A review of very-high-temperature Nb-silicide-based composites

The temperatures of airfoil surfaces in advanced turbine engines are approaching the limits of nickel-based superalloys. Innovations in refractory metal-intermetallic composites (RMICs) are being pursued, with particular emphasis on systems based on Nb-Si and Mo-Si-B alloys. These systems have the potential for service at surface temperatures >1350 °C. The present article will review the most recent progress in the development of Nb-silicide-based in-situ composites for very-high-temperature applications. Nb-silicide-based composites contain high-strength silicides that are toughened by a ductile Nb-based solid solution. Simple composites are based on binary Nb-Si alloys; more complex systems are alloyed with Ti, Hf, Cr, and Al. In higher-order silicide-based systems, alloying elements have been added to stabilize intermetallics, such as Laves phases, for additional oxidation resistance. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, high-temperature creep performance, and oxidation resistance. Recent progress in the development of composite processing-structure-property relationships in Nb-silicide-based in-situ composites will be described, with emphasis on rupture resistance and oxidation performance. The Nb-silicide composite properties will be compared with those of advanced Ni-based superalloys. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Cyclic oxidation response of multiphase niobium-based alloys

Cyclic oxidation tests were performed on multiphase Nb-based alloys containing silicide, Laves, and Nb solid solution phases. In particular, the oxidation resistance of six alloys with various compositions (Nb, Ti, Hf, Cr, Ge, and Si) and microstructures was characterized by thermal cycling from ambient temperature to a peak temperature that ranges from 900 °C to 1400 °C. Weight change data were obtained and the corresponding spalled oxides were collected and identified by X-ray diffraction. The results indicated that Nb-based alloys formed a mixture of CrNbO₄, Nb₂O₅, and Nb₂O₅ · TiO₂, with possibly small amounts of SiO₂ or GeO₂. The oxidation resistance was improved when CrNbO₄ formed instead of Nb₂O₅ and Nb₂O₅ · TiO₂. These results were used to assess the influence of microstructure and composition on the oxidation resistance of multiphase Nb-based alloys.     

Mechanical properties of NiAl-Y2O3-based powdered alloys produced by directional recrystallization

The mechanical properties of NiAl-Y₂O₃-based powdered composite alloys (0.5–7.5 vol %), including those with an NiAl intermetallic matrix alloyed with 0.5 wt % Fe and 0.1 wt % La have been studied. Structures with various aspect ratios (AR, the ratio of the grain length to the grain diameter) are formed using deformation and subsequent annealing. A combination of the optimum amount of strengthening phase (2.5 vol % Y₂O₃) and a quasi-single-crystalline structure with a sharp axial texture with the (100) main orientation and AR ≈ 20–40 provides the maximum short-term strength and life at temperatures up to 1400–1500°C. An NiAl-Y₂O₃ alloy (2.5 vol %) has the best strength properties among all known nickel superalloys at temperatures higher than 1200°C and can operate under moderate loads at temperatures higher than the working temperatures of nickel superalloys (by 100–400°C) and their melting points. Additional alloying with 10 wt % Co and 2 wt % Nb makes it possible to increase the ultimate tensile strength of an intermetallic NiAl matrix at 1100°C by a factor of 1.3–1.4.     

High-Temperature Oxidation Behavior of Two Nickel-Based Superalloys Produced by Metal Injection Molding for Aero Engine Applications

For different high-temperature applications like aero engines or turbochargers, metal injection molding (MIM) of superalloys is an interesting processing alternative. For operation at high temperatures, oxidation behavior of superalloys produced by MIM needs to match the standard of cast or forged material. The oxidation behavior of nickel-based superalloys Inconel 713 and MAR-M247 in the temperature interval from 1073 K to 1373 K (800 °C to 1100 °C) is investigated and compared to cast material. Weight gain is measured discontinuously at different oxidation temperatures and times. Analysis of oxidized samples is done via SEM and EDX-measurements. MIM samples exhibit homogeneous oxide layers with a thickness up to 4 µm. After processing by MIM, Inconel 713 exhibits lower weight gain and thinner oxide layers than MAR-M247.     

Microstructural Stability and Hot Deformation of γ–γ′–δ Ni-Base Superalloys

Nickel-base superalloys exhibit excellent high-temperature mechanical and physical properties and remain the first choice for structural components in advanced gas turbine engines for the aerospace propulsion and power generation applications. In response to the increasing demand for more efficient solutions and tighter requirements linked to gas turbine technologies, the properties of nickel-base superalloys can be improved by modification of their thermo-mechanical and/or compositional attributes. Recent investigations have revealed the potential use of ternary eutectic γ–γ′–δ Ni-base superalloys in advanced gas turbines due to high temperature mechanical properties that are comparable to state-of-the-art polycrystalline Ni-base superalloys. With properties largely dependent on microstructural strengthening mechanisms, both the composition and thermo-mechanical processing parameters of this novel class of alloys need to be optimized concurrently. The hot deformation characteristics of four γ–γ′–δ Ni-base superalloys with varying levels of Nb were evaluated at temperatures and strain rates between 1353 K and 1433 K (1080 °C and 1160 °C) and 0.01 to 0.001/s, respectively. Evidence of dislocation-based plasticity was observed following deformation at low temperatures and high strain rates, while high temperatures and low strain rates promoted superplasticity in these alloys. The extent of the microstructural changes and the magnitude of the cavitation damage which occurred during deformation was found to vary as a function of the alloy composition.     

