Nb Ti Si Research Articles

Formation of near core-shell-like structure and dual-phase nanoprecipitation behavior in Nb–Si–Ti based alloys

Dual-phase metal materials can usually take into account the advantages of both phases, but the heterogeneity of dual-phase properties will cause flow instability. Therefore, it is difficult to further improve the microstructure and properties of Nb–Si–Ti based alloys containing ductile Nbss and hard silicides through hot working. Here, we propose a method for interface modification by precipitating an intermediate phase and forming a core-shell-like structure with silicides. A novel near core-shell structure (β-Nb5Si3/Nb4FeSi) is obtained in Nb–Si–Ti based alloy by deep cryogenic quenching and annealing. The precipitation of Nb4FeSi is closely related to stacking faults, where high-density stacking faults can develop into the nanotwins during annealing at 1200 °C. In addition, two types of nanoprecipitation behaviors are observed in the Nbss and β-Nb5Si3. The nanoscale plate-like δ-Nb11Si4 is precipitated along the [100]Nbss and [010]Nbss in the Nbss matrix, while the precipitation of granular Nb4FeSi from β-Nb5Si3 satisfies (1‾10)β//(01‾0)Nb4FeSi. In addition to the twinning induced plasticity of Nb4FeSi (shell), a phase transformation from β-Nb5Si3 to α-Nb5Si3 and Nbss that occurs within the core-shell structure is beneficial for the hot forming of the alloy.

Microstructure characterization of high temperature mechanisms in a Nb–Ti–Si alloy

The deformation mechanisms of an Nb–20Si–25Ti–6Al–3Cr–3Mo alloy deformed in compression at 1000 °C and at a strain rate of 10−4 s−1 have been investigated, using focused ion beam and transmission electron microscopy techniques. The initial microstructure of the alloy was constituted of a solid solution bcc Nbss matrix and of Nb5Si3 phases of β (tetragonal) and γ (hexagonal) crystallographic structures, with also hcp-Ti precipitates. As expected, the only phase which exhibited plastic deformation at 1000 °C was the Nbss matrix. Glide of b→=1/2111 dislocations has been observed, in planes compatible with {112} and {123} planes, or with cross-slip in {110} planes, as classically reported in bcc structures. Conversely, climb contribution has not been evidenced, probably because of rearrangement of the climbing dislocations in sub-boundaries. Fine δ-Nb11Si4 precipitates have been detected in the samples compressed at 1000 °C. Interactions of the δ-Nb11Si4 precipitates with the dislocations have been observed, which can potentially lead to strengthening of the Nbss phase.

Microstructure evolution and fracture toughness of Nb–Si–Ti based alloy with Cr, Mo and W elements addition

This paper deals with microstructural evolutions, phase constitutions and fracture toughness of Nb–16Si–22Ti-(6Mo/8Cr/2W) alloys using the vacuum non-consumable arc-melting. The results show that the addition of 6 at.% Mo or 8 at.% Cr promotes the eutectic reaction of (Nb, X)3Si phase → Nbss/(Nb, X)5Si3 eutectic. The common addition of 8 at.% Cr and 6 at.% Mo leads to the microstructure transform from hypoeutectic microstructure to near-eutectic microstructure. The Nb–16Si–22Ti–8Cr alloy with coupled structure of Nbss/(Nb, X)5Si3 eutectic has the maximum fracture toughness of 13.4 MPa m1/2. The 6 at.% Mo or 2 at.% W addition has little influence on the room-temperature fracture toughness KQ value, but the common addition of 8 at.% Cr, 6 at.% Mo and 2 at.% W can improve room-temperature fracture toughness KQ value. The high content of Nbss phase and Nbss dendritic crystal in the matrix can contribute to fracture toughness of NbSi alloy. The coupled structure of Nbss/(Nb, X)5Si3 eutectic can prevent the crack propagation and improve the fracture toughness.

