The role of Sn and Ti additions in the microstructure of Nb–18Si base alloys

The effects of Sn and Ti on the microstructure and hardness of the as cast and heat treated Nb–18Si–5Sn (NV9) and Nb–24Ti–18Si–5Sn (NV6) alloys were studied. In both alloys the phases present in the as cast and heat treated microstructures were Nb_ss, Nb₃Sn and Nb₅Si_3. In NV9, Sn suppressed the formation of Nb₃Si, partitioned in Nb_ss stronger than in Nb₅Si₃ and did not affect significantly the solubility of Si in the Nb_ss. In NV6, the solubility of Ti in (Nb,Ti)_ss increased in the presence of Sn, the concentration of Ti in Nb₅Si₃ was sensitive to cooling rate and the solubility of Sn in Nb₅Si₃ decreased as the concentration of Ti increased. The Ti controlled the partitioning of Si between (Nb,Ti)_ss and Nb₃Sn and was considered responsible for the macrosegregation of Si in the as cast ingot. The transformation of β to Nb₅Si₃ was enhanced by the synergy of Sn and Ti. The addition of Ti did not destabilise the Nb₃Sn. Silicon increased the hardness of Nb₃Sn significantly, Sn did not affect the hardness of Nb₅Si₃ and Ti reduced the hardness of Nb₃Sn and Nb₅Si₃ significantly. The hardness of NV9 and NV6 decreased and increased, respectively, by heat treatment. The reduction of the hardness of NV6-AC compared to NV9-AC is attributed to the strong effect of Ti on the hardness of Nb₃Sn and Nb₅Si₃.     

Effect of Mo on microstructure and mechanical behaviour of as-cast Nbss–Nb5Si3 in situ composites

The effect of Mo on the structure–property relationships of arc-melted cast in situ composites of Nb solid solution (Nb_ss) and -(Nb,Mo)₅Si₃, with hypereutectic Nb–19.1Si–5.2Mo (A) and Nb–17.9Si–26.3Mo (B), and hypoeutectic Nb–12.8Si–4.1Mo (C) and Nb–12.3Si–14.8Mo (D) compositions has been studied. The influence of Mo concentration on lattice constants and microhardness of the constituent phases has been analyzed. The morphology, volume fraction and size distribution of the primary phase and the eutectic have been critically examined and related to the elastic modulus, hardness and fracture toughness of the composites. Fracture toughness values, determined through three-point bend and indentation tests, increase with higher amount of coarse eutectic but decrease with increasing Mo content. Indentation cracking exhibits R-curve type behaviour, promoted through bridging or arrest of cracks by the coarse Nb_ss in the eutectic. The study suggests promise for near-eutectic compositions of Nb–Si–Mo ternary system with low concentration of Mo for structural applications.     

Phase diagram of the Nb–Al–Si ternary system

A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Al–Si ternary system which is very valuable for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Aluminum is an alloying element for these composites, thus the Nb–Al–Si phase diagram, especially solubility of Al in Nb₅Si₃, is important information for the composite design. An isothermal section at 1000 °C was constructed from the results obtained from a diffusion multiple using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A ternary phase Nb₃Si₅Al₂ was observed. The solubility data of Al in Nb₅Si₃ and NbSi₂ as well as Si solubility in Nb₃Al, Nb₂Al and NbAl₃ were obtained. The new isothermal section helps to judge the reliability of the existing literature results and to add new data to the Nb–Al–Si phase equilibria.     

Influence of tin on the structure and properties of as-cast Ti-rich Ti–Si alloys

By the methods of DTA, X-ray diffraction, metallography and microprobe analysis, phase equilibria in the Ti-corner (more than 50 at.% Ti) of the Ti–Si–Sn system were studied. The solidus projection and the melting diagram (solidus+liquidus) were constructed. A new ternary compound T of composition Ti₅Si_1.2–1.6Sn_1.8–1.4 was found to form with the crystal structure of W₅Si₃-type. The ternary eutectic equilibrium L↔β-Ti+Ti₅Si₃+Ti₃Sn was established to occur at 1460 °C with the composition of the invariant point E at 77Ti–9Si–14Sn. Microhardness measurements were carried out for the primary grains of the alloys with 5 at.% Si.     