Fabrication and evaluation of Nb/Nb5Si3 microlaminate foils

Alloys of Nb and Nb₅Si₃, and in particular Nb/Nb₅Si₃ microlaminates, have potential as high-temperature materials. In this study, microlaminates of Nb and amorphous Nb-37.5 at. pct Si are magnetron sputter deposited from elemental Nb and polycrystalline Nb₅Si₃ targets. The microlaminates are heat treated at high temperatures to produce crystalline layers of Nb and Nb₅Si₃ that are flat, distinct, and stable for at least 3 hours at 1200 °C. The layers consist of textured Nb grains and equiaxed submicron Nb₅Si₃ grains. Initial room-temperature tensile tests indicate that the microlaminates have strengths similar to cast and extruded alloys of Nb and Nb₅Si₃. The fracture mode of the Nb layers is dependent on the Nb layer thickness, with thin layers failing in a ductile manner and thick layers failing by cleavage. The Nb layers bridge periodic cracks in the Nb₅Si₃ layers, and using a shear lag analysis, the tensile strength of Nb₅Si₃ is estimated. The results indicate that microstructurally stable and mechanically robust microlaminates of Nb and Nb₅Si₃ can be fabricated by sputter deposition with a high-temperature heat treatment. The processing, microstructure, and mechanical properties of these microlaminates are discussed.     

The stability of Nb/Nb5Si3 microlaminates at high temperatures

The microstructural, phase, and chemical stability of Nb/Nb₅Si₃ microlaminates was investigated at temperatures ranging from 1200 °C to 1600 °C. Freestanding Nb/Nb₅Si₃ microlaminates were prepared by sputter deposition and their stability was investigated by annealing either in vacuum or in an Ar atmosphere. The microlaminates were generally structurally stable, with no evidence of layer pinchoff, even after annealing at 1600 °C. However, a small volume fraction (<2 pct) of voids formed in the silicide layers at 1500 °C and 1600 °C, which are attributed either to the Kirkendall diffusion of Si or to the growth of silicide grains. In terms of phase stability, there was no discernible dissolution of the Nb₅Si₃ layers and no silicide precipitates in the Nb layers following anneals at 1400 °C. Annealing at higher temperatures, though, resulted in the formation of non-equilibrium Nb₃Si on the Nb/Nb₅Si₃ interfaces. This phase is thought to precipitate from the supersaturated Nb-Si solid solution on cooling, and is stabilized by the development of tensile stresses in the Nb layers. The most pervasive observed high-temperature breakdown mechanism was chemical in nature, namely, the loss of Si via sublimation to the environment. The Si loss was partially suppressed either by annealing in a Si-rich atmosphere or by annealing in Ar.     

On the Effect of Nb on the Microstructure and Properties of Next Generation Polycrystalline Powder Metallurgy Ni-Based Superalloys

The effect of Nb on the properties and microstructure of two novel powder metallurgy (P/M) Ni-based superalloys was evaluated, and the results critically compared with the Rolls-Royce alloy RR1000. The Nb-containing alloy was found to exhibit improved tensile and creep properties as well as superior oxidation resistance compared with both RR1000 and the Nb-free variant tested. The beneficial effect of Nb on the tensile and creep properties was due to the microstructures obtained following the post-solution heat treatments, which led to a higher γ′ volume fraction and a finer tertiary γ′ distribution. In addition, an increase in the anti-phase-boundary energy of the γ′ phase is also expected with the addition of Nb, further contributing to the strength of the material. However, these modifications in the γ′ distribution detrimentally affect the dwell fatigue crack-growth behavior of the material, although this behavior can be improved through modified heat treatments. The oxidation resistance of the Nb-containing alloy was also enhanced as Nb is believed to accelerate the formation of a defect-free Cr₂O₃ scale. Overall, both developmental alloys, with and without the addition of Nb, were found to exhibit superior properties than RR1000.     

