Tensile properties of a refractory metal base in situ composite consisting of an Nb solid solution and hexagonal Nb5Si3

Tensile properties and fracture behavior of an Nb_SS/Nb₅Si₃ in situ composite in which the silicide has a hexagonal structure has been investigated. Excellent tensile strength of 460 MPa is obtained at 1470 K. The fracture is attributed to cleavage of the Nb₅Si₃, Nb_SS/Nb₅Si₃ interface separation and ductile rupture of Nb_SS.     

Nb-16Si-10Ti-10Mo-5Hf原位复合材料拉伸性能及其变形机制

采用机械合金化和放电等离子烧结方法制备了Nb-16Si-10Ti-10Mo-5Hf原位复合材料。采用不同温度下的拉伸试验评价了其力学性能,结合其不同温度下的断口形貌研究了其变形机制。结果表明,复合材料的微观组织由Nbss(铌固溶体)、金属间化合物Nb_5Si_3和少量的Nb_3Si相组成,晶粒呈等轴状。室温和1200℃抗拉强度分别为413和496 MPa。从室温到1200℃拉伸断裂方式为Nb5Si3相解理脆性断裂,1200℃拉伸延伸率仅为1.2%;然而,在1300℃拉伸试验中,其拉伸延伸率为27%,这归因于Nbss延性的增加和界面/晶界强度的降低;在1400℃和更高的温度,材料具有极大的塑性或超塑性,塑性变形机制由晶内滑移转变为晶界滑移。晶界滑移在三叉晶界处产生的应力集中通过软化的Nbss协调而释放,从而避免了早期断裂。     

Study of the role of Ta and Cr additions in the microstructure of Nb–Ti–Si–Al in situ composites

Niobium silicide-based in situ composites are Nb-base alloys with high Si content that have the potential for higher temperature capability than the Ni-base superalloys. Microstructure-property studies of these alloys have been the subject of many research programmes, where the differentiation between the αNb₅Si₃ and βNb₅Si₃ is usually not clear, even though it is essential to understanding the solidification of the alloys and the stability of their microstructures at high temperatures. In this work, the effects of Cr (5 or 8 at.%) and Ta (6 at.%) in the microstructures of as-cast and heat-treated Nb–24Ti–18Si–5Al in situ composites have been studied. The main phases observed in the as-cast and heat-treated (100 h at 1400 or 1500 °C) alloys were the niobium solid solution, (Nb,Ti)_ss, the niobium 5–3 silicides, αNb₅Si₃ and βNb₅Si₃, and a Cr-rich C14 silicide Laves phase. During solidification, Al additions promoted the formation of βNb₅Si₃, while the Cr additions caused the appearance of the C14 silicide Laves phase that was probably formed congruently from the remaining liquid. During heat treatment, the βNb₅Si₃ phase transformed to αNb₅Si₃ according to the reaction βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss. The Cr addition lowered the melting temperature of the alloys as liquation was observed after 100 h at 1500 °C in the two Cr-rich alloys. Ta and Cr retard the βNb₅Si₃→αNb₅Si₃+(Nb,Ti)_ss transformation. Solid state diffusion was sluggish in the presence of Ta, but the Ta addition did not destabilize the three-phase equilibrium among (Nb,Ti)_ss, αNb₅Si₃ and the C14 silicide Laves phase, in the Nb–24Ti–18Si–6Ta–8Cr–4Al alloy.     

A study of the effects of Hf and Sn additions on the microstructure of Nbss/Nb5Si3 based in situ composites