Microstructure evolution and mechanical properties of ZrC-added Nb–16Si–20Ti alloys

The effect of ZrC addition on the phase composition, microstructure, and room-temperature mechanical properties of arc melted Nb–Si–Ti based alloys has been systematically investigated. The results indicated that the addition of ZrC promotes the decomposition of (Nb,X)3Si phase. The addition of ZrC promotes the microstructure transformation from hypoeutectic to hypereutectic, and the eutectoid reaction of (Nb,X)3Si to Nbss/(Nb,X)5Si3 eutectic structure. The KQ value of Nb–16Si–20Ti–1ZrC alloy is the highest, reaching 13.9 MPa m1/2. The decomposition of (Nb,X)3Si phase can increase the room-temperature fracture toughness. However, the precipitation of primary γ-(Nb,X)5Si3g-b phase deteriorates the room-temperature fracture toughness. Meanwhile, the more the content primary γ-(Nb,X)5Si3g-b is, the lower the room-temperature fracture toughness is. The maximum compressive strength of Nb–16Si–20Ti–1ZrC alloy is 2331 MPa, and the fracture strain of Nb–16Si–20Ti–3ZrC alloy is 11.4%. The fracture morphology of Nb–16Si–20Ti based alloys with ZrC additions is a brittle quasi-cleavage fracture.

Improvement of oxidation resistance of Nb–Ti–Si based alloys with additions of Al, Cr and B at different temperatures

The effects of Cr, Al and B addition on the microstructure and high-temperature oxidation behaviors (at 1200, 1250 and 1300 ​°C) of Nb–Ti–Si based alloys were investigated. The results showed that the addition of Cr stabilized α-Nb5Si3, while Al promoted the formation of β-Nb5Si3 and adding B promoted the formation of γ-Nb5Si3. Among the three elements, Al and Cr were beneficial to oxidation resistance at 1200 ​°C, and B was favorable to the oxidation resistance at 1300 ​°C. At 1250 ​°C, Al and B had the same effects on the improvement of oxidation resistance. The ratio of these alloying elements might play an important role in enhancing oxidation resistance. The oxidation resistance of the three kinds of silicides was compared, and the sequence was: γ-Nb5Si3> β-Nb5Si3> α-Nb5Si3. To predict the effects of the investigated alloying elements on the oxidation resistance of Nb–Ti–Si based alloys in a wider range of concentration, an artificial neural network (ANN) model was established, showing excellent accuracy and generalization ability. With the instructions of the ANN model, the oxidation resistance can be optimized with less additions of alloying elements.

Effect of C addition on microstructure and mechanical properties of Nb–Si–Ti based alloys

To improve the mechanical properties of Nb–Si alloy, the influence of C addition on microstructural evolution and mechanical properties of Nb–16Si–24Ti based alloys is studied. Results show that with the C content increase, the microstructure of Nb–16Si–24Ti- x C transforms from hypoeutectic structure to nearly eutectic structure, and the phase constitutions transform from Nbss + (Nb, Ti) 3 Si to Nbss + γ-(Nb, Ti) 5 Si 3 . The microstructure transforms into fully eutectic structure and TiC phase is precipitated by addition of 4 at.% C. The element C is mainly dissolved in the Nbss phase, and the nanoindentation hardness of Nbss phase obviously increases with the increasing of C content, which is attributed to the solid solution strengthening by C addition. The fracture toughness increases from 6.3 MPa m 1/2 to 14.9 MPa m 1/2 , as the C content increases from 0 at.% to 3 at.%, since with the C content increase, the c/a ratio decreases, the Nbss phase content increases, and the Nbss phase becomes more continuous. The addition of 4 at.% C induces the formation of TiC phase in the matrix, and thus decreases the fracture toughness.

Rapid directionally solidified microstructure characteristic and fracture behaviour of laser melting deposited Nb–Si–Ti alloy