Determination of the Nb–Cr–Si phase diagram using diffusion multiples

A high-efficiency diffusion-multiple approach was employed to determine the phase diagram of the Nb–Cr–Si ternary system which is critical for the design of niobium silicide-based in situ composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. The formation of the Nb(Cr,Si)₂ Laves phase is beneficial to the high oxidation resistance of the composites and the Nb–Cr–Si system serves as the base for understanding the Laves phase formation. The results clearly demonstrate the applicability of the diffusion-multiple approach in determining such complex phase diagrams as Nb–Cr–Si which contains 14 phases. Two isothermal sections at 1000 and 1150 °C were constructed from the results obtained from diffusion multiples using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and electron backscatter diffraction (EBSD). Three ternary compounds, CrNbSi, (Cr,Nb)₆Si₅ and (Cr,Nb)₁₁Si₈, were observed at both temperatures, and the C14 Laves phase of the Cr–Nb binary system was stabilized by Si to lower temperatures.     

The ternary system: silicon–titanium–uranium

Phase equilibria in the system Si–Ti–U were established at 1000 °C by optical microscopy, EMPA and X-ray diffraction. Two ternary compounds were observed and were characterised by X-ray powder data refinement: (1) stoichiometric U₂Ti₃Si₄ (U₂Mo₃Si₄-type) with a small homogeneity region of about 3 at.% exchange U/Ti and (2) U_2−xTi_3+xSi₄ (Zr₅Si₄-type) extending at 1000 °C for 0.7<x<1.3. Mutual solubility of U-silicides and Ti-silicides was found to be below about 1 at.%. The Ti,U-rich part of the diagram was also investigated at 850 °C establishing the tie-lines to the low temperature compounds U₂Ti and U₃Si. U₂Ti₃Si₄ is weakly paramagnetic following a Curie–Weiss law above 50 K with μ_eff.=2.67 μ_B/U, Θ_P=−150 K and χ₀=1.45×10⁻³ emu/mol (18.2×10⁻⁹ m³/mol).     

High temperature strength of Nb3Al-base alloys

High temperature strength was investigated as a function of volume percent of Nb₃Al using ternary alloys with controlled microstructures of equiaxed Nb₃Al and Nb_ss (Nb solid solution) grains. Creep strength was examined in Mo-added Nb₃Al-base alloy with two types of different microstructures, equiaxed grains and directionally elongated grains. Mo addition increases high temperature strength at all the volume percents of Nb₃Al, while Ta addition is effective only at high volume percents of Nb₃Al. Ti addition decreases high temperature strength at all the volume percents of Nb₃Al. Mo-added Nb₃Al-base alloy consisting of directionally elongated grains has high creep strength compared to other refractory intermetallic alloys such as MoSi₂ alloy and (Cr,Mo)₃Si/(Cr,Mo)₅Si₃ alloy. Creep strength is decreased under a low applied stress in Mo-added Nb₃Al-base alloy with equiaxed grains probably because of easy grain boundary sliding. The obtained results are discussed in terms of solid solution strengthening of the constituent phases.     

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1₂/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1₂, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1₂ and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

Microstructure and mechanical behaviour of reaction hot pressed multiphase Mo–Si–B and Mo–Si–B–Al intermetallic alloys

Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown -Mo, Mo₃Si, and Mo₅SiB₂ phases. In addition, particles of SiO₂ formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while -Al₂O₃ formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of -Mo and Mo₃Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by -Mo, crack deflection and branching. Flexural strength is related to volume fraction of the -Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.     

