Microstructural characterization of Nb–B–Si alloys with composition in the Nb−Nb 5Si 2B (T 2-phase) vertical section (original) (raw)
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On the T 2-phase formation in mechanically alloyed Nb–Si and Nb–Si–B powders
Journal of Alloys and Compounds, 2007
The present work reports on the T 2 -phase formation in mechanically alloyed Nb-37.5Si, Nb-25Si-12.5B, Nb-12.5Si-25B powders. Results indicated that Si and B atoms were dissolved into the Nb lattice to form supersaturated solid solutions during ball milling. The T 2 -phase was formed in Nb-37.5Si powders after milling for 30 h. A large amount of T 2 -phase was formed in Nb-25Si-12.5B and Nb-12.5Si-25B powders after heat treatment at 1600 • C for 1 h. The following lattice parameters of the T 2 -phase were measured in Nb-37.5Si powders: a = 6.5625 nm and c = 11.9017 nm. In Nb-12.5Si-25B powders, it was noted a reduction of a and c values to 6.2317 and 11.6159 nm, respectively.
Isothermal Section of the Nb-Si-B System at 1700 °C in the Nb-NbSi2-NbB2 Region
Journal of Phase Equilibria and Diffusion, 2011
Me-Si-B (Me-metal) alloy systems have been evaluated to aid the development of ultra high temperature structural materials. In the present work, the phase relations at 1700°C in the Nb-NbSi 2-NbB 2 region of the Nb-Si-B system have been experimentally determined. The alloys were prepared via arc melting and powder metallurgy routes. All samples were characterized via scanning electron microscopy (SEM) and x-ray diffraction (XRD) and selected samples were also scanned via wavelength dispersive spectroscopy (WDS). The high B solubility in the aNb 5 Si 3 phase has been confirmed with solubility range larger than that proposed in the 1600°C isothermal section of Nowotny et al. (Aufbau und Zunderverhalten von Niob-Bor-Silicium-Legierungen, Mon. Chem., 1960, 91, p 975-990). In contrast to Nowotny's isothermal section, it has been observed that the D8 8-phase should be nearly stoichiometric with composition close to 57Nb35Si8B (at.%). It was also found that the D8 8-phase does not equilibrate with NbB 2-phase at 1700°C. A negligible Si solubility in the boride phases as well as that of B in NbSi 2 has been noticed.
Intermetallics, 2004
The lattice parameters and thermal expansion coefficients ( a and c ) of the T 2 -phase of the Nb-Si-B system with different boron contents were determined from high temperature X-ray diffraction data (298-1473 K). Alloys with nominal compositions Nb 62.5 Si 37.5 , Nb 64 Si 30 B 6 and Nb 64 Si 14.4 B 21.6 (at.%) were prepared from high-purity materials through arc melting and heat-treatment under vacuum. Both a and c lattice parameters decrease with increasing boron content. A significant decrease in c is observed with increasing boron content while a remains practically unchanged, diminishing the anisotropy ratio c / a from approximately 1.2 at the binary composition to approximately 0.7 at Nb 64 Si 14.4 B 21.6 .
Microstructural characterization of multicomponent Nb-Ti-Si-Cr-Al-X alloys
Metallurgical and Materials Transactions A, 2006
Si system have become of interest for high-temperature structural applications. In the present work, the microstructure of multicomponent Nb-30Ti-8Si-10Cr-10Al-X (in at. pct) alloys in the as-cast and heat-treated conditions was studied using X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy. The effect of temperature and time on phase evolution was examined in detail. The as-cast microstructure was found to be composed of three phases: b (bcc), silicides (M 5 Si 3 type), and a Cr-rich Laves phase. The b phase was found to display B2-type ordering. The silicides in these alloys were generally quite stable during heat treatment, whereas the Cr-rich Laves phase was observed to dissolve on solutionization at temperatures above 1300°C. Aging of the solutionized materials between 900°C and 1100°C led to the precipitation of fine particles of another Laves phase in the b matrix. In addition, the b matrix revealed a tendency toward phase separation into Nb-rich (b 1) and Ti-rich (b 2) regions. The volume percentage and chemical composition of each phase has been determined as a function of time and temperature and the changes in microstructure have been rationalized in terms of the distribution of elements in various phases. The role of different alloying elements on the formation of these phases has also been critically examined.