Aiming at achieving fine and directionally-solidified microstructure of Nb–Si based alloy, Nb, Si and Ti powder particles were utilized as the raw materials, and laser melting deposition (LMD) experiments were conducted with 1500 ​W and 2000 W laser power, respectively. The microstructure characteristic, micro-hardness, and indentation fracture toughness were examined by scanning electron microscope (SEM), X-ray energy dispersive spectrometer (XEDS) and X-ray diffraction (XRD). The results showed that the two kinds of as-deposited Nb–17Si–23Ti alloy samples were mainly composed of NbSS, (Nb, Ti)3Si and Ti-rich NbSS, and the microstructure presents rapid directionally-solidified characteristic, in which the <100> crystallographic direction of NbSS and <110> crystallographic direction of (Nb, Ti)3Si tend to be parallel to the LMD building direction. With the laser power increasing from 1500 ​W to 2000 W, the microstructure became more oriented and homogeneous. For the sample at 2000 W, the dendrite spacing was only about 1–2 ​μm, and NbSS and (Nb, Ti)3Si couples grow alternately and they are parallel to the building direction strictly. In the meantime the micro-hardness and indentation fracture toughness reached 1075 HV and 20.3 ​MPa·m1/2, respectively, indicating significant improvement by the directional growth of fine NbSS and (Nb, Ti)3Si phases, and the dimple fracture mode of the NbSS dendrites.

НИОБИЙ, АРМИРОВАННЫЙ ВОЛОКНАМИ α-Al2O3 Часть 1. Двухкомпонентные композиции

The paper reviews the results of development of a new class of high-temperature composites based on niobium and various types of reinforcers continuous monocrystalline fibers (MCF) α-Al2O3, with TiN, Mo, W barrier coatings, and with controlled (Si, Ti) and uncontrolled (O, C) impurities. The analysis of Nb–Si, Nb–C, Nb–O binary diagrams and Nb–Si–Ti, Nb–Fe–Ti ternary diagrams was performed, on the basis of which the matrix compositions were selected. The basis for the preparation of composites was the powder method of mechanical alloying of the mixture preparing, followed by its pressing together with α-Al2O3 MCF by spark plasma sintering (SPS) and further preparation of experimental samples. An analysis of the interaction of fibers with a matrix was carried out, where the matrix was Nb or system on the basis of the above mentioned binary or ternary diagrams.

Influence of molybdenum contents on the microstructure, mechanical properties and oxidation behavior of multi-elemental Nb–Si based ultrahigh temperature alloys

The mechanical properties, including room-temperature fracture toughness, microhardness and compressive strength at 1250 °C, and oxidation resistance at 1250 °C of Nb–Si based alloys with different Mo contents (0, 2, 5 and 10 at.% Mo additions) were systematically evaluated. The roles of Mo content additions in the relationships between these properties and microstructures in a multi-elemental system of Nb–Ti–Si–Cr–Al–V-Hf-Zr-Mo were revealed. The results show that the four alloys are all composed of primary γ(Nb,X)5Si3 blocks, three types of Nbss/γ(Nb,X)5Si3 eutectic and a small amount of Nbss/Cr2Nb eutectic. Alloying with Mo obviously decreases the amount and size of primary γ(Nb,X)5Si3 blocks, and correspondingly increases the concentration of Si in the remaining liquid during solidification, which results in preferable formation of regular eutectic. Both the microhardness of microstructural constituents at room temperature and the compressive strength at 1250 °C of the alloys are obviously improved due to the solid solution strengthening effect by Mo addition in the alloys. Adding 2 at.% Mo in the alloys simultaneously ameliorates the room-temperature fracture toughness as well as the oxidation resistance at 1250 °C of the alloys, while the additions of 5 and 10 at.% Mo in the alloys deteriorate them. Moreover, the adhesion of oxide scales is significantly improved by alloying with Mo.

Phase equilibria and multi-element interaction study in the metal-rich side of the Nb–Ti–Si–Al quaternary system

The knowledge of phase diagram is theoretical principle for compositional design. Since Ti and Al are important alloying elements for Nb–Si alloys, and Nb and Si are important alloying elements for Ti–Al alloys, Nb–Ti–Si–Al phase equilibria is prerequisite for optimal microstructural design of both Nb–Si and Ti–Al systems. In this work, key alloys were selected firstly according to calculated isothermal section based on extrapolated thermodynamic description, then key alloy ingots were prepared by arc-melting and heat treated to reach equilibrium state. Currently obtained phase equilibria data coupled with critically reviewed literature data were employed to study the element interaction of relative phases (α-Nb5Si3, β-Nb5Si3, Ti5Si3, Nb3Al and TiAl et al.) in the Nb–Ti–Si–Al system. Thermodynamic parameters of these phases were modified accordingly to reproduce the determined phase relationships. Finally, the important phase equilibria features of the Nb–Ti–Si–Al system were described with several isothermal sections. The phase equilibria extending from ternary systems and new phase equilibria appearing in the quaternary space were demonstrated in this work.