Investigation of the interaction of the components in the Y–Si–Sb system at 670 K

Phase equilibria were established in the Y–Si–Sb ternary system at 670 K. The investigation of the phase relations was based on X-ray diffraction experiments made on arc-melted alloys, which were annealed up to 720 h. The 670 K isothermal section consists of 8 three-phase, 12 two-phase and 11 single-phase regions. The formation of a solid solution of Si in the binary YSb compound (8 at.% Si) has been observed. In the Y–Si–Sb system solid solutions between the isostructural binary compounds Y₅Si₃–Y₅Sb₃ form a continuous series. One ternary compound was observed: Y₅Si₂Sb₂ (Tm₅Si₂Sb₂ str. type, Cmca space group, a=1.4971(2), b=0.7855(2) and c=0.7820(2) nm).     

Structural chemistry and phase relations in intermetallic systems Ti–{Pd,Pt}–Al

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing arc melting, levitation melting under argon or by powder reaction sintering in closed crucibles. Three ternary compounds were observed at 950°C in the Ti–Pd–Al system: τ₃-(Ti,Pd)(Ti,Pd,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁ with a filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pd,Al)(Ti,Pd,Al)₃ with AuCu₃-type. Due to the wide extension of the Laves phase field, there is no compatibility among γTiAl and τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁. The Ti–Pt–Al system at 950°C contains three ternary compounds: τ₃-(Ti,Al)(Ti,Pt,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁ with the filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pt,Al) with Cu-type. Compatibility exists for Al-rich γTiAl and τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁. The typical feature for both alloy systems studied is the three-phase equilibrium: ₂Ti₃Al+γTiAl+τ₃-(Ti,Pd/Al)(Ti,Pd/Pt,Al)₂. The solid solubility of palladium and platinum in the binary titanium aluminides, as observed from EMPA and X-ray data, is rather small and at 950°C accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new oxide compounds Ti₃PdAl₂O_x and Ti₃PtAl₂O_x with a filled Ti₂Ni-type are observed in both quaternary systems.     

Nb–20%Ta alloy powder by the hydriding–dehydriding technique

Nb–Ta alloys have been used in the chemical industry to substitute pure Ta in corrosive environments (inorganic acids at high temperature). The production of components from these alloys does not show important technical problems due to the high ductility of these materials. The present work is aimed at the production of Nb–20%Ta (wt%) alloy powders by the hydriding–dehydriding technique. The alloy was produced in ingot form by the aluminothermic reduction of oxides (Nb₂O₅/Ta₂O₅) and electron beam melting. The hydriding step has been carried out in a hydrogen gas atmosphere at different temperatures using chips machined from the ingot. No significant hydriding has been observed in the experiments carried out below 500 °C, meaning that it is the lowest possible hydriding temperature of the material through the adopted experimental procedure. The XRD patterns of the hydride and Nb–20%Ta powders coincide with those of β-NbH_0.89 and Nb XRD standards, respectively. The powders were of angular and irregular morphology. The specific masses of the hydride and Nb–20%Ta powder were determined as approximately 8.55 and 9.57 g/cm³, respectively. The apparent and TAP specific masses of the hydride and Nb–20%Ta powders were (4.30/5.60) and (4.65/6.10) g/cm³, respectively.     

Production of niobium carbide ceramic composites derived from polymer/filler mixtures: preliminary results

Studies have shown that Al₂O₃–NbC composites present a good potential to be used for metalworking. Manufacturing of composite ceramic material derived from polymer reactive filler mixtures were investigated. The present study reports the preliminary results of reaction bonded niobium carbide derived from polymer (polysiloxane), inert filler (Al₂O₃) and reactive filler (Nb). Niobium powder, alumina and polysiloxane mixtures were homogenized in a planetary ball milling and pressureless sintered in inert atmosphere at temperatures up to 1600 °C. Depending on the niobium content and pyrolysis conditions, ceramic materials with a porosity of 20–40%, a weight loss of 5–15%, a linear shrinkage of 2–4% and a flexural strength of maximum 80 MPa were obtained. X-ray diffraction (XRD) of a sample containing 60 wt% polymer + 40 wt% Nb showed the presence of new crystalline phases such as NbC, Nb₃Si and Nb₅Si₃.     