Liquidus projection of the Nb–Si–B system in the Nb-rich region
Intermetallics, 2003
Alloys of the Nb-Si-B system have been evaluated aiming its use as high temperature structural materials. In this work the liquidus projection of the Nb-Si-B system in the Nb-rich region has been established based on the microstructural characterization of arc-melted alloys through X-ray diffraction (XRD) and scanning electron microscopy (SEM). Six different primary solidification regions were observed: Nb ss , NbB, T 2 (aNb 5 Si 3 ), Nb 3 Si, T 1 (bNb 5 Si 3 ) and D8 8 . The following ternary invariant reactions are proposed to occur in the region of study: II 1 : L+NbB()Nb ss +T 2 ; II 2 : L+T 1 ()Nb 3 Si+ T 2 ; II 3 : L+Nb 3 Si()Nb ss +T 2 ; III 1 : L+NbB+D8 8 ()T 2 ; III 2 : L+T 1 +D8 8 ()T 2 .
Thermodynamic and Microstructural Modeling of Nb-Si Based Alloys
Journal of Phase Equilibria and Diffusion, 2007
Nb-Si alloys have gained much attention over the last decade as the next generation alloys for high-temperature aero-engine applications due to their low density and improved mechanical properties. However, the microstructures of these alloys are quite complex and vary significantly with the addition of elements such as Ti and Hf. Hence, an improved understanding of the phase stability and the microstructural evolution of these alloys is essential for alloy design for advanced high-temperature applications. In the present paper, we describe the microstructural evolution modeling results of the dendritic and eutectic solidification of the binary Nb-16 at.% Si alloy, obtained using a Phase-Field simulations performed with MICRESS. The effect of parameters; such as heat extraction rate, the ratio of the diffusivity of the solute in liquid to solid, and the interfacial energy of liquid and solid interface, on the microstructural evolution during dendritic solidification is discussed in detail.
Advanced Composites Letters, 2018
The Nb/Nb 5 Si 3 based composites were fabricated by conventional casting (CC) and directional solidification (DS) methods. Microstructural characteristics, compressive properties and fracture toughness of the CC and DS composites were investigated by SEM, XRD, TEM, bending and compression tests. The results demonstrate that in the CC Nb/Nb 5 Si 3 based composite, the intergrowth of fine (Nb,Ti)ss and α-(Nb,Ti) 5 Si 3 phases leads to the formation of eutectic structure and the coarse α-(Nb,Ti) 5 Si 3 dendritic phase prefers to grow along eutectic cell boundary. The (Nb,Ti) 3 Si, (Ti,Nb) 5 Si 3 and Dy 2 O 3 phases mainly segregate along the eutectic cell boundary and moreover there is an orientation relationship between the (Nb,Ti) 3 Si and (Nb,Ti)ss phases: [001] (Nb,Ti)3Si //[112] (Nb,Ti)ss and (110) (Nb,Ti)3Si //(110) (Nb,Ti)ss. The DS processing promotes the formation of coarse primary α-(Nb,Ti) 5 Si 3 phase, (Ti,Nb) 5 Si 3 /(Nb,Ti)ss eutectic and α-(Nb,Ti) 5 Si 3 /(Nb,Ti)ss eutectic in the DS Nb/Nb 5 Si 3 based composite. Moreover, the (Nb,Ti)ss and α-(Nb,Ti) 5 Si 3 phases are aligned paralleling to the DS direction and exhibits strong crystal orientation preference. In addition, an orientation relationship between the (Nb,Ti) ss and α-(Nb,Ti) 5 Si 3 phases is observed: [310] α-(Nb,Ti)5Si3 //[110] (Nb,Ti)ss. Compared with the CC Nb/Nb 5 Si 3 based composite, the DS Nb/Nb 5 Si 3 based composite possesses the higher yield strength and fracture toughness, which should be ascribed to the microstructure optimization.
Microstructural Properties of Nb-Si Based Alloys Manufactured by Powder Metallurgy
Advanced Materials Research, 2011
An NbTiHfCrAlSi niobium silicide based atomized powder has been compacted by a conventional technique (hot extrusion) and by spark plasma sintering to nearly fully dense alloys. Both materials exhibit a metastable fine micrometer-sized microstructure that has been coarsened by a subsequent heat treatment. The densification of the SPS sample takes place between ca. 800°C and 1300°C.
Containerless processing and rapid solidification of Nb-Si alloys of hypereutectic composition
Metallurgical Transactions A, 1991
Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetically levitated drops were melted and subsequently splat quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Microstructures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the a-Nb + Nb3Si eutectic are also suggested.