On the oxidation of the third-generation single-crystal superalloy CMSX-10

The oxidation behavior of the third-generation nickel-base single-crystal superalloy CMSX-10 is examined. Since the in-service performance of the alloy is of the greatest practical significance, a detailed study is made of the microstructural degradation of a turbine blade that had been removed prematurely from a commercial gas turbine engine. The results are augmented with isothermal oxidation tests conducted in the laboratory for 100 hours, at temperatures of 800 °C, 900 °C, and 1000 °C. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, microhardness testing, and X-ray diffraction (XRD) were employed. It was found that the oxidation of CMSX-10 at temperatures below 1000 °C does not produce either Al₂O₃ or the spinel Ni(Cr,Al)₂O₄, both of which are found in the internal oxidation zone of the earlier generations of superalloys. Surprisingly, it is demonstrated conclusively that the oxidation of CMSX-10 generates the β phase (NiAl). This reaction, which to the authors’ knowledge has not yet been reported, is termed self-aluminization. The XRD studies demonstrate that the internal oxidation of CMSX-10 produces (Ni,Co)Ta₂O₆, (Ni,Co)WO₄, CrTaO₄, and Cr(W,Mo)O₄. There is indication that the formation of the δ phase (Ni₂Al₃) slows the oxidation rate at 1000 °C.     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute     

Tensile Properties and Deformation Characteristics of a Ni-Fe-Base Superalloy for Steam Boiler Applications

Ni-Fe-base superalloys due to their good manufacturability and low cost are the proper candidates for boiler materials in advanced power plants. The major concerns with Ni-Fe-base superalloys are the insufficient mechanical properties at elevated temperatures. In this paper, tensile properties, deformation, and fracture characteristics of a Ni-Fe-base superalloy primarily strengthened by γ′ precipitates have been investigated from room temperature to 1073 K (800 °C). The results showed a gradual decrease in the strength up to about 973 K (700 °C) followed by a rapid drop above this temperature and a ductility minimum at around 973 K (700 °C). The fracture surfaces were studied using scanning electron microscopy and the deformation mechanisms were determined by the observation of deformed microstructures using transmission electron microscopy. An attempt has been made to correlate the tensile properties and fracture characteristics at different temperatures with the observed deformation mechanisms.     

Isothermal oxidation of TiAl alloy

Isothermal oxidation behavior of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range 850 °C to 1000 °C. The oxidation was essentially parabolic at all temperatures with significant increase in the rate at 1000 °C. Effective activation energy of 404 kJ/mol was deduced. The oxidation products were a mixture of TiO₂ (rutile) and α-Al₂O₃ at all temperatures. An external protective layer of alumina was not observed on this alloy at any of the temperatures studied. A layered structure of oxides was formed on the alloy at 1000 °C.     

Unconstrained and constrained tensile flow and fracture behavior of an Nb-1.24 At. Pct Si alloy

The unnotched and notched tensile behavior of the β-phase constituent (Nb with Si in solid solution) of the (Nb)/Nb₅Si₃ composite has been investigated at room temperature and -196 °C. At room temperature, the unnotched tensile behavior comprises significant strengthening due to Si, low strain-rate sensitivity, low strain hardening, extensive ductility, and ductile microvoid coalescence fracture, even at strain rates as high as 1.1 s⁻¹. At −196 °C, the unnotched alloy exhibited much higher strength, good ductility, and cleavage fracture. At room temperature, the notched specimens exhibited cleavagelike fracture with significant plasticity, and at −-196 °C, they exhibited cleavagelike fracture with much lower plasticity at the notch. A finite-element analysis (FEA) of stress and strain fields in the vicinity of the notch root, together with un-notched tensile behavior, indicates that plasticity plays an important role in nucleating cracks, while the high-axial tensile stress component governs crack propagation. These results are used to rationalize the observed toughening and fracture behavior of a (Nb)/Nb₅Si₃ composite.     

On the embrittlement of a rapidly solidified Al-Fe-V-Si alloy after high-Temperature exposure

The effects of high-temperature exposure on the mechanical properties and the microstructure of a rapidly solidified Al-Fe-V-Si alloy were examined in order to identify the critical factors controlling the thermal stability of the alloy, particularly at temperatures above 400 °C. Room-temperature (RT) tensile tests were conducted on specimens exposed to temperatures ranging from 150 °C to 482 °C for 100 hours. The microstructure of the extrusion is characterized by a banded structure consisting of a layer containing fine silicide dispersoids and a layer containing coarse silicide dispersoids, which is a replication of the microstructure of the melt-spun ribbon. The alloy did not show any significant change in tensile properties after 100 hours exposure up to 427 °C due to the stability of the microstructure. However, after exposure above 427 °C, tensile elongation decreased significantly and the brittle cleavage regions were observed on the fracture surface. The occurrence of brittle cleavage fracture is due to the presence of coarse equilibrium Al₁₃Fe₄ or Al₃Fe phase, which forms by the transformation of the coarse silicide dispersoids above 427 °C.     