Thermodynamic assessment of the Ti–B system

A consistent thermodynamic data set for the Ti–B system is obtained by means of CALPHAD technology. The sublattice model is used to describe the solid solution phases: (Ti%)₁(B, Va%)_0.5 and (Ti%)₁(B, Va%)₃ for the terminal solution (Ti) and (βTi), and Ti₁(B%, Ti)₁ and (B, Ti%)₁(B%, Ti)₂ for the compound solution TiB and TiB₂, respectively. The intermetallic compound Ti₃B₄ is treated as a stoichiometric compound. The liquid solution phase is assumed to be a substitutional solution with Redlich–Kister formula for the expression of its excess Gibbs energy. The complete T–x phase diagram for the Ti–B binary system is given. The calculation results agree well with experiments.     

Constitution, structural chemistry and magnetism in the ternary system Ce–Ag–Si

Phase relations were established for the Ce–Ag–Si system at 850°C by means of X-ray diffraction, light optical microscopy and quantitative electron probe microanalysis. Phase equilibria are characterised by the existence of extended solid solutions starting from the binaries: Ce(Ag_xSi_1−x)_2−y (ThSi₂-type), Ce(Ag_1−xSi_x)_1−y (unknown structure type) and Ce(Ag_1−xSi_x)_2−y (unknown structure type). Three ternary phases were found to exist, CeAg₂Si₂ (ThCr₂Si₂-type), Ce(Ag_xSi_1−x)_2−y (AlB₂-type) and the new ternary compound CeAgSi₂ with unknown structure type. Magnetic behaviour was studied from magnetic susceptibility and magnetisation measurements down to 1.7 K and employing magnetic fields up to 5 T. Soft ferromagnetism is observed for CeAg_xSi_2−x (AlB₂-type) below 5 K. Alloys Ce(Ag_xSi_1−x)_2−y with 0.08<x_Ag<0.30 (ThSi₂-type) encounter ferromagnetic order below 7 K. For x_Ag=0.31 the ferromagnetic interaction changes to antiferromagnetism with T_N=5.7 K. For CeAgSi₂ ferrimagnetic or canted antiferromagnetic order is indicated below 7 K.     

激光3D打印方法制备Nb/SiC体系梯度材料的微观组织及反应机理

采用激光3D打印的方法制备了Nb/SiC体系梯度材料,获得了从纯金属铌逐渐过渡到40%Nb+60%SiC(体积分数)复合层的梯度材料. 通过扫描电子显微镜(SEM)、电子探针(EPMA)和X射线衍射仪(XRD)分析了梯度复合层的微观组织. 结果表明,层与层之间结合良好,激光熔敷过程中铌和SiC之间发生了反应,生成了Nb₅Si₃,Nb₂C,NbC和NbSi₂等二元化合物,并且随着SiC含量的逐渐增加,梯度材料内部产物呈现如下变化趋势:Nb+Nb₅Si₃+Nb₂C+NbC→Nb₅Si₃+NbSi₂+SiC+NbC→NbSi₂+SiC+NbC. 当SiC含量达到30%时,梯度复合层中残留金属铌消失,且开始出现残余的SiC颗粒,随着原始SiC比例的增加,复合层中不仅残余SiC的含量增加,而且粒度也逐渐增大.     