The phase stability and microstructures of Nb_ss/Nb₅Si₃ based in situ composites alloyed with Hf and Sn have been investigated. The Nb₅Si₃ silicide in the β and γ forms was the primary phase in the Nb–24Ti–18Si–5Cr–5Al–5Hf–2Mo and Nb–24Ti–18Si–5Cr–5Al–5Hf–5Sn–2Mo alloys studied in this work. In the as cast alloys, the formation of the Nb₃Si phase was suppressed by the addition of Hf. The structure of the Nb₅Si₃ phase was mainly affected by the Hf addition. The Hf-rich regions in the 5–3 silicide probably corresponded to the γNb₅Si₃. This silicide was stable up to 1500 °C in the presence of Hf. The Ti solubility in the Hf-rich Nb₅Si₃ was higher than that in the Nb₅Si₃. The Si concentration in the Nb₅Si₃ phase increased slightly and shifted closer to its stoichiometric composition by the addition of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb₅Si₃ and Nb_ss, respectively. The Sn addition had a significant effect on the niobium solid solution leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in Nb_ss increased considerably whilst the Cr solubility decreased. As a result of the decrease of the Cr solubility in Nb_ss, the Sn addition promoted the formation of Si-rich C14 Cr₂Nb Laves phase and stabilised this phase at 1300 °C. TiN and HfO₂ were formed below the surfaces of the heat treated alloys.     

Mechanical properties and fracture behavior of an Nb-Silicide in situ composite

An Nb-Silicide in situ composite with a nominal composition of Nb-16Si-10Ti-10Mo-5Hf (at. %) was fabricated by mechanical alloying followed by hot-pressing sintering. The microstructure consisted of an Nb solid solution, Nb₅Si₃ and a small amount of Nb₃Si. This in-situ composite exhibited good balance of strength between ambient temperature and high temperatures; the ultimate tensile strength was 413 and 496 MPa at room temperature and 1200 °C, respectively. The tensile fracture behavior was dominated by cleavage of the Nbss and Nb₅Si₃ at 1200 °C and lower temperatures. However, the fracture behavior was governed by ductile rupture of Nbss at 1300 °C and higher temperature, which was ascribed to both the increased ductility of Nbss and the decreased interface strength. At 1400 °C and higher temperature, the material exhibited extensive plasticity or superplasticity; the dominant deformation mechanism was grain boundary sliding at 1400 °C and higher temperature.     

The effects of Ti and Mo additions on the microstructure of Nb-silicide based in situ composites

The effects of Ti and Mo additions on phase selection, phase transformations and microstructure development in as cast and heat-treated Nb–Si–Cr–Al in situ composites have been investigated. The βNb₅Si₃ was formed in all the as cast alloys, which are classified as hyper-eutectic alloys. After heat treatment at 1500 °C, the βNb₅Si₃ transformed to αNb₅Si₃ completely or partially. The lattice parameter of the bcc Nb solid solution (Nb_ss) decreased with the addition of Ti and with increasing Mo concentration. The C14–Cr₂Nb Laves phase was absent in the alloys with Ti addition at 24at.% in the presence of Mo (≤5 at.%). The eutectoid decomposition of Nb₃Si to Nb_ss and αNb₅Si₃ was very sluggish in the alloy without Ti but it was enhanced with the addition of Ti. The partitioning of Ti between the Nb_ss and (Nb,Ti)₅Si₃ led to the formation of Ti-rich (Nb,Ti)₅Si₃, where the concentration of Ti was about 30.3 at.% in the heat treated microstructure. Nb_ss particles precipitated inside αNb₅Si₃ in the heat treated alloys.     

Reactive growth of niobium silicides in bulk diffusion couples

The diffusion-controlled growth of niobium silicides (NbSi₂ and Nb₅Si₃) was studied in Nb/Si and Nb/NbSi₂ bulk diffusion couples annealed at 1200–1350 °C for 2–24 h. Both compounds were found to grow as parallel layers, according to the parabolic rate law. The concept of the integrated diffusion coefficient is used to describe the growth kinetics of the two silicides. The corresponding activation energy is 263 kJ/mol for Nb₅Si₃ and 304 kJ/mol for NbSi₂. The activation energy (in eV) scales as 0.98T_m(K)/1000 for Nb₅Si₃ and as 1.4T_m(K)/1000 for NbSi₂ in agreement with the general behavior observed for many transition metal silicides. The position of the Kirkendall plane inside the Nb₅Si₃ layer developed in Nb/NbSi₂ couples indicates that, in the present temperature range, the diffusion of Si in Nb₅Si₃ is considerably faster than that of Nb.     