Tensile properties and microstructure of haynes 25 alloy after aging at elevated temperatures for extended times

The tensile properties of Haynes 25 alloy have been measured after various aging treatments, time, and temperature: as received; and aged at 600 °C for three months; 800 °C for 6 and 12 months; and 1000 °C for 3 and 6 months. Contour plots in temperature-ln (time) space were constructed based on the literature and our own data, detailing changes in yield strength, ultimate tensile strength, and tensile elongation. Scanning electron microscopy and transmission electron microscopy observations of the Haynes 25 alloy microstructure provided an explanation of why the properties changed with aging. Intense lattice distortions after aging at 600 °C, the presence of an α-Co₃W, a L1₂-ordered, fcc phase, a=0.357 nm, after aging at 800 °C, and the nucleation and growth of W₃Co₃C carbides from aging at 800 °C and 1000 °C produced the changes in tensile properties. We did not observe either the Co₂W Laves phase or Co₇W₆ γ phase in any of the material conditions we examined, using TEM of thin foils: as received and aged at 600 °C, 800 °C, and 1000 °C. Other researchers believe these phases cause a loss of ductility in the Haynes 25 alloy with prolonged high-temperature exposure.     

The effect of thermal exposure on the microstructure and mechanical properties of nickel-base superalloys

The three nickel-base superalloys B-1900, TRW-NASA VIA, and René 80 were studied utilizing metallographic and residue analysis techniques in conjuction with mechanical property tests to determine the effect of thermal exposure on the microstructure and mechanical properties. Exposure times of 10, 100, 1000, and 5000 h at temperatures from 1400 to 2000°F (760 to 1093°C) were evaluated. Four minor phases-MC, M₆C, M₂₃C₆, and M₃B₂-plus gamma-prime were observed in the gamma matrix of these alloys. Significant variations in the mechanical properties were observed to occur with thermal exposure. Microstructural evaluation indicated that these variations in properties were due primarily to gamma-prime agglomeration or ripening. Perturbations noted in a number of the mechanical property vs exposure temperature curves in the 1500 to 1900°F (816 to 1038°C) temperature range appeared to be due to the precipitation and growth of M₆C and/or M₂₃C₆ carbides.     

Phase Evolution in and Creep Properties of Nb-Rich Nb-Si-Cr Eutectics

In this work, the Nb-rich ternary eutectic in the Nb-Si-Cr system has been experimentally determined to be Nb-10.9Si-28.4Cr (in at. pct). The eutectic is composed of three main phases: Nb solid solution (Nb_ss), β-Cr₂Nb, and Nb₉(Si,Cr)₅. The ternary eutectic microstructure remains stable for several hundred hours at a temperature up to 1473 K (1200 °C). At 1573 K (1300 °C) and above, the silicide phase Nb₉(Si,Cr)₅ decomposes into α-Nb₅Si₃, Nb_ss, and β-Cr₂Nb. Under creep conditions at 1473 K (1200 °C), the alloy deforms by dislocation creep while the major creep resistance is provided by the silicide matrix. If the silicide phase is fragmented and, thus, its matrix character is destroyed by prior heat treatment [e.g., at 1773 K (1500 °C) for 100 hours], creep is mainly controlled by the Laves phase β-Cr₂Nb, resulting in increased minimum strain rates. Compared to state of the art Ni-based superalloys, the creep resistance of this three-phase eutectic alloy is significantly higher.     

Microstructures and High-Temperature Mechanical Properties of a Martensitic Heat-Resistant Stainless Steel 403Nb Processed by Thermo-Mechanical Treatment

Thermo-mechanical treatments (TMT) at different rolling deformation temperatures were utilized to process a martensitic heat-resistant stainless steel 403Nb containing 12 wt pct Cr and small additions of Nb and V. Microstructures and mechanical properties at room and elevated temperatures were characterized by scanning electron microscopy, transmission electron microscopy, and hardness, tensile, and creep tests. The results showed that high-temperature mechanical behavior after TMT can be greatly improved and microstructures with refined martensitic lath and finely dispersed nanosized MX carbides could be produced. The particle sizes of M₂₃C₆ and MX carbides in 403Nb steel after conventional normalizing and tempering (NT) treatments are about 50 to 160 and 10 to 20 nm, respectively, while those after TMT at 1123 K (850 °C) and subsequent tempering at 923 K (650 °C) for 2 hours reach about 25 to 85 and 5 to 10 nm, respectively. Under the condition of 260 MPa and 873 K (600 °C), the tensile creep rupture life of 403Nb steel after TMT at 1123 K (850 °C) is 455 hours, more than 3 times that after conventional NT processes. The mechanisms for improving mechanical properties at elevated temperature were analyzed in association with the existence of finely dispersed nanosized MX particles within martensitic lath. It is the nanosized MX particles having the higher stability at elevated temperature that assist both dislocation hardening and sub-grain hardening for longer duration by pinning the movement of dislocations and sub-grain boundary migration.