The ternary system Al–Ni–Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation

Phase relations in the ternary system Al–Ni–Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1x_Al0.7. The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam melting of proper elemental powder blends. The existence of four ternary compounds, τ₁ to τ₄, is confirmed, however, in contrast to earlier investigations at significantly different compositions and with different shape of the homogeneity regions. This is particularly true for the phase regions of τ₃-Al₃NiTi₂ with the MgZn₂-type structure ranging from Al₃₀Ni₂₈Ti₄₂ (composition lowest in Al) to Al₅₀Ni₁₆Ti₃₄ (composition richest in Al) and for τ₂-Al₂NiTi. The complex atom site substitution mechanism in τ₃ changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diffraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{Al_xTi_1-x}₂+τ₃, we observed two closely adjoining three-phase equilibria: ₂-AlTi₃+Ni{Al_xTi_1-x}₂+ τ₄-AlNi₂Ti and ₂-AlTi₃+τ₃-Al₂NiTi₂+τ₄-AlNi₂Ti. The earlier reported “homogeneous phase at Al₂₃Ni₂₆Ti₅₁′” was shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic. The experimental results are in fine agreement with the thermodynamic calculation.     

Effect of immersion Ag surface finish on interfacial reaction and mechanical reliability of Sn–3.5Ag–0.7Cu solder joint

A comparative study on the preparation of specific titanium silicides (including Ti₃Si, Ti₅Si₃, Ti₅Si₄, TiSi, and TiSi₂) in the Ti–Si system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of their corresponding stoichiometries. The effect of initial sample stoichiometry was investigated on combustion characteristics and product composition. Experimental evidence shows a distinct combustion front traversing the entire reactant compact in a self-sustaining manner for all of the samples adopted in this study. However, as a result of the eutectic reaction, test samples composed of Ti:Si = 3:1 and 1:1 experienced excessive melting during the propagation of the reaction front. Moreover, the flame-front propagation rate and combustion temperature were found to vary significantly with starting stoichiometry of the powder compact. The flame-front velocity up to 50 mm/s was observed in the samples of Ti:Si = 5:4 and 5:3, in contrast to the values in the range of 2.6–3.5 mm/s for the compact of Ti:Si = 1:2. Reaction front temperatures between 1380 and 1460 °C were achieved by the powder compacts with Ti:Si = 5:4 and 5:3. For the samples of Ti:Si = 3:1 and 1:1, their flame-front temperatures were comparable and about 1330 °C. Consistent with its slow reaction front, the Ti + 2Si sample had the lowest combustion temperature close to 1150 °C. The XRD analysis confirms complete conversion from the reactant with Ti:Si = 5:3 to a single-phase silicide Ti₅Si₃. Powder compacts of Ti:Si = 5:4 and 1:1 yielded two silicide phases, Ti₅Si₄ and TiSi, in their end products. In spite of formation of a minor amount of TiSi, the disilicide TiSi₂ was identified as the dominant composition in the product obtained from the Ti + 2Si sample. Combustion products containing Ti₅Si₃ and a large amount of unreacted Ti were synthesized from the powder compacts of Ti:Si = 3:1, implying a poor degree of phase conversion.     

TiZrNiCu钎焊(Cf-SiCf)/SiBCN与Nb的界面产物及机理分析

采用TiZrNiCu钎料来实现改良的超高温陶瓷（C_f-SiC_f）/SiBCN与金属Nb的钎焊连接,研究了温度、时间对界面组织及力学性能的影响规律,对连接机理进行了分析. 结果表明,在900 ℃/20 min的工艺参数下,（C_f-SiC_f）/SiBCN-Nb接头室温抗剪强度最高达到36 MPa,接头典型的界面结构为Nb/Ti-Nb固溶体/（Ti, Zr）₂（Cu, Ni）/Zr₅Si₃ + Ti₅Si₃/TiC + ZrC/（C_f-SiC_f）/SiBCN. Cu元素在钎焊过程中逐渐从钎料扩散陶瓷母材中,通过与SiC反应生成Cu-Si脆性化合物进一步促进（C_f-SiC_f）/SiBCN陶瓷的分解,同时Cu-Si相是接头断裂路径由钎料层扩展到陶瓷侧的主要原因;保温时间过高时,陶瓷的分解程度增加,接头断裂在陶瓷内部;而温度过高时,固溶体前端与钎料层物相差异增大而引起了贯穿钎料层的裂纹.