Si合金化对C15 NbCr2 Laves相稳定性和断裂韧性影响的第一性原理研究

基于第一性原理计算方法,通过对形成焓、结合能、原子自由体积和电子结构的计算,研究了Si合金化对C15 NbCr₂Laves相稳定性和断裂韧性的影响。位点占据能表示Si原子倾向于占据Cr位点。形成焓和结合能计算表明,随着Si含量的增加,Nb₈Cr_16-xSi_x(X= 0~ 5)相的形成能力和稳定性均得到提升且与Si含量保持线性相关性。原子自由体积计算表明,Nb₈Cr_16-xSi_x相的原子自由体积较NbCr₂基体相均得到增加,其中在Si含量为8.33 at%(Nb₈Cr₁₄Si₂)时,原子自由体积取得最大值,断裂韧性达到最优。电子结构计算表明,Si合金化使得DOS曲线右移,费米能级向赝能隙峰谷靠近,稳定了NbCr₂基体相,同时所有的成键峰变得下降和展宽,削弱了Nb-Cr原子的键合强度,使得剪切变形易于进行,从而提高韧性。     

HRTEM observation of silicides and Laves phase precipitates in Nb-Ti-Si based alloys

The morphologies, crystallographic characteristics and formation processes of silicides and Laves phase precipitates in Nb-Ti-Si based alloys have been systematically investigated by high-resolution transmission electron microscopy (HRTEM) observation. Silicide precipitate of δNb₁₁Si₄ usually exhibits homogeneous distribution with acicular and tetrapod morphologies, possessing uniform orientation relationships (ORs) with Nbss. The occurrence of these morphologies might be caused by the symmetry decrease during the phase transformation of Nbss ⟶ δNb₁₁Si₄ + Nbss₁. In some cases, the δNb₁₁Si₄ precipitates also adopt heterogeneous distribution with plate morphology and identical ORs as tetrapod precipitates with Nbss. The Laves phase precipitate, Cr₂Nb, possesses C15 structure and contains high densities of stacking faults (SFs). It always forms along defects or Nbss/γNb₅Si₃ interfaces. The coupling precipitation behaviors and precipitation reactions of δNb₁₁Si₄, Cr₂Nb and γNb₅Si₃ in the Nb-Ti-Si based alloys have been discussed.     

Oxidation of niobium particles embedded in a sintered ceramic matrix – a proposed model

Ceramics display superior properties of hardness, chemical stability and refractoriness. However, their brittleness often limits their application as engineering components. The addition of second‐phase inclusions that influence crack propagation has been exhaustively studied. One of the main toughness enhancement theories proposes it is caused by the plastic deformation of ductile inclusions during crack propagation. This article proposes a model to explain the oxidation mechanisms of niobium particles embedded in an Al₂O₃ matrix, based on a thermogravimetric analysis. Two types of oxidation kinetics are identified here: linear and parabolic kinetics, with activation energies of 12.4 kJ/mol and 24.9 kJ/mol, respectively. Based on our results, a kinetic model is proposed to describe the oxidation of niobium particles embedded in alumina ceramic composite matrix.     

Thermodynamic modeling of the Nb–Si system

Optimized coefficients of the Gibbs free energy expression for each stable phase of the binary Nb–Si have been obtained, using the Thermo-Calc program for this purpose. The Nb₃Si, αNb₅Si₃, NbSi₂ and Diamond (Si) have been modeled as stoichiometric phases and the liquid L, BCC (Nb) and the βNb₅Si₃ phases as solutions, with the excess term described using the Redlich–Kister formalism. The Si solubility in the βNb₅Si₃ phase has been modeled according to two possibilities: (i) Si substituting for Nb and (ii) vacancies in the Nb positions. The calculated diagrams compare well with the experimental information taken from the literature.     

Effects of alloying elements on phase stability in Nb–Si system intermetallics materials

The effects of alloying elements, such as Ti, Cr, Al and Hf on phase stability in Nb–Si system intermetallics materials have been investigated by scanning electron microscopy (SEM), X-ray energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The binary, ternary and multicomponent alloys have been fabricated by vacuum non-consumable arc melting method. The results showed that Ti and Hf tend to stabilize Nb₃Si phase to lower temperatures, while Cr and Al promote the direct formation of Nb₅Si₃ phase. The coordinate effects of alloying elements on microstructures of Nb–Si system intermetallics materials have also been examined.     