Strength and Corrosion-Fatigue Crack-Growth Resistance of Alloys of the Ti–Nb–Zr–Si System Intended for Biomedical Purposes

We study the influence of the chemical composition and the conditions of thermomechanical and thermal treatments on the structural strength determined by the characteristics of strength and cyclic crack-growth resistance, as well as on the corrosion and corrosion-fatigue properties of alloys of the Ti–Nb–Si and Ti–Nb–Zr–Si systems. These characteristics are compared with the characteristics determined for the extensively used Ti–Al–V alloy intended for biomedical applications. It is shown that, under the conditions of reliable control over the formation and development of mechanical defects (fatigue cracks) in the products made of Ti–18.7Nb–1.0Si alloy, this alloy may serve as a good alternative to Ti–6Al–4V alloy.

Study of Interfacial Bonding in Composite Materials Based on NbSi Eutetic Reinforced with Single-Crystal α-Al2O3 Fibers with a W Coating

Abstract—The relationship between the Nb–Si–Ti matrix and single-crystal α-Al2O3 fibers with a W barrier coating at the interphase matrix–coating–fiber boundaries is studied. It is established that at 1300°C the bending strength of the composite material reinforced with fibers with a coating is 2 times higher than that of the matrix and is 1.3 times higher than the strength of the composite material reinforced with fibers without a coating.

Формування структури литих біосумісних стопів системи Ti–Nb–Si

Формування структури литих біосумісних стопів системи Ti–Nb–Si

Microstructure and oxidation behaviors at 800°C and 1250°C of MoSi2/ReSi2/NbSi2 compound coating prepared by electrodeposition and then pack cementation

Abstract Mo Re duplex film has been electrodeposited from aqueous solution innovatively and then applied to fabricate protective silicide coating for the Nb Ti Si based alloy successively. The XRD, SEM and EDS results indicated the formation of MoSi2/ReSi2/NbSi2 compound coating after halide reactivated pack cementation (HAPC) treatment. Pesting oxidation of MoSi2 layer was suppressed and the Nb Ti Si based alloy was effectively protected at 800 °C with a mass gain of 0.48 mg cm−2 after oxidation for 100 h. At 1250 °C, continuous mass gain was observed with a mass gain of 2.6 mg cm−2 after exposure of 10 h. The synergistic combination of MoSi2 and NbSi2 layer enabled admirable protection of the substrate, in which MoSi2 layer worked as the major anti-oxidation coating and the NbSi2 layer could be a reservoir for Si. The contrast experiments indicate that ReSi2 layer could improve the integrity and adherence of MoSi2 layer, reduce the outward diffusion of alloying elements and oxidation of NbSi2 layer, and then improve oxidation resistance of the compound coating ultimately.

Investigation of phase boundaries in composition materials based on Nb–Si eutectics reinforced by monocrystal fibers of α-Al2O3 with Mo coating

Interaction of the Nb–Si–Ti matrix and monocrystalline fibers of α-Al2O3 with barrier Mo coating at the phase boundaries matrix–coating–fiber is investigated. It is determined that the bending strength at 1300°C of the composition material reinforced by fibers with coating is two times higher than that of the matrix and higher than that of the composition material reinforced by fibers without coating by 50%.

