispersive spectroscopy (EDS). Results show that the five alloys are all composed of primary (Nb,Ti) solid solution ((Nb,Ti)SS) phase and eutectic (Nb,Ti)SS/(Nb,Ti)5Si3 structure. After oxidation at 1250 °C for 100 h in air, the surfaces of the five alloys are covered by the oxides of Nb2O5, TiNb2O7, Ti2Nb10O29, TiO2 and amorphous SiO2. It is found that the alloying elements of V and W are detrimental for oxidation resistance and the addition of Ta has no obvious effect. Zr addition obviously benefits the oxidation resistance at high temperature, by decreasing the weight gain from 242.75 to 184.83 mg·cm−2. The oxidation mechanism of Nb–Si-based alloys and the effects of different alloying elements on the oxidation resistance of Nb–Si-based alloys were discussed. [ABSTRACT FROM AUTHOR]

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 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 + NbSi 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 + NbSi eutectic cells. With the increase in withdrawal rates, the petaloid Nb + NbSi eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nb is formed gradually which isolates the NbSi, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of NbSi exist in the heat-treated (HT) samples identified by crystal form and element composition, which are supposed as α-NbSi and γ-NbSi, respectively. This study exhibits significant merits in guiding the optimization of Nb-Si-based alloys' mechanical properties. [ABSTRACT FROM AUTHOR]

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere. Microstructural characteristics and oxidation resistance of the alloys at 1250 °C were investigated. The results show that, when the Cr content is 4 at%, the microstructures consist of (Nb,Ti) and NbSi; as Cr content increases to 8 at%, C14 Laves phase CrNb is formed. The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features. The weight gains of the two alloys after oxidation at 1250 °C for 100 h are 235.61 and 198.50 mg·cm, respectively. During oxidation, SiO, TiO, NbO and CrNbO are formed at first. Then, TiNbO is formed after oxidation for 20 min and begins to change into TiNbO as the oxidation proceeds. SiO is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale. After oxidation for 100 h, oxidation products consist of SiO, TiNbO, NbO and CrNbO. [ABSTRACT FROM AUTHOR]

Based on annual runoff data collected from several hydrological stations in the Nen River Basin from 1956 to 2004, the cumulative filter method, Mann-Kendall method and Morlet wavelet analysis were used to analyze variations in the characteristics and factors influencing runoff. Specifically, the general characteristics list as: The distribution of runoff was found to be uneven within a year, and the annual variation showed an overall decreasing trend. The abrupt change points of runoff were found to be in the early 1960s, middle 1980s and late 1990s. Multiple time scales analysis revealed three time-scale cycles, a long-term cycle of about 20-35 years with a scale center of 25 years, another cycle of about 8-15 years with a scale center of 11 years and a short-term cycle of about 5 years. Based on the Morlet wavelet transform coefficients figure of the 25-year time scale, it is preliminarily estimated that the Nen River Basin will enter a high flow period in 2013. The results obtained using various methods were consistent with each other. The physical causes of the results were also analyzed to confirm their accuracy. [ABSTRACT FROM AUTHOR]