Microstrucural characterization of gas atomized Fe73.5Si13.5B9Nb3Cu1 and Fe97Si3 alloys

Powder particles of Fe_73.5Si_13.5B₉Nb₃Cu₁ and Fe₉₇Si₃ soft magnetic alloys have been prepared by gas atomization. The gas atomized powder was microstructurally characterized and the dependence of coercivity with the composition and powder particle size investigated. As-atomized powder particles of both compositions were constituted by a bcc α-Fe (Si) solid solution. The Fe_73.5Si_13.5B₉Nb₃Cu₁ powder particles presented a grain microstructure with dendrite structure, which dendrite arms were enriched in Nb. The coercivity increased as the particle size decreased, with a minimum coercivity, of 5 Oe, measured in the Fe₉₇Si₃ alloy in the range of 50–100 μm powder particle size. The coercive fields were quite higher in the Fe_73.5Si_13.5B₉Nb₃Cu₁ than in the Fe₉₇Si₃ powder, due to the Nb addition, which produced a phase segregation that leads to a noticeable magnetic hardening.     

Effects of alloying on the microstructures and mechanical property of Nb-Mo-Si based in situ composites

Nb-Mo-Si based alloys have been prepared by arc melting in a water-cooled copper crucible under an argon atmosphere. The effects of Al, Cr, Hf and Ti additions on the phase components and stability, microstructures and mechanical property of Nb-Mo-Si based alloys have been studied. The results indicated that the additions of Al, Cr and Ti elements do not change the phase components of Nb-20Si-6Mo alloys, which are composed of Nb solid solution (Nb_ss) and β-Nb₅Si₃. The phase component is α-Nb₅Si₃ and Nb_ss in Nb-20Si-6Mo-3Hf alloy. The additions of Cr and Ti element make the Nb_ss/Nb₅Si₃ eutectic morphology anomalous and coarsening. The element segregation is obvious found with the additions of Hf and Ti. The enrichments of Hf and Ti change the compositions of retained melt and promote the formation of fine eutectic structure. After heating treatment at 1873K for 10 h, β-Nb₅Si₃ phase translates into α-Nb₅Si₃ phase and γ-Nb₅Si₃ phase. The eutectic structures tend to anomalous and coarsening. The Ti solid solution (Ti_ss) phase was found in Nb-20Si-6Mo-20Ti alloy and the formation mechanism of Ti_ss phase was discussed. The high temperature (1523 K) compression strength of as cast Nb-Mo-Si based alloys increased with the additions of Al, Cr, Hf, and decreased with Ti addition.     

Effect of tin addition on Nb–Si-based in situ composites. Part I: Structural modifications

This paper reports the effect of adding 2 to 8 at.%Sn on the microstructure of Nb–25Ti–8Hf–2Cr–2Al–16Si. The samples were synthesised by casting and were heat treated for 5 days at 1200 °C. The as-cast and heat-treated microstructures were characterised using X-ray diffraction, scanning electron microscopy and microprobe analysis. The results revealed strong microstructural changes when Sn content exceeded 2 at.% Sn. The two-phase (Nb,Ti)_SS/γ-M₅Si₃ composite evolves towards a quaternary phase equilibrium: (Nb,Ti)_SS saturated with Sn/α-M₅Si₃/(Nb_1−xTi_x)₃(Sn_1−yTi_y)/γ′-M₅Si₃. The γ′-M₅Si₃ differs from the γ-M₅Si₃ through its Hf and Ti contents. The (Nb,Ti)_SS fraction decreases strongly, benefitting the (Nb_1−xTi_x)₃(Sn_1−yTi_y) fraction. Therefore, adding greater than 2 at.% Sn addition may dramatically affect the mechanical properties of Nb/Nb₅Si₃ composites because the solid solution (Nb,Ti)_SS has been shown to dominate the toughness of such materials.     