Phase equilibria in the Nb–Ti side of the Nb–Si–Ti system at 1200 °C and its oxidation behavior

Abstract Nb–Si based composites are one of the most promising superalloy for the next generation turbine airfoil materials. Alloying Ti can efficiently improve their poor oxidation resistance. In this work, five equilibrium alloys were used to determine the unclear but underlying phase equilibria in the Nb–Ti rich region of the Nb–Si–Ti system at 1200 °C. Convincing evidences on three-phase equilibria of bcc(Nb,Si,Ti) + αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si, αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si + Ti(Nb) 5 Si 3 and αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si + Ti(Nb) 5 Si 3 at 1200 °C were obtained. The important tie-triangle of bcc(Nb,Si,Ti) + αNb(Ti) 5 Si 3 + (Nb,Ti) 3 Si turns out to be much narrower than the previous calculation. An optimized thermodynamic description validated in the Nb–Ti rich (≤37.5 at.% Si) region between 1000 and 1500 °C of the Nb–Si–Ti system was subsequently acquired based on calculation of phase diagram (Calphad) method. Furthermore, the oxidation kinetics of three equilibrated alloys with increasing Ti content (5–30 at.%) were evaluated at 1050 °C. The one with bcc(Nb,Ti,Si) + (Nb,Ti) 3 Si equilibrium performed the lowest oxidation rate, which might due to the formation of compact TiO 2 film and the absent of fine eutectic structure bcc(Nb,Si,Ti) + αNb(Ti) 5 Si 3 . These knowledge is contributive to understanding the Ti effect on oxidation resistance of Nb–Si based alloys along with the variation of phase constitutions and microstructures.

SHS metallurgy of Nb–Si composites

Niobium-based composites doped with functional and alloying additives (Si, Hf, Ti, Al, etc.) have prospects for industrial applications such as aircraft engine building. Previously the authors demonstrated that such composites can be synthesized in autowave mode (combustion mode) using highly exothermic Nb2O5 mixtures with Al, Si, Hf and Ti. It was shown that hafnium actively participates in Nb2O5 reduction, and this makes it difficult to introduce it into the composite. This paper focuses on the possibility to synthesize Nb composites doped with a high amount of Hf using centrifugal SHS metallurgy. Experiments on a centrifugal unit under 40 g force demonstrated that reactive Hf replaced by its less reactive compounds Hf–Al or Hf–Ti–Si–Al in Nb2O5/Al mixtures enabled combustion in a steady frontal mode rather than in an explosive one. With the increasing size of Hf–Al granules (from 0–40 to 160–300 μm), the Hf content of resultant composites was found to grow from 1,3 to 3,8 wt.%. In case of Hf–Ti–Si–Al granules 1–3 mm in size introduced to the charge, the Hf content of synthesized composites based on niobium silicides attained a value of up to 8,1 wt.%. Electron microscopy and X-ray diffraction analysis were used to determine the integral composition and distribution of basic and doping elements in the structural components of synthesized composites as well as their phase composition. Composites with a maximum content of Hf (8,1 wt.%) contain three structural constituents: (1) a metal Nb–Si–Ti matrix; (2) intergrain boundaries containing Nb, Ti, and Al; and (3) hafnia-based inclusions. The XRD pattern showed the presence of three phases in the composite: Nb and Nb5Si3 solid solutions as well as minor amounts of Nb3Si.

Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nbss and γ-Nb5Si3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb3Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.

ИССЛЕДОВАНИЕ ВЫСОКОТЕМПЕРАТУРНОЙ ПРОЧНОСТИ in-situ-КОМПОЗИТОВ НА ОСНОВЕ Nb, АРМИРОВАННЫХ МОНОКРИСТАЛЛИЧЕСКИМИ ВОЛОКНАМИ α-Al2O3

ИССЛЕДОВАНИЕ ВЫСОКОТЕМПЕРАТУРНОЙ ПРОЧНОСТИ in-situ-КОМПОЗИТОВ НА ОСНОВЕ Nb, АРМИРОВАННЫХ МОНОКРИСТАЛЛИЧЕСКИМИ ВОЛОКНАМИ α-Al2O3

Study of NbSi composites reinforced by single-crystal fibers of α-Al2O3 with and without interfacial coating

The interaction between the Nb–Si–Ti matrix and single-crystal α-Al2O3 fibers with and without the interfacial coating on interphase boundaries is studied: fiber–coating–matrix and fiber–matrix. It is established that at 1300°C the bending strength of the composite material reinforced by fibers without coating is 1.5 times higher than that of the matrix, and the bending strength of the material reinforced by fibers with coating is 2.5 times higher.