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 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.     

The role of Fe and Ti additions in the microstructure of Nb–18Si–5Sn silicide-based alloys

The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)_ss, (Nb,Ti)₃Sn, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si, FeNb₄Si, and Fe₂Nb₃ and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)₃Si, (Nb,Ti)₃Sn and the Ti rich oxide. In NV8 the formation of Nb₅Si₃ was destabilised, the stability of Nb₃Si was enhanced and the eutectic between Nb₅Si₃ and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb₄Si and Fe₂Nb₃ phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)₃Sn, β(Ti,Nb)_ss, (Ti,Nb)₃Si and (Ti,Nb)₅Si₃ phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)_ss + (Ti,Nb)₅Si₃ → (Ti,Nb)₃Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb₃Sn, Ti₃Sn, and Nb₃Si, Ti₃Si, and Ti and Nb base 5-3 silicides are discussed.     

Phase relations in the Al–Pr–Sb system at 773 K

The isothermal section of the phase diagram of the Al–Pr–Sb ternary system at 773 K over the whole concentration region has been investigated mainly by powder X-ray diffraction (XRD) with the aid of scanning electron microscopy (SEM). A new ternary compound Al₁₁Pr₂₄Sb₆₅ has been found.     

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.     

Solid-state phase equilibria in the Gd–Co–Al ternary system at 1173 K

A portion of the isothermal section (1173 K) of the phase diagram of the Gd–Co–Al ternary system (up to 33.3 at.% Gd) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. Fourteen single-phase regions (including solid solution regions of the binary compounds), twenty-five two-phase regions and twelve three-phase regions were found to exist at this isothermal section. In the GdCo₂–GdAl₂ system, the existence of the GdCo_0.74Al_1.26 phase is identified and it has a composition range of 30–45 at.% Al, the maximum solid solubility of Al in GdCo₂ is about 15 at.%, and Co in GdAl₂ is about 16 at.%. Besides, the maximum solid solubility of Al in Co, Gd₂Co₁₇ and GdCo₅ is about 6, 17 and 25 at.%, respectively, the maximum solid solubility of Gd in Co, CoAl is below 2 at.% and the solid solubility range is from 47 to 61 at.% Al in CoAl phase. In this work, no new ternary compounds were observed.     

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).     

Phase equilibria in the Fe–Al–Mo system – Part I: Stability of the Laves phase Fe2Mo and isothermal section at 800 °C

Phase equilibria in the Fe–Al–Mo system were experimentally determined at 800 °C. From metallography, X-ray diffraction and electron probe microanalysis on equilibrated alloys and diffusion couples a complete isothermal section has been established. It is shown that the Laves phase Fe₂Mo is a stable phase. The phase Al₄Mo, which only becomes stable above 942 °C in the binary system, is the only ternary compound found at 800 °C. For all binary phases the solid solubility ranges for the third component have been established. The D0₃/B2 and B2/A2 transition temperatures have been determined for a selected alloy by differential thermal analysis and transmission electron microscopy. The results confirm that the D0₃/B2 transition temperature substantially increases by the addition of Mo, while the B2/A2 transition temperature is about that for a binary alloy with the same Al content.     

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.     

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.     

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.     

Microstructures and mechanical properties of Fe3Al-based Fe–Al–C alloys

In this paper results on the microstructures and mechanical properties of Fe₃Al-based Fe–Al–C alloys with strengthening precipitates of the perovskite-type κ-phase Fe₃AlC_x are presented. The alloys are prepared by vacuum induction melting and cast into Cu-moulds. The composition of the Fe₃Al matrix of the investigated Fe–Al–C alloys varies between 23 and 29 at.% Al. The ternary C-additions range from 1 to 3 at.%. The microstructures of the alloys are characterised by means of light optical microscopy (LOM). Phase identification is performed by means of X-ray diffraction (XRD). The strength of the alloys as a function of temperature is determined through compression tests. The room-temperature ductility is evaluated by tensile tests. The fracture surfaces of the tensile specimens are analysed using scanning electron microscopy (SEM).     

Atomic-scale microstructure and elastic properties of quaternary Zr–Al–Si–C ceramics

Transmission electron microscopy characterizations and elastic properties of two quaternary carbides, i.e. Zr₂(Al(Si))₄C₅ and Zr₃(Al(Si))₄C₆ are reported. The space group and atomic-scale microstructures of both compounds were determined using a combination of selected area electron diffraction, convergent beam electron diffraction, high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. In addition, the combined experimental and theoretical studies on elastic properties for Zr₂(Al(Si))₄C₅ are presented. A full set of second-order elastic constants, bulk modulus, shear modulus, and Young’s modulus were calculated using first-principles calculations. Both experimental and theoretical works demonstrated that quaternary Zr–Al–Si–C ceramics possess close elastic properties to ZrC. Furthermore, Zr₂(Al(Si))₄C₅ retained a high Young’s modulus up to about 1580 °C, which can be attributed to its comparable activation energy of lattice drag process to that of ZrC.     

The 773 K isothermal section of the Nd–Ni–V ternary system

The 773 K isothermal section of the phase diagram of Nd–Ni–V ternary system was investigated by X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. There are in total 14 single-phase regions, 25 two-phase regions and 12 three-phase regions in the 773 K isothermal section. The maximum solid solubilities of V in Ni, NdNi₅, Nd₂Ni₇ and NdNi₂, is about 16.5 at.% V, 2.0 at.% V, 1.0 at.% V and 3.0 at.% V at 773 K, respectively, and that of Nd in Ni, Ni₃V, Ni₂V, Ni₂V₃, NiV₃ and V does not exceed 1 at.% Nd. No ternary compounds have been observed in this work.     

Phase evolution of Mg–Al–Zr nanophase alloys prepared by mechanical alloying

Al–Mg and Al–Mg–Zr alloys were processed by mechanical alloying. The phase constitution of the powders was strongly dependent on the composition of the starting mixture. In as-milled powders, an Al(Mg) solid solution was formed with up to 40 at% Al, which after annealing transformed to the equilibrium β-Al₃Mg₂ phase. For high Mg concentrations (60–90 at%) the dominant phase was γ-Al₁₂Mg₁₇ in accordance with the equilibrium phase diagram. The addition of Zr led to the appearance of Zr–Al intermetallics causing Mg to precipitate out of the Al(Mg) solution. The effect of zirconium was also to refine the structure and to retard grain growth.     

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).     

Structure and transformation behaviour of rapidly solidified Ni–Al–Hf alloys

Rapidly solidified Ni–Al–Hf alloys of ternary eutectic compositions were studied by X-ray diffractometry, transmission electron microscopy and differential scanning calorimetry. Ni₆₆Hf₂₀Al₁₄ amorphous alloy with relatively high Hf/Al ratio showed high tensile strength of 1600 MPa and high thermal stability against crystallization. The formation of a nanoscale metastable intermetallic compound having a body-centered cubic lattice with a lattice parameter of a=1.22 nm was observed in the alloys with higher Al than Hf content. The transformation of an amorphous+metastable cubic mixture to AlNi₃ produces AlNi₃ of the same composition as the AlNi₃ phase formed on rapid solidification. Pm m AlNi₃ phase can dissolve up to 11 at.% Hf.     

New ternary aluminides T5M2Al having W5Si3-type structure

Six new ternary aluminides having W₅Si₃-type structure were found. These are: Zr₅Sn₂Al, Hf₅Sn₂Al, Ti₅Pb₂Al, Zr₅Pb₂Al, Hf₅Pb₂Al and Nb₅Sn₂Al.     

Solid-State Diffusion Bonding of Nb_(SS)/Nb_5Si_3 Composite Using Ni/Al and Ti/Al Nanolayers

Diffusion bonding of refractory Nb–Si-based alloy was performed with Ni/Al and Ti/Al nanolayers under the condition of 1473 K/30 MPa/60 min. The Nb_(SS)/Nb_5Si_3 in situ composite with the nominal composition of Nb–22 Ti–16 Si–3 Cr–3 Al–2 Hf was used as the parent material. The joint microstructures were examined by using a scanning electron microscope equipped with an X-ray energy dispersive spectrometer. Shear test was conducted for the bonded joints at room temperature.Within the joint bonded with Ni/Al multilayer, element diffusion occurred between the base metal and the nanolayer, with the reaction products of AlNb_2 + Ni_3 Al, NiAl and AlNi_2 Ti phases. The average shear strength was 182 MPa. While using Ti/Al multilayer, the interface mainly consisted of TiAl,(Ti,Nb)Al and(Ti,Nb)_2 Al phases, and the corresponding joints exhibited an increased strength of 228 MPa. In this case, the fracture mainly took place in the TiAl phase and presented a typical brittle characteristic.     

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.