Influence of vibrational entropy on structural stability of Nb–Si and Mo–Si systems at elevated temperatures

The heats of formation of stable and metastable phases of the Nb–Si and Mo–Si systems were studied using density functional theory (DFT). The high-temperature behavior of the competing phases was studied by performing additional phonon calculations. Our theoretical results rationalize the major differences observed in the behavior of the Nb–Si and Mo–Si systems: Nb₃Si is only stable at temperatures above 2043 K, whereas Mo₃Si is always stable; Nb₅Si₃ and MoSi₂ undergo phase changes at elevated temperatures, in contrast to Mo₅Si₃ and NbSi₂. These differences are qualitatively explained by including the vibrational entropy to the free energies within the harmonic approximation. In particular, the softer shear moduli of the Nb₅Si₃ and MoSi₂ βphases cause their stabilities over the α phases at elevated temperature.     

Micromagnetic simulation of the distribution of magnetization in semiinfinite single crystals

The enrichment of the Fe_73.5Si_13.5B₉Cu₁Nb₃ amorphous alloy in iron was found to change the kinetic regime of primary crystallization, which becomes two-stage rather than single-stage. The observed peculiarities of the crystallization behavior of the alloy were explained assuming that niobium plays a key role in the formation of its amorphous structure.     

Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

The microstructure and oxidation resistance of NbSi₂ coating formed on Nb-based alloy C-103 by a pack siliconization process have been studied. The as-formed coating consists of an outer NbSi₂ layer and an inner Nb₅Si₃ layer. A NbSi₂–Nb₅Si₃ two-phase zone is also present between the above two layers. Weight-change data obtained under isothermal and cyclic oxidation in air at 1100 and 1300°C, suggests that the coating gives oxidation protection up to about 4 h. The oxide scale that formed on the coating during oxidation exposure consists of an outer glassy silica layer and an inner Nb₂O₅-silica mixed layer. Nb₂O₅ phase is also present in the outer silica scale in the form of elongated particles. Oxidation protection is achieved primarily by the presence of the glassy silica layer on the surface. Spallation of this layer during thermal cycling causes significant reduction in the protective life of the coating.     

Morphology, structure and formation mechanism of silicide coating by pack cementation process

The MoSi2 coating on C 103 niobium based alloy was prepared by pack cementation method. The formative mechanism, morphology and structure of coating were investigated. The silicide coating was formed by reactive diffusion obeying parabolic rule during pack cementation process. It is found that the composite structural coating is composed of three inferior layers as follows. The main layer is composed of MoSi2, the two phases＇ transitional layer consists of NbSi2 and a few NbsSi3 and the diffuse layer is composed of NbsSi3. The dense amorphous glass layer formed on the surface at high temperature oxidation circumstance can effectively prevent the diffusion of oxygen into coating.     

Study of three-phase equilibrium in the Nb-rich corner of Nb–Si–Cr system

The full understanding of the Nb–Si–Cr ternary system is important for the development of Nb silicide based composites, which show great potential for high temperature applications. There was, however, disparity in experimental observations of phase equilibrium in the vicinity of the Nb-corner. Two kinds of three-phase equilibrium, Nb_ss+C14+αNb₅Si₃ and Nb_ss+CrNbSi+αNb₅Si₃, in the Nb corner of the Nb–Si–Cr system have been reported in the literature. This work aims to clarify the three-phase equilibrium near the Nb-corner, by studying phase equilibrium in the Nb–18Si–15Cr ingot. Such a composition was chosen with the assistance of CALPHAD calculations to avoid unnecessary load of work. The alloy ingot was prepared by clean melting followed by heat treatment at 1000 and 1500 °C. The C14 Laves phase formed in all the samples and was stable at both temperatures. The results confirmed that Si has the effect of stabilising the C14 Laves phase down to at least 1000 °C. The three-phase equilibrium Nb_ss+C14+αNb₅Si₃, instead of Nb_ss+CrNbSi+αNb₅Si₃, was observed in this work. The current work demonstrates that ingot metallurgy is necessary to check the reliability of the information obtained about the ternary and higher-order phase diagrams, especially for the regions where the exact phase boundaries are in question. The investigation of the selected alloy was greatly helpful to clarify the confusion of the three-phase field near the Nb corner in the Nb–Si–Cr ternary system. The work confirmed the CALPHAD prediction of phase equilibrium near the Nb corner, showing the power to combine phase diagram predictions with experimental work for cost effective